A good forum for radiant barrier

[gsubrec]

I have searched ebay and yahoo, and I see wildly different
claims on radiant barrier insulation and many vendors bad-mouthing their competition

Is there any forum where I can find some unbiased information

[pstu]

The very best research I have found from FSEC, the Florida Solar Energy Center. It is head and shoulders better than anything we have in Texas, Louisiana, or the other Gulf states. And nothing I have seen from them makes my baloney detector go off. A Google search will give you lots more than the few links that I could offer you.

I believe another decent source of info is Oak Ridge National Laboratory (ORNL). One thing about government research, they are reasonably objective.

Any info from Lstiburek is well done but he is riding a hobby horse to have sealed and condtioned attics in hot climates, so he won't give full coverage to what he considers second-best alternatives. Personally I think it is wiser to believe there is more than one way to build.

I will testify nearly every new house I see in my neighborhood is using radiant barrier in the form of special roof decking with foil on one side. It is well accepted as good practice in my neighborhood far west of Houston. And personally if you get under RB when a roof is partway done, you can really feel that it is cooler.

Hope this helps -- Pstu

[mark beiser]

No scientific stuff to back it up, but having worked in almost every concievable attic situation you can find in Texas, these are my observations.

Roof decking with the radient barrier as part of it works the best.

A radient barrier stapled to the underside of the roof rafters works about as well as the decking, but is more prone to damage, and I have seen some radient materials that degraded to the point that the layers were seperating and pealing off.

The only spray on barriers I have seen were done by a company called Efficient Attic Solutions. I have had several customers that had it done, and the difference in attic air temperature, the temperature of metal surfaces in the attic, and the reduction in heat gain in the ductwork was amazing.
EAS will tell you up front that the spray on barrier is only about 70% as effective as a "real" radient barrier, but it is a whole lot less expensive to install as a retrofit in a finished house.

People that install radient barriers laying down on the floor of the attic need to be shot. It makes it very difficult, even dangerous, to work in the attic. If the barrier doesn't cover the equipment and duct system, it does nothing to reduce heat gain in the ductwork. If it does cover the equipment and duct system, it makes service a PITA.

[cem-bsee]

RB is rendered almost worthless by dust --

[pstu]

A couple of FSEC publications which might answer many questions. I believe FSEC research is honestly and thoroughly done, if any other source contradicts FSEC research, I would at first distrust the other source. If any source can *credibly* contradict FSEC, then things are getting extremely interesting and I would want to know all the details.


"CONSUMER FACTS ABOUT RADIANT BARRIERS", FSEC pub.FS-37 1994
http://www.p2pays.org/ref/08/07694.pdf

"Radiant Barriers: A Question and Answer Primer" FSEC-EN-15
http://www.fsec.ucf.edu/Pubs/energynotes/en-15.htm

That second publication I believe has been recommended recently by another poster. It bears repeating. One detail I disapprove of, is documenting that RB can be laid flat over insulation vs. attached to the roof rafters. Sure this works initially when everything is clean, but as FSEC notes in the details, RB loses effectiveness when dust collects on the top surface. When RB is at the roofline, the *bottom* surface is pretty much guaranteed not to collect dust.

Hope this helps -- Pstu

++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++
P.S. Here is an example of IMO hucksterism, advertising false claims about RB chips:
http://www.savenrg.com/1testpag.htm

Note the claims of electricity savings "a 25% to 40% decrease in total monthly energy costs", truly incredible claim because the entire ceiling and duct heat gain will not be this big!
Note that FSEC and several utilities forbid their research being used by this company, IMO because they know it will be used for false claims.
And finally note the bizarre claim that roof mounted RB *increases* energy use. Any consumer shopping for info on the internet really needs to have a working baloney detector.

[Edited by pstu on 06-06-2006 at 10:23 AM]

[jdb52]

gsubrec

Most of the responders to your question are from Texas as I am. Loading of attics via sun is something we deal with. In my locale, expected temp today is 101.

At some point, you simply have to make a judgement about this stuff. Yes, there is some hype. You cannot lay it over insulation like they claim you can. But, in my judgement, I wish I had it. I have a west facing house, dark roof, and 101 today.

The research from FSEC and anecdotal reports like from Mark Beiser are enough to swing judgement toward believing in the value of radiant barrier, both rafter placement and paint for a retrofit.

I can't remember the claimed percentage of heat load reducation due to radiant barrier, but I do remember thinking that it was enough to make it worthwhile if you have significant loading from the sun. Reducing heat load is the result of taking many small steps which cumulatively make a huge difference when you consider their effects over a long period of time.

I vote yes for radiant barrier.

Good luck
jdb

[fat bob]

http://www.ornl.gov/sci/roofs+walls/radiant/index.html

This is prepared by the Oak Ridge National Laboratory, one of the most respected labs in the country. They used to build nuclear weapons.

[jdb52]

The few comments on this thread giving links and anecdotal information provide the most concise and helpful treatment of radiant barrier that I have seen.

Nice work
jdb

[cem-bsee]

just read article by Oak Ridge Labs -- excellent

RB must face vented open space, gable ends to be covered --

see BUILDINGSCIENCE.com about vents & rainwater carried by high winds for long periods!

I had wondered about affect upon roofing -- got my answer

at 101F, what is the highest temp inside attic?

[uktra]

ptsu---the "hobby horse" that you claim Dr. Lstiburek is riding is Secretariat. If you really took the time to study the man, you know that Joe gives several ways to do things depending on climate and building costs. Building a sealed attic is one of the best ways to save energy and increase comfort, when HVAC equipment is placed there, as it is in many homes. Remember, Joe is one of the pioneers in building healthy, durable, energy effecient homes at little or no increase in initial costs. Give the man the respect he deserves, and don't try to explain what he understands and you don't.

[jdb52]

cem-bsee

at 101F, what is the highest temp inside attic?

I don't know, but have read that generally we are dealing with attic temps of 150. Whatever it is, anything that holds promise of reducing those temps is worth a look.

As you have pointed out many times, anything that can provide shade is the starting point. But, in many homes, especially new construction, the roofs are open to direct sunlight. In my particular case, I have a large mulberry and pretty large red oak in the front yard, but the house positioning is such that the west setting sun is able to glide down between the opening between these two trees and hit the roof squarely. So, I look at:

proper attic ventilation
insulation
radiant barrier

jdb

[pstu]

I am pretty sure we are NOT dealing with attic temperatures in the 150 degree range. If we were, the house would not even need a water heater, just a tank in the attic. I cannot claim my own house is typical, but its attic is not particularly well vented and all last summer as I remember the maximum temperature seen was in the 115-120 degree range. There were a couple of days over 100F in this location. Plenty hot enough to kill a man, I don't want to minimize the hazard to HVAC people who must go up there in the summer. But not as high as some people say.

Regarding Lstiburek, I have a ton of regard for his research, but at the detail level I have some small reservations in my mind. His discussion of ventilation always seems to center on the FanCycler, surely a clever enough device but not the only way to get the job well done. Not all his ideas work the way he thinks they will, that means he is learning as he goes (and honest enough to tell the world when things won't work out).

I do believe reading some of the reports on the Cinco Ranch Building America project, that there have been some moisture issues which resulted in changing his building recipe. That would be extremely serious if true. But I don't know the whole story on that.

It is my opinion that RB and attic ventilation should receive more coverage since it would apply to 99.9% of the houses in my region. In my net searches for RB info, in my opinion Lstiburek and company come up a little short where they should say more. I don't mean to discredit Lstiburek's unique and fine work. But any person should be able to offer dissenting ideas.

Hope this helps -- Pstu

[jdb52]

ptsu,
Greetings from hot but less humid West Texas. All I know is what I read, but have read stuff that suggests attic temps can get to 150 with outside temp 100+. Here's a link to a bulletin from Rheem on attic temps and water heaters.

http://www.rheem.com/Documents/ResourceLibrary/TSB_StandingPilot1400/1411.pdf#search='attic%20temperatures'

I don't know what mine is today as we climb toward 100, but I think we agree that anything we can do to get that temp down is advisable.

take care,
jdb

[uktra]

pstu--Joe likes the Aircycler because it is very low cost mechanical ventilation that he can sell to production builders. Does he say that is the only way to mechanically ventillate? No. Does he say it is the most energy effecient way to mechanically ventilate? No. He recommends it for certain types of homes in certain climates with initial cost/ energy savings ratio's in mind.
I have recommended it in the same type of climate as Houston and it does a great job.

Has Joe made mistakes when trying new things--of course he has, he is on the "bleeding edge", and no one is perfect. In fact I gave him details of a low cost ventilation product I found in Canada and asked if he would check it out for me and let me know what he found. Have not heard from him. Very disapointed. Perfect--no. One of the best, if not the best in his field,--absolutely. Any one can queation Joe, but you had better have done your homework.

In terms of radiant barriers, Joe likes them where they do the most good--on the top of the roof (metal roofs, and white tile roofs). For custom homes, where cost is not as much of an issue, he recommends them.

Finally, ventillating roofs in Houston, Texas does not make any sense.

[Bill Lee]

RB decking will make working in an attic much more bearable. Two summers ago, we were installing a geo system in a new house. The framers had used conventional OSB sheathing on the first row of decking over a garage of the house and RB OSB decking on the second row. The roof was brown shingles. It faced south. We got to talking with the framers during lunch about the advantages of RB. I got my IR thermometer and took surface temperatures of both decking materials. The RB decking was 30 deg cooler than the conventional OSB decking. This really made a beliver out of everyone! Sherwin-Williams makes a RB paint that can be sprayed on the underside of decking. It is called "E Barrier".

[mark beiser]

A note on attic temps, since I actually measure them fairly often while I'm in the attic checking temp drops through coils and such.

The very hottest attic I have ever experienced was 170º at 3:30ish pm on a sunney 74º spring day. It was hot enough up there that the air inside the furnace was heating up enough to cause the fan/limit control in thier furnace to turn the heating speed of the blower on.
It wasn't a typical attic though. The house origonally had a wood roof, and they changed to a black composition roof, no ventalation was installed, and the house was built with no soffet ventalation.

The usual peak attic temp I see in a ventalated attic with no RB is 115-130º if it has light colored shingles. If it is dark colored shingles, I typically see 125-150º+. Ventalation helps, but a LOT of air has to be moved through to significantly reduce the temperature.
Keep in mind that this is just the air temperature, there is also the raidient heat from the roof heating surfaces in the attic, especially metal, to much higher temperatures.

With a good radient barrier at the roof line, and decent ventalation, I normally get attic temps that are 5-15º above the outdoor ambient temps, a little higher with the spray on barriers.

I have a customer that has a black roof with a spray on radient barrier. Before the barrier, I have measured temps as high as 145º. I havn't been up in his attic on a really hot day since he had the barrier installed, but when I did the PM on his systems recently it was late in the afternoon, around 90º and sunney out, and it wasn't even 100º in his attic.

People with radient barriers at the roof line get more thorough PM service from me and higher quality repairs/installations. ;)

[pstu]

It is terrific to see somebody reporting results which are actually measured! Directly measuring something trumps reading about it, if you have to choose one over the other.

My dissatisfaction with Lstiburek on attics right now, stems from his choosing to be relatively useless with advice on what to do with existing housing. His "Building Guide for Hot-Humid Climate" is a very nice book, but it has exactly one mention of radiant barrier, on page 94. He recommends the foil type, attaching it above (for new construction) or below the roof rafters.

The insistence that all attics should be sealed in a hot-humid climate is shall we say, interesting. This is contrary to many building codes and contrary to 99.9% of existing housing. What is much more relevant to most of us is, how to fix problems in what is already built? It surely seems to me that good craftsmanship in traditional methods, is a worthy alternative to living on "the bleeding edge" with research that is not finished yet.

The following statement was made: "...ventillating roofs in Houston, Texas does not make any sense". I propose that Pulte homes should be in a perfect position to verify this, since they built sealed-attic homes under the Building America program. Having this experience, has Pulte seen the superior method and stopped building ventilated attics? I honestly do not know the answer to this question but hope to find out in the near future.

In the past Emerald Homes has made dramatic guarantees of energy efficiency, I think similar to the Pulte Building America claims, and they did it with a vented attic design of course employing radiant barrier. I once posed the question to a Department of Energy person, on a website advising on housing, which is superior: the sealed attic per Lstiburek design, or the ventilated attic many others insist on? His reply was you could get excellent results with either design philosophy, provided attention was paid to the details. I think that reply was wise.

I very highly respect Lstiburek, just am of the opinion right now that he is letting himself get into an ivory tower on some subjects, and into a commercial sales position on some others. Still wish there were more like him.

Hope this helps -- Pstu

[uktra]

pstu--I don't uderstand your "dissatisfacton" with Lstiburek advising what to do with existing home attic construction. Joe is not in the renovation business, he is in the new construction and forensic diagnostic business. Besides, every existing home is different and has to be done on a case by case, cost/ROI consideration. Did you bother to read why Joe recommends sealed attics in hot humid climates for new construction? Trying to use the code and that "99.9" percent on existing home attics are vented, is a completely bogus reason to choose vented over non-vented. The venting of attics in the code was due to ice dams in cold climates. Codes get changed frequently, the laws of thermodynamics don't. Finally, Joe is not the one in the "ivory tower", I believe that space is occupied by the person I am addressing in this thread.

[Panama]

My house has dark asphalt shingles over a vented attic with R-30 insulation. Wrightsoft (MJ8) says the sensible load of the top floor is 15,300 Btuh.

When I change the roof in Wrightsoft to include a radiant barrier the load drops only 3%.

When I change the roof to white asphalt shingles without a radiant barrier the sensible load also drops 3%.

When I change the roof to white tiles with or without a radiant barrier the sensible load drops 12%.

Perhaps radiant barriers may not be worth the cost. Of course Manual J, 8th Edition could be way off.

[paul42]

Originally posted by Panama
My house has dark asphalt shingles over a vented attic with R-30 insulation. Wrightsoft (MJ8) says the sensible load of the top floor is 15,300 Btuh.

When I change the roof in Wrightsoft to include a radiant barrier the load drops only 3%.

Perhaps radiant barriers may not be worth the cost. Of course Manual J, 8th Edition could be way off.



At the electric rates in my part of the country (12 cents a kwh) that works out to about a nickel per hour. For new construction, there is no difference in the labor costs so it might very well be cost effective.

[uktra]

It also involves how many heating degree days in your location.

[pstu]

>>Did you bother to read why Joe recommends sealed attics in hot humid climates for new construction?

I sure have, and keep it printed for future reference. It is good stuff. Much of it stems from his losing hope that contractors could ever build ducts with less than 15% leakage. Another issue is sealing against attic-ceiling infiltration, something I think can be reasonably corrected for old and new construction. If I ever build new construction I will surely consider a sealed attic, however for now I am addressing the issues in the house where I live.

>>Trying to use the code and that "99.9" percent on existing home attics are vented, is a completely bogus reason to choose vented over non-vented.

It is becoming clear that my focus is on existing housing and yours is on new construction as it "should" be. I am not choosing attics so much as I am working with the houses out there. And to a lesser extent, co-existing with today's builders who continue to use vented attic construction. I am not trying to revolutionize the world of housing.

>>Finally, Joe is not the one in the "ivory tower", I believe that space is occupied by the person I am addressing in this thread.

Funny I should read this after 4 hours working in the attic this morning. I do try to connect academic learning with results wherever I can. Lstiburek does too but his definition of "results" is more narrowly focused than I would prefer. I think that to be reallly educated one must be able to discuss alternatives, both pros and cons of a plan.


Uktra, you have some good ideas and I believe you have expressed them pretty well. If you get some new information then I hope you will present it so I can learn from you on this board. I will report what I find, if I can identify whether Pulte continues to build according to the Building America pattern.

Best wishes -- Pstu

[pstu]

>>My house has dark asphalt shingles over a vented attic with R-30 insulation. Wrightsoft (MJ8) says the sensible load of the top floor is 15,300 Btuh.
>>When I change the roof in Wrightsoft to include a radiant barrier the load drops only 3%.
>>When I change the roof to white asphalt shingles without a radiant barrier the sensible load also drops 3%.
>>When I change the roof to white tiles with or without a radiant barrier the sensible load drops 12%.
>>Perhaps radiant barriers may not be worth the cost. Of course Manual J, 8th Edition could be way off.

I want to praise Panama for running these trials through his Wrightsoft software and sharing the results with us. This is advanced stuff and we ought to appreciate it.

Of course Manual J is a model, and I think it is possible for error to exist in any model. This does not ruin the usefulness of the model, just trims it a little if the model contains errors. I cannot guess why the load drops only 3% using RB, can only point out that there have been several good studies and I *think* FSEC says it best when estimating 8-12% reduction in cooling load (in their Florida climate). But that's just an opinion based on a little bit of experience and a much larger amount of reading others' research.

I'm not trying to flame Uktra, but I have been told that Pulte stopped building to the Building America standard after the formal experiment closed. In its place (which is probably inferior) they build to Energy Star standards. I have my doubts that the EPA has our best interests at heart in our hot-humid Houston climate. Remember that Pulte has won lots of quality awards, they surely are among the better builders. It seems that despite direct experience, they have not been sold on the idea that a vented attic makes no sense in a hot-humid climate. Surely dollars plays a role here.

I am still respectful of Lstiburek's research which is superior to most all of us, but I keep looking for some kind of skeleton in his closet. Being on the bleeding edge means there probably are some to be found, at least in past practice. Personally I will give myself a couple years to look for and identify the downsides, and will sincerely consider a sealed attic if/when I buy another house. In the meantime I am convinced that attic ventilation with radiant barrier, is not too bad a practice. Is that fair enough?

Best wishes all -- Pstu

[Edited by pstu on 06-08-2006 at 11:26 AM]

[jdb52]

http://www.hvac-talk.com/vbb/showthread.php?threadid=106598

Above is a link to a current thread which generally hits on the theme of attic ventilation

As PSTU has stated, for most of us the problem of attic temps revolve around the issues of radiant barrier and ventilation. Add in insulation and we have three themes.

1. Block a significant source of heat-radiant barrier
2. Remove the heat-ventilation
3. Absord the heat-insulation

All designed to keep the heat from reaching the conditioned space

Wish there was a little bit more clarity on the subjects.


jdb

[gsubrec]

I have a question on radiant barrier paints

I am looking at 3 choices
1. Sherwin Williams e-barrier paint
2. Hy-tech aluminum and ceramic compound in paint
3. Thermakool, - ceramic compound in paint

Has anyone actually used this stuff and which brand is better

Thermakool also sells a R-100 roofing package
and claim each layer of paint of their ceramic compound mix in paint is R-12
Some other vendors claim R-7 per paint layer

[mark beiser]

Originally posted by pstu
Remember that Pulte has won lots of quality awards, they surely are among the better builders.

This made me laugh. ;)

I'm sure they are the cream of the trash heap when it comes to the big national builders, but in the end, all of them still suck, no matter how many awards they get.

[uktra]

pstu--I am not trying to start a flame war either--however, you keep making statements like Pulte is not doing sealed attics anymore to try to prove Lstiburek is wrong, and "I keep looking for a skeleton in his closet" because he is on the bleeding edge and you are sure you will find some. Spend less time looking for skeltons and more time reading what the man has wrote and done.

I don't know what the status situation is on Pulte concerning the Building America program in Houston. But I can tell you for sure they have built over 748 homes with sealed attics in 12 different communities. Check Creek Bend Estates where they have Building America homes and are still building and let me know if they have changed and why.

[Bill Lee]

gsu

The only paint that I have seen used was the Sherwin-Williams E-barrier. It does work. How well as compared to RB decking, I don't know. Haven't taken under side temps with IR thermometer. The attic is cooler than decking without RB. R values for insulation ratings are somewhat a myth. What is the R value of a styrofoam cup that you get McDonald's coffee in?? It's thin enought that you can almost see through it, but your fingers don't get burned!!
The R values for insulations are determined by a steady-state test and doesn't really relate to real world conditions. The manufacturers can "claim" any rating that they want--but I have doubts about R-12 per coat of paint.

[pstu]

>>I have a question on radiant barrier paints
>>
>>I am looking at 3 choices
>>1. Sherwin Williams e-barrier paint
>>2. Hy-tech aluminum and ceramic compound in paint
>>3. Thermakool, - ceramic compound in paint
>>
>>Has anyone actually used this stuff and which brand is better

Can you obtain a number for reflectivity or emissivity for any of these 3 paints? You want a reflectivity (defined as 1.0 - emissivity, I think) higher rather than lower for performance reasons. The talk I have been hearing is that 0.75 is the highest you can get with paints, and that is a pro-only paint which has two parts to mix. Wish I could tell you the name but I forget. The earliest RB paints I know of had reflectivity around 0.5, so you might be getting numbers in between with the 3 choices above.

Foil type RB has reflectivity 0.95 or better, it is the highest performing by that metric. But of course aftermarket foil is so labor intensive that you probably cannot find a contractor to do the job (RB decking has no added labor). Paint is easy enough for the contractor with the right equipment, though not a cheap job. If you go ahead with a DIY project, you might give us a note or two to tell us how you do!

Best of luck -- Pstu

[pstu]

A followup on Pulte current building practices, from a conversation with their builder. The sealed attic method is used in their "Platinum" series of homes, which sounds prestigious but they do not build them in Houston any more. Their next level is "Gold" which has vented soffits and radiant barrier as an *option*. Then lower down is the "Energy Star" level which sounds good, but I do not really think is superior in terms of today's best practices.

Their reasons may have been economic rather than performance, but that's the way it is today. I believe even Creek Bend lacks the "Platinum" method of construction.

We may wish for Pulte to do things differently, I certainly think it borderline scandalous for the "Gold" models to not offer radiant barrier as standard. I am ready to give this whole subject of sealed vs. ventilated attics, a rest for awhile. I will grant the proponents that it has some big advantages, it's just not the world we live in and buy houses in.

Hope this helps -- Pstu

[jdb52]

A question for those commenting about heat gain from radiant energy. I think you are saying that thermal radiation (rays-waves) are traveling thru or are being emitted from the undersurface of the roof decking. My question is this, What happens when those rays (waves) hit the top surface of the insulation? Are they all absorbed creating heat? Do some travel thru the insulation hitting the top side of the ceiling where they absorbed creating heat? Do some travel directly into the living space? What happens?

jdb

[pstu]

Sincerely wish I had some undisputable research to back up what I say, but regrettably I don't have any links to offer right now, and I don't have it on paper. But I believe the following is true, and ask any pro who knows better, to straighten me out if I am wrong:

During the day I do believe the heat radiation is mostly absorbed in the top layer of insulation and then some of it re-radiated into the air. All the non-insulated structures in the attic will slowly get hot up to the ambient attic temperature, perhaps a few degrees hotter due to radiation. Then after sunset the roof will radiate its absorbed energy off toward the sky, and will no longer be a source of heat energy toward the attic interior. The contents of the attic (wood, metal and insulation itself) will do their process of radiating energy outward and the attic interior will slowly cool down. Sometime in the night the main heat source will be the top layer of attic insulation -- though at that time in the early morning you probably aren't worrying too much about heat.

Last summer I measured AC runtime using a data logger, and was surprised to learn how long were the hourly runtimes, in the evening until after midnight. This was with no radiant barrier in that section of the house (slow DIY job on my part). It is my expectation that this summer I will see significantly shorter runtimes in the 9pm-3am period, because that part of the house is well covered in radiant barrier.

Hope this helps -- Pstu

[jdb52]

thanks PSTU,

That helps. The way I take statements like "radiant energy is the main attic source of heat to the conditioned space", it almost sounds like the waves are going thru everything right into the house. At least, I was confused.

The way you explain it leads me to understand that heat (attic temps) is important, but the source of that heat is a certain part of the spectrum that consists of waves which travel through space, hit stuff, and cause heat. In the case of attic temp the structures that cause it to rise having been hit by the waves are shingles, decking, insulation, rafters, etc. The issue with insulation is can it unload before the heat travels thru and hits the ceiling. It would seem to me that reducing air temp at the surface of the insulation would help it unload.

So, for me treatment would be: block the waves, get the heat out, and keep heat off the ceiling. That's radiant barrier, insulation and passive venting for those of us who don't have sealed attics. I hope this is correct. If not, let me know. I would like to get this straight.

thanks,
jdb

[uktra]

jdb52--Insulation does not "unlode heat". It just slows the heat transfer. There are only three ways heat can transfer in an attic assembly. 1. Radiant (the dominant method). 2. Conductive (hot air hitting insulation). 3. convective (warm air leaking from attic to rooms below through penetrations).

Since radiant transfers the most heat of the three, this is why insulation is important in summer. The second highest heat transfer ususally is convective, with the hot air bypassing or going through the insulation. This will of course be dependant on total area of "holes" in the attic assembly vs insulation levels and type.

If you are looking to save energy in both summer AND winter, proper insulation levels and sealing the holes is the best investment. Reducing the radiant transfer by installing a radiant barrier depends on cost, geographics, unit energy costs, and how long will you be in the home.

[jdb52]

thanks uktra for the info.

a question

Talking about summer.

As attic air temps decline in the evening and night (aided by good ventilation), wouldn't there be a heat transfer from the upper levels of the insulation into the cooler air at the surface of the that insulation. That's what I meant by unloading.

It's hard to see how radiant barrier wouldn't be advisable for the attic heat loads we experience in Texas.

jdb

[uktra]

Yes, once temperatures decline that will happen. Radiant barriers in Texas in new construction ae cost effective, in existing construction it all depends on cost of install and how long you will be in home to recoup initial investment.

[jdb52]

Thanks again, uktra,

I have seen your comments about radiant energy being the dominant factor in attics. From other threads I have read of what I think of as two different understandings of heat transfer in attics.

1. Solar rays hit shingles, they heat up as does underlying decking. The decking heats up the attic air. That super hot air heats up the insulation which transfers heat via conduction to the ceiling and into the interior. Convection thru insulation and holes is also involved.

2. Solar rays hit shingles, they heat up as does the underlying decking. The decking heats up attic air BUT, it also emits thermal radiation (waves-rays) and those waves hit the upper surface of the insulation and heat it up. That heat transfers thru the insulation, ceilings and ultimately into the living space. In this view, radiant(waves not air)plays the major role in the production of heat in insulation. Convection and conduction play a role also.



Presently, I understand the latter view as being accurate, but would want to be corrected if the above is not roughly accurate. I believe that it's important to get this down because it relates to what measures one takes to deal with the problem


jdb

[uktra]

Number 1 is incorrect, number 2 is correct. That is why even if you lower the attic temperature to the same as the outside the dominant energy transfer will still take place. There is no question there is energy transfer from hot air by conductivity through the insulation. the amount of energy transfered, at a difference between 100 degree air and 130 degree air doesn't effect the lower room temps as much as a sunny day vs a cloudy day. The reason a/c's run even after the air is cooler in the attic after the sun goes down is that the heat in the sheathing and wood frame is still radiating heat to the insulation below. That is why in winter when you stand next to a large single pane window, you feel colder even though the air is 70 degrees between you and the window. Your body is radiating heat out to the cold surface.

[pstu]

On the radio I recently heard advertised a PITTSBURGH PAINTS "HEAT BLOCK 75" product which anyone can buy, i.e. not limited to professionals. It is said to reflect 75% of the radiant energy which is pretty good for a paint.

Much less labor than retro-fit of foil type RB.

Hope this helps -- Pstu

[Edited by pstu on 06-10-2006 at 11:55 AM]

[jdb52]

I'm beginning to like the idea of paint, more and more. If all these structures in the attic are either emitting rays or absorbing rays and producing heat, and maybe even re-radiating after absorption, then why not get up in the attic in fall or early winter and paint everything in sight.

I'm even wondering about painting interior ceilings and the interior portion of exterior walls, especially west facing walls.

The main issue with radiant barrier, whether foil or paint, is labor. A DIY removes that problem.

jdb

[pstu]

I think you would be glad to see these products then:
http://www.chemrex.com/

Then type "radiant" in the search box, and click on "go".

These are three interior paints from BASF, which claim emissivity in the .71-.73 range. Two have data sheets:
http://www.chemrex.com/documents/icp_tdg.pdf
http://www.chemrex.com/documents/ifw_tdg.pdf

With any luck there are competitors and then you will have a choice. I do believe you will need to visit a real paint store, in other words not shop for this at Home Depot etc.

Hope this helps -- Pstu

P.S. corrected my link to something that works, added links to .pdf data sheets.

[Edited by pstu on 06-10-2006 at 03:21 PM]

[jdb52]

thanks pstu,

The link did not work on my computer, but I found one of the paints. Link below to specs

http://www.chemrex.com/documents/abc_tdg.pdf

When I first came to this site a few months ago, I was looking for what many do-the best most energy efficient brand. Now, I think that given the cost of equipment, install and energy, the best strategy is just not use much of the stuff. That means retrofitting our homes which is time consuming and somewhat of a pain. Better to build then right from the start.

Then find a good and knowledgeable contractor who can create a system that will provide comfort in the most energy efficient way and pay for what that is worth.

Brand becomes a distant third, especially if a person stays above builder grade stuff. Having said all of that, I guess I still favor Trane for heat pumps, but it's not that big a deal.

regards, jdb

[jdb52]

pstu,

thanks for the edit on the paints

I had not found what you were talking about.

jdb

[jdb52]

example and question

It's 6:00 pm on a 100 degree Texas afternoon. Clear skies. It was 90 degrees at noon. It will still be 90 at 9pm. In other words, the shingles-decking have been loading up the attic and insulation for some time and will continue to do so.

Does anyone know what the temperature will be in the upper layers of insulation (first 3 or 4 inches)?


thanks,
jdb

[cem-bsee]

temp in middle of insul can be calculated, but who cares?
of importance is what will be the max temp at the inner surface of the ceiling just under the attic.

[jdb52]

cem-bsee

Point well taken. I was just curious. In keeping with the theme of radiant energy being emitted from the decking and being absorbed by the upper levels of insulation, I was just curious about temp levels on a very hot day.

jdb

[gsubrec]

Has anyone use Lomit Radiant barrier attic paint ?

[pstu]

The "Home Energy Magazine" has been published for a number of years and has some good info on RB. This one shows an experiment based in Austin TX and concludes that 80% of the benefit comes from reducing duct losses in an unconditioned attic. It also illustrates how it is always beneficial to locate ducts in conditioned space if you can.

http://homeenergy.org/archive/hem.dis.anl.gov/eehem/96/960704.html

"...The authors found that daytime cooling energy use dropped by 16%, and that 80% of this was attributable to improved duct performance in the attic, where average ambient temperatures were lowered by 18oF. On the preretrofit test date, the A/C ran from 2:50 pm to 9:00 pm; on the postinstallation date, it ran only from 5:30 pm to 8:00 pm. "


Hope this helps -- Pstu

[cem-bsee]

do you really think that a reporter could get the facts correct? if so, I have some slightly damp ground that I would like to discuss selling!

study showed leakage to be 5 changes per hour, when recommended is 1 per 3h? What if they studied an identical house & caulked= EVERYWHERE? [ I caulked around all win & doors & penetrations of envelope outside. I caulked all wall seams & trim & elec boxes inside. I foamed around all pipes under sinks inside. I caulked almost all holes in attic (except at chimmey, vent pipes thru exterior wall.]

were the ducts sealed?

[pstu]

Cem-Bsee, you had me going for a little while there. I have no basic trouble believing a reporter could mis-state technical facts, but in this case he apparently did not. The study showed 5+ air changes per hour UNDER TEST CONDITIONS:
"...The 1530 ft2 house was equipped with a 2.5-ton air conditioner and an R-38 attic, and showed envelope leakage of 5.2 air changes per hour (ACH) at 50 Pascals (Pa)."

My own house had a blower door test done and this is a standard way they report how airtight is the structure. My air changes at 50 Pa were measured as 5.274, and that translates to estimated Manual J air change/hour rate of 0.40 winter. 0.24 summer. Good but not great.

Hope this helps -- Pstu

[uktra]

Pstu--the article in Home Energy also re-inforces why Lstiburek likes sealed conditioned attics!

[pstu]

That is nice. I happen to think that every car and truck should have side curtain air bags, traction control, electronic stability control, and antilock brakes. But you won't find me scolding people who still own vehicles that don't have them.

Best wishes -- Pstu

[jmbeam]

I wish I would have known about radiant barrier solutions before my roof was constructed. I thought I did my research when I decided tobuild ICF and have spray foamed and sealed attic.

My question now is Can I spray the radiant paint ontop of the spray foam insulation? My foam is a soy based product?

I also stick built my garage for a couple of reasons that I will not bore you with. The walls are foamed and the roofline is also. I am getting ready to install siding, Does it make sense to paint the walls with Radiant barrier paint? It is too late for the front of my home because I already had a brick facade installed.

Could a product like this work on the interior before I prime and paint it? Or instead of priming and painting it?

Is a ceramic product better than one with metallic content?
Hopefully some of the members of this site can answer a couple of these questions. Thanks

[jdb52]

If I had to choose between ceramic or aluminum paint, I would choose the metallic paint. But here's a link to a company whose prime technology is ceramic, but makes a paint with both ceramic and metal in it.

http://www.hytechsales.com/prod85.html

[txborn]

jmbeam, I am using Icynene foam for my house and sealed attic. I have talked to the many building science gurus the past few months and they all said no need to install a radiant barrier with foam. In fact it is discouraged.

Are you having heat issues in the conditioned attic with the foam? Everyone I have talked to said the attic temps are only 5 - 10 degrees warmer than the occupied space with a non-vented attic. Bottom line is there is no need to bother with a RB if you use foam.

[dfw_lr]

I wouldn't go as far to say that with foam there is no need for a radiant barrier. Foam insulation, like most insulating materials, basically works like a sponge absorbing heat. The higher grades of foam just take longer to reach the saturation point. Even with aviation grade foam, which is too expensive to ever be used in a home, eventually it would become saturated and would be powerless to stop BTUs of heat.

A radiant barrier will reflect 97% of the heat, regardless of how long it is exposed to heat. Additionally, with a radiant barrier, the foam will only be under 3% of the radiant heat load and perform almost twice as well.

If you've ever seen the radiant barrier and bubble insulation, the bubbles are essentially 8 R Value in half an inch, but because it's R Value is tested with the radiant barrier, 97% of the radiant heat never reaches the bubbles and the R Value is regestered at about R15.

Adding a radiant barrier to any ammount of insulation will significantly increase its efficiency by almost double.

[Carnak]

I have white metal roofing on top of 1.5 inches of foam, on top of the roof 'deck', stops a lot of heat

[pstu]

I wouldn't go as far to say that with foam there is no need for a radiant barrier. Foam insulation, like most insulating materials, basically works like a sponge absorbing heat. The higher grades of foam just take longer to reach the saturation point. Even with aviation grade foam, which is too expensive to ever be used in a home, eventually it would become saturated and would be powerless to stop BTUs of heat.

A radiant barrier will reflect 97% of the heat, regardless of how long it is exposed to heat. Additionally, with a radiant barrier, the foam will only be under 3% of the radiant heat load and perform almost twice as well.

If you've ever seen the radiant barrier and bubble insulation, the bubbles are essentially 8 R Value in half an inch, but because it's R Value is tested with the radiant barrier, 97% of the radiant heat never reaches the bubbles and the R Value is regestered at about R15.

Adding a radiant barrier to any ammount of insulation will significantly increase its efficiency by almost double.
DFW, you really think so? I would seriously like to question your opinion, there might be a variety of reasons but let me start with personal experience. I completely believe in the usefulness of radiant barrier (RB) for a typical Texas attic, which is ventilated to outdoors and probably contains the AC equipment and ductwork. But in the case of a foamed attic (which is normally sealed to outdoors) I think RB is far less useful, not worth the cost.

FWIW, I am a Texas homeowner who grew up in Dallas and now lives outside of Houston. My own attic is the typical ventilated sort, and has foil type RB installed as a laborious DIY project. You can really feel the benefit there, and my electric bills seem to support the 8-12% claim of savings on cooling. My neighbor has recently foamed and sealed his attic, in a house that was similar construction before that work. You can really feel the coolness in the attic!

My thinking is my type house has roof decking at some gawdawful high temperature, which I estimate at maybe 150F. That roof decking is the major source of BTUs (heat) in the attic, and it radiates onto anything that is not shaded by another solid object. My RB reflects 97% of that back toward the decking, making it even hotter, but the net result is much less radiant energy in the attic when the sun is out. Over the night hours things slowly cool down so that by dawn the attic materials are approximately ambient temperature.

With my neighbor's house, the foam never radiates heat like my roof decking does. I may not be able to explain where all the BTUs go, but they are clearly not radiating heat into the attic, not to a degree you can feel. I can agree the foam material heats up *some*, but it never even approaches the temperature of my roof decking. Again I believe everything cools off overnight and each dawn sees all the materials at ambient outdoor temperature.

If the above is true, to add RB to my neighbor's house would be reflecting back 97% of a tiny amount of heat. That would mean a tiny difference in AC load, if you counted cost and benefit there would be no practical payback time.

Whaddaya think?

Best wishes -- Pstu

[dfw_lr]

I have to admit, you raise a good point. Solely based on the physics of radiant barrier, I know that the heat load will be decreased and the R Value improved dispite the ammount of insulation, so that's what I based my opinion on. However, if you get to a point where the current insulation NEVER reaches saturation point, which may also be a result of the dead air in a sealed attic acting as additional insulation, the difference may NOT be worth the cost.

I guess it's like, if all you need is R30, there is no need to increase it to R60. No matter how cheap it would be, it wouldn't be worth it.

[Carnak]

Can you explain insulation saturation.

[dfw_lr]

Can you explain insulation saturation.

Ok, let's go back to the analogy ofinsulation being like a sponge absorbing and trapping BTU's of heat. Eventually it will get to a point where the insulation can't hold any more, like a sponge becomes saturated. At that point, every BTU of heat that hits it creates an equal BTU of heat emmitting from it. All insulation has a saturation point. R Value is a numeric calculation of how long it takes for heat to penetrate it. That's why higher R Values are better.

A radiant barrier doesn't absorb heat, but reflects it at 95% to 97% (depending on the product). Now the 3% to 5% gets through immeaditally, which is why radiant barrier has no R Value. Most of the heat will never reach the insulation and the little that does will be easily absorbed and it will take longer for the insulation to reach the saturation point.

Although the radiant barrier is a cheap way to increase the efficiency of the existing insulation in a home, there can get to a point where the existing insulation is enough and saturation point will seldom be reached as is, so adding a radiant barrier is not necessary.

[jrbenny]

Can't says as I've ever thought of heat transfer in that manner. Trying to decide if I agree with it.

[zupchuck]

With my home requiring a new roof in a couple of years (stripping down to the deck), putting up a radiant barrier on top would be really easy. However, all the docs I've seen for an RB attached to the roof deck panel before they are put down are perforated and are on the underside.

Can it be safely applied to the top side?

Putting a radiant barrier in the attic on the underside is fairly straightforward. The perforated stuff applied between the rafters reduces my concerns about moisture between deck and RB and can be done a little at a time (instead of the larger job of painting everything all at once).

Also, would it not make sense to use a radiant barrier paint on my upper floor ceilings (which all need painting, anyway)? And, since I have no vapor barrier above the ceiling (6" of cellulose from 1975 and then 8" of fiberglass bats on top running perpendicular to the joists) perhaps a vapor barrier paint might work well, too (a system like this: http://www.befreetech.com/thermaguard.htm, or http://www.hytechsales.com/prod85.html).

The north side of my roof has de-laminating roof decking and visible condensation issues (some mold, too). Putting in spacers in the lower 4' of the slopes has lowered the attic roof temp about 15ºF in the summer and markedly increased air flow (you could feel it as each spacer went in). When a ridge vent goes in (gable vents now), I'm sure it will be even better. I've worked hard to seal off the penetrations through the attic plane, but getting to the ones at the bottom of the slopes will likely be impossible unless the roof deck come off down there.

Any downside to either vapor barrier or RB paint on the upper floor ceiling? Diminishing the slight texture of the ceiling does not bother us at all.

[Carnak]

Ok, let's go back to the analogy ofinsulation being like a sponge absorbing and trapping BTU's of heat. Eventually it will get to a point where the insulation can't hold any more, like a sponge becomes saturated. At that point, every BTU of heat that hits it creates an equal BTU of heat emmitting from it. All insulation has a saturation point. R Value is a numeric calculation of how long it takes for heat to penetrate it. That's why higher R Values are better.

A radiant barrier doesn't absorb heat, but reflects it at 95% to 97% (depending on the product). Now the 3% to 5% gets through immeaditally, which is why radiant barrier has no R Value. Most of the heat will never reach the insulation and the little that does will be easily absorbed and it will take longer for the insulation to reach the saturation point.

Although the radiant barrier is a cheap way to increase the efficiency of the existing insulation in a home, there can get to a point where the existing insulation is enough and saturation point will seldom be reached as is, so adding a radiant barrier is not necessary.


Well that is a new one to me, sounds like conduction has hit a steady state and the temperature on the 'cool side' is hot enough to significantly radiate. I don't see that happening with 5 inches of icynene though, doubt the surface temperature would be much more than a degree or so above ambinet.

[Shophound]

Ok, let's go back to the analogy ofinsulation being like a sponge absorbing and trapping BTU's of heat. Eventually it will get to a point where the insulation can't hold any more, like a sponge becomes saturated. At that point, every BTU of heat that hits it creates an equal BTU of heat emmitting from it. All insulation has a saturation point. R Value is a numeric calculation of how long it takes for heat to penetrate it. That's why higher R Values are better.



Insulation in an attic is a mechanism for trapping air, since air itself is a slow conductor of heat. The insulating material is also a slow conductor, but can gain heat over time, since it has mass. This will in turn conduct and radiate heat toward the ceiling drywall, since it is cooler than the material that is being heated by the high heat gain of the attic.

Insulation does not stop heat transfer, obviously. It provides a lag effect to keep the interior of the house from becoming too quickly overheated. In a perfect world the lag would allow the house to coast through the peak solar heating of the day with little transfer to the interior of attic heat, and just when such a transfer rate picked up, the sun would set and the attic would begin to lose heat rapidly to the cooling outdoor air. This does occur to an extent, but often the effect of the delay (heat gain into the structure) continues well into the night, requiring the a/c to continue running to offset this gain long after it has cooled down to more tolerable conditions outside.

A radiant barrier makes complete sense in that it greatly reduces the amount of heat reaching the insulation in the first place. Along with that would include less heat gain to the attic mounted ductwork and any air handlers present. Less heat in means less heat transferred to the structure, plain as can be. This in effect boosts system capacity, as with typical attics with no barrier and attic mounted ductwork, there can be a significant capacity penalty paid via heat gain through these ducts, and heat gain through attic mounted air handlers.

[pstu]

I do want to add that all the RB studies I have seen, are in cooling-intensive climates like Texas or Florida. I have never seen much discussion about the relative payback in a heating dominated climate. Your results might be different from a Texas house in ways I would not be able to guess.

Carnak has described his concept of putting RB on the upper side of the roof, as I understand it this basically means reflective roofing. I hope he will elaborate for the benefit of this thread. Sad to say some communities would have a problem with that on the dubious grounds of looks, in practical terms some of us cannot do that.

Hope this helps -- Pstu

[Carnak]

I do want to add that all the RB studies I have seen, are in cooling-intensive climates like Texas or Florida. I have never seen much discussion about the relative payback in a heating dominated climate. Your results might be different from a Texas house in ways I would not be able to guess.

Carnak has described his concept of putting RB on the upper side of the roof, as I understand it this basically means reflective roofing. I hope he will elaborate for the benefit of this thread. Sad to say some communities would have a problem with that on the dubious grounds of looks, in practical terms some of us cannot do that.

Hope this helps -- Pstu

I have white metal roofing on top of foam insulation on top of my roof deck. I have R7 insulation on my roof, and I still have not conditioned my attic.

A sheet rock ceiling separates my unconditioned attic from the conditioned space.

If I keep the conditioned space at 79F, the attic averages just over 82F(peaks at 85), if I keep the conditioned space at 75F, my attic averages 81F(peaks t 83). Humidity in attic, low 50% range.

Attic is hottest after the sunsets, there is thermal mass involved plus, it traps heat from surface mounted lights on the ceiling.

As a worst case, the small differential in temperature between they attic and the conditoned space drives heat through that ceiling at a rate of 1.7 Btu/hr/sq ft. R7 with reflected heat blows away R30 against a superheated sauna. It is really dumb to let all that solar heat and humidity into an attic.

Multimillionaires got white roofs here. The metal roofing is referred to as 'standing seam', and it performed the best against a Cat 5 storm that hit us.

[ericrocks]

I live in TX myself, I believe strongly in the possitive effect of RB.
BUT, does anybody knows something about the effect on the roofing material (e.g. asphalt shingles). It must be that the roof temperature increases when a RB is installed between the rafters.
Higher temperature leads to shorter roof lifespan.

Before you know, you spend $$$ on installing a radiant barrier in your attic to safe $$ on cooling cost in the Summer months and after 6 years or so you have to replace the roof at $$$$$

[mark beiser]

Before you know, you spend $$$ on installing a radiant barrier in your attic to safe $$ on cooling cost in the Summer months and after 6 years or so you have to replace the roof at $$$$$

I'm pretty sure Owens-Corning, and other large roofing material manufacturers, have all looked at the effect of radiant barriers on their product.

I'm guessing there is negligible impact, if any, on the life of the materials, or the manufacturers would have come out with something.
They do have 20-50 year warranties to look out for...

[mark beiser]

Did some digging, only thing I found about roofing material temperatures was this.

"Effect of radiant barriers on roof temperatures

Field tests have shown that radiant barriers can cause a small increase in roof temperatures. Roof mounted radiant barriers may increase shingle temperatures by 2 to 10oF, while radiant barriers on the attic floor may cause smaller increases of 2F or less. The effects of these increased temperatures on roof life, if any, are not known."

From a DoE fact sheet: http://www.ornl.gov/sci/roofs+walls/radiant/rb_03.html

[Carnak]

Time to redesign the shingles or use something else.

[pstu]

Shouldn't let those delicate things lay out in the sun!

[nadsab]

I have a possible scenario for installing 2 radiant barriers in a Michigan cathedral ceiling that I would like to run by you folks to see if it makes sense.

The home is in southern Michigan, and has a very low sloped cathedral ceiling, 15 degree pitch. It is constructed out of 2x8 rafters. All there is, is ceiling drywall attached to rafters, then the rafters, then roof decking and then shingles.

I was thinking about installing either Dow Corning R-19 or R-25 in the rafter cavities, then using a bubble wrap type radiant barrier that would act as both a radiant barrier and vapor barrier between the ceiling drywall and the drywall side of the insulation. The manufacturer suggests that there be placed a ¾ inch air gap between the barrier and drywall which makes sense.

This is what I was also thinking about doing – the local code requires that rafter vents be placed between the top of the insulation and the underside of the roof decking to provide a continuous 1 inch gap - path between the sofits and the ridge vent. Instead of using the garden variety plastic or Styrofoam rafter vents, I was thinking about instead stapling an additional layer of the bubble wrap radiant barrier between the underside of the roof decking and the top of the insulation, an inch away from the decking, in a twofold scenario to create both the rafter vent, and also for purpose to reflect heat away from the top of the insulation in the summer time to reduce AC loads.

In other words, the insulation would be sandwiched between the two radiant barriers. The lower vapor barrier between the drywall and the bottom of the insulation in the wintertime would reflect and help to retain heat inside the home during the cold Michigan winters and (hopefully) save on heating bills, and the radiant barrier on top of the insulation would reflect the summer heat away from the top of the insulation during our scorching Michigan summers to save on the AC bill.

As far as price is concerned, if the radiant barrier on top is used as both a rafter vent and RB, the price difference between the RB and the plastic rafter vents is minimal, about an extra 30 or 40 bucks extra for the part of the house I am insulating/drywalling.

Does this scenario make sense? Or would it be a bad idea?

Thanks for any comments.

[tuccillo]

Not quite true. The reason you feel cold next to a large single pane of glass is that the net long wave radiation ( what you radiate minus what you receive from the pane of glass is a negative number). If you stand next to a warm wall, the wall is radiating more than the cold pane of glass so the net radiation is a smaller negative number. Remember, all materials radiate. The net (what you radiate minus what you receive from other materials) is what is important.


Number 1 is incorrect, number 2 is correct. That is why even if you lower the attic temperature to the same as the outside the dominant energy transfer will still take place. There is no question there is energy transfer from hot air by conductivity through the insulation. the amount of energy transfered, at a difference between 100 degree air and 130 degree air doesn't effect the lower room temps as much as a sunny day vs a cloudy day. The reason a/c's run even after the air is cooler in the attic after the sun goes down is that the heat in the sheathing and wood frame is still radiating heat to the insulation below. That is why in winter when you stand next to a large single pane window, you feel colder even though the air is 70 degrees between you and the window. Your body is radiating heat out to the cold surface.

[nadsab]

Correction to my previous post – the ceiling in my earlier question is actually a Vaulted ceiling, which is a sloped ceiling with a single sloping side, supported by unequal height load bearing exterior walls, 15 degree pitch.

It's kind of like "half of a cathedral ceiling"

[pstu]

I have a possible scenario for installing 2 radiant barriers in a Michigan cathedral ceiling that I would like to run by you folks to see if it makes sense.

The home is in southern Michigan, and has a very low sloped cathedral ceiling, 15 degree pitch. It is constructed out of 2x8 rafters. All there is, is ceiling drywall attached to rafters, then the rafters, then roof decking and then shingles.

I was thinking about installing either Dow Corning R-19 or R-25 in the rafter cavities, then using a bubble wrap type radiant barrier that would act as both a radiant barrier and vapor barrier between the ceiling drywall and the drywall side of the insulation. The manufacturer suggests that there be placed a ¾ inch air gap between the barrier and drywall which makes sense.

This is what I was also thinking about doing – the local code requires that rafter vents be placed between the top of the insulation and the underside of the roof decking to provide a continuous 1 inch gap - path between the sofits and the ridge vent. Instead of using the garden variety plastic or Styrofoam rafter vents, I was thinking about instead stapling an additional layer of the bubble wrap radiant barrier between the underside of the roof decking and the top of the insulation, an inch away from the decking, in a twofold scenario to create both the rafter vent, and also for purpose to reflect heat away from the top of the insulation in the summer time to reduce AC loads.

In other words, the insulation would be sandwiched between the two radiant barriers. The lower vapor barrier between the drywall and the bottom of the insulation in the wintertime would reflect and help to retain heat inside the home during the cold Michigan winters and (hopefully) save on heating bills, and the radiant barrier on top of the insulation would reflect the summer heat away from the top of the insulation during our scorching Michigan summers to save on the AC bill.

As far as price is concerned, if the radiant barrier on top is used as both a rafter vent and RB, the price difference between the RB and the plastic rafter vents is minimal, about an extra 30 or 40 bucks extra for the part of the house I am insulating/drywalling.

Does this scenario make sense? Or would it be a bad idea?

Thanks for any comments.
I have seen much research supporting RB in the Sun Belt, for reducing AC demand and energy. Have seen little or nothing advocating it in a heating climate, can you find any research supporting usage in Michigan at all?

The direction I am thinking is, solar heating through the roof may be a benefit in a heating climate, and RB would work against that. Could there be another direction of house improvement which would be more beneficial, I wonder. But I'm from Texas and would like to see someone more expert try to judge this.

Best of luck -- Pstu

[nadsab]

I have seen much research supporting RB in the Sun Belt, for reducing AC demand and energy. Have seen little or nothing advocating it in a heating climate, can you find any research supporting usage in Michigan at all?

Thanks for insight pstu.

I had been talking to an employee of a well known manufacturer of radiant barriers. He was very helpful and knowledgeable about the product and the industry, and he claims that my scenario would work great for both summer and winter seasons and would not raise the shingle temperature more than 5 degrees on hot summer days, and the top layer would help block heat in summer, the bottom layer would help keep heat in in winter.

Like you pstu, I have found no research as to radiant barriers in northern US climates. I was hoping to find any independent info or opinions as to this scenario in northern US states.

In Michigan, we have a wide range of seasons, it can easily get down to single digit F temperatures for weeks on end, and it’s not uncommon for the temp to drop below zero. And in the summer, mid 90 degree temps for days or even weeks at a time are not uncommon from late June to late august. 100+ temps are not unheard of either, although we don’t get 100 F plus days for several day or week stretches, which I assume many of the southern states can get. It’s also very humid here at times.

One thing I am concerned about, if I install the second radiant barrier on top of the insulation in the Vaulted ceiling, just an inch below the decking (also for purpose of creating that rafter vent), I am wondering if the shingle temperature would be hotter than if the radiant barrier were installed on the underside of the rafters in a no insulation scenario with several inches of dead air space between the bottom of the rafters and the decking? Many manufacturers in warm climates recommend installing the RB at rafter bottom. Would that small 1 inch air vent space get super heated, as opposed to an 8 or 12 inch empty air space?

The manufacturer guy I talked to said it would not, that radiant barriers do not work that way and don't worry your shingles won't get cooked.

My other concern is moisture - if I have the insulation sandwiched between two radiant barriers which also will act as vapor barriers, am I creating a big moisture problem in that rafter cavity?

A second radiant barrier manufacturer just told me that no, there is really no need to add an additional RB to the top of the insulation because the barrier on the bottom of the insulation would block heat migration in both directions - winter and summer - no matter which side the air space between barrier and insulation is on, and that adding a second layer of RB on top of insulation would not give me more than a few percent extra value for the dollar, and that if I live in Michigan to just install his product between the drywall and insulation. So now I'm a bit confused as to what to do - only have one chance to get this right once I start laying drywall.

[blackfeather]

...My other concern is moisture - if I have the insulation sandwiched between two radiant barriers which also will act as vapor barriers, am I creating a big moisture problem in that rafter cavity?

A second radiant barrier manufacturer just told me that no, there is really no need to add an additional RB to the top of the insulation because the barrier on the bottom of the insulation would block heat migration in both directions - winter and summer - no matter which side the air space between barrier and insulation is on ...

RB is also available perforated, to allow moisture to pass through.

I disagree with the second part there - everything I've read about RBs say that they are only effective with an air gap on the HOT side. So in the summer, to save on cooling, the shiny side must point up to reflect the heat back towards the roof. And vice-versa in the winter: there must be an air gap BELOW the shiny side to reflect the heat back into the house.

I'd like to do something similar: I'm considering making my own rafter vents (aka insulation baffles) out of cardboard lined with foil. The foil would face the 1" to 2" air gap, reflecting heat back towards the underside of the roof deck. The rafters are only 2x6's so I haven't much room for a second RB on the underside. I also wonder about the fire safety of using cardboard up there.

I live on Long Island, NY where the winters are bitter and the summers are brutal. It's hard to find test results about RB's benefits / drawbacks in the winter months. My solution would only be effective reducing AC costs in summer.

Does anyone know of a commercially available product like that? A baffle with a built-in radiant barrier?

[paul42]

RB is also available perforated, to allow moisture to pass through.

I disagree with the second part there - everything I've read about RBs say that they are only effective with an air gap on the HOT side. So in the summer, to save on cooling, the shiny side must point up to reflect the heat back towards the roof. And vice-versa in the winter: there must be an air gap BELOW the shiny side to reflect the heat back into the house.

I'd like to do something similar: I'm considering making my own rafter vents (aka insulation baffles) out of cardboard lined with foil. The foil would face the 1" to 2" air gap, reflecting heat back towards the underside of the roof deck. The rafters are only 2x6's so I haven't much room for a second RB on the underside. I also wonder about the fire safety of using cardboard up there.

I live on Long Island, NY where the winters are bitter and the summers are brutal. It's hard to find test results about RB's benefits / drawbacks in the winter months. My solution would only be effective reducing AC costs in summer.

Does anyone know of a commercially available product like that? A baffle with a built-in radiant barrier?

radiant barrier is effective whether it is on the hot or the cold side. The problem with facing it up is that it collects dust and is no longer effective.

[mark beiser]

everything I've read about RBs say that they are only effective with an air gap on the HOT side.

The DoE building scientists at Oak Ridge Labs disagree, so do the various utilities that have studied them, the entire radiant barrier industry, and my own personal observations of a variety of radiant barrier products in action.

Duno what you have been reading....

http://www.ornl.gov/sci/roofs+walls/radiant/index.html

[blackfeather]

The DoE building scientists at Oak Ridge Labs disagree, so do the various utilities that have studied them, the entire radiant barrier industry, and my own personal observations of a variety of radiant barrier products in action.

Duno what you have been reading....

http://www.ornl.gov/sci/roofs+walls/radiant/index.html


Taken directly from the page you linked to:

What are the characteristics of a radiant barrier?

All radiant barriers have at least one reflective (or low emissivity) surface, usually a sheet or coating of aluminum. Some radiant barriers have a reflective surface on both sides. Both types work about equally well, but if a one-sided radiant barrier is used, the reflective surface must face the open air space. For example, if a one-sided radiant barrier is laid on top of the insulation with the reflective side facing down and touching the insulation, the radiant barrier will lose most of its effectiveness in reducing heating and cooling loads.

[blackfeather]

...only effective with an air gap on the HOT side.

Perhaps I should clarify: The air gap aught to be on the side of the RB that you want the heat to stay on. I assumed you understood I meant a "one-sided" RB - that is only reflective on one side, facing an air gap.

I'll accept criticism for speaking too quickly. But this seems like basic thermodynamics to me.

[pstu]

Blackfeather, have you ever thought about the roof decking material which has RB on the down facing side? Widely used where I live, which is a hot climate.

I submit that the decking gets really hot but the foil blocks it from radiating that heat into the attic. I know this seems counterintuitive, but all the science I've read says it works. Perhaps someone else can explain it better.

For your own house, it probably will be easier to use foil type RB, or hire someone to do spray-on, will it not? This sounds like an opportunity for you do do something easier than you proposed.

Best wishes -- Pstu

[OOC]

Hey pstu, I wonder if a foil only RB would transfer heat through it through conduction if it were in contact with a hot material (such as insulation, decking or shingles), thus reducing it's total efficiency of stopping the transfer of heat. Having an air gap would greatly reduce the conduction of heat through the foil RB and improve the overall efficiency of the RB at slowing the transfer of heat. Anyone have a good diagram of 3 objects and air gaps showing the transfer of heat through the 3 mechanisms of conduction, convection, and radiation? That may help show that folks are likely talking very similar things, but don't realize it.

Someone on here a while back pasted a link to a very bad scanned copy of a report done by TVA (I think) that showed dust did not hurt a RB's effect by much. This seems very counterintuitive to me, unless the dust only covers a small fraction of total reflective area and dust material is a very poor conductor of heat. I can't find if I downloaded a copy of that report, but a search of this forum may turn it up.

[dn0121]

I spent the weekend installing radiant barrier. I had it in when I lived in Houston. Well moved to Maryland this summer and the temps up on the 2nd floor were much hotter then the first. I decided on installing it about a month ago. For about $100 and a weekend of work I think it will be worth it.

I stapled it up to the rafters. After installing some on Saturday and then going back up on Sunday I could feel the heat going up and out the ridge vent. This was on a 50 degree day. I am also adding some insulation to take my attic up to R-49 now that the foil is up.

Oh here are some pictures that you may have been referring too.

http://www.atticfoil.com/foilgraphics.htm

http://sol.sci.uop.edu/~jfalward/heattransfer/heattransfer.html

http://biocab.org/Heat_Transfer.html

[OOC]

Was the atticfoil the brand you used? I don't know about anyone else, but you can't bore me with as much details as you can provide about your install. I'm thinking about it and now that the attic is tolerable enough, I may do it.

[dn0121]

Yea it was, it was the best value I could fine. Wasn't hard and doesn't look perfect, but that is the beauty of it. It doesn't need to. I just cut and stapled. I followed what they had for instructions on the website, and looked at the pictures they had. I made sure to cut holes for the heat to escape up when I did side walls.

I also have a section where the house juts out and creates a mini attic in the attic. I have 2 of these, I know there are real names but I can't think of them now. Here there was little soffit ventilation so I got creative. It ended up with good airflow while still providing some protection.

As they say on the website it doesn't need to be taped or look perfect. Just make sure you have good airflow from the bottom to the top and you will be fine.

This stuff was great, cut easily, and was very strong. Hard to tear so that made it very easy to work with. As a side benefit the light that is up there now lights up the whole attic.

[mark beiser]

Taken directly from the page you linked to:

What are the characteristics of a radiant barrier?

All radiant barriers have at least one reflective (or low emissivity) surface, usually a sheet or coating of aluminum. Some radiant barriers have a reflective surface on both sides. Both types work about equally well, but if a one-sided radiant barrier is used, the reflective surface must face the open air space. For example, if a one-sided radiant barrier is laid on top of the insulation with the reflective side facing down and touching the insulation, the radiant barrier will lose most of its effectiveness in reducing heating and cooling loads.


So what is your point? All that passage is saying is that the reflective side of the barrier has to be facing the open air space, then gave an example of what they mean.
They could just as well have used radiant barrier roof decking as an example. The foil side would go down because that is where the open air space is.
I work in a lot of attics in Texas, and can tell you from experience that radiant barrier roof decking makes an amazing difference in the amount of heat entering an attic.
I have more before and after experience with the spray on products. They are not quite as effective, but still make a huge difference.

All that is required for RB materials to work is for one side to not be touching anything. They will block radiant heat transfer regardless of if the heat source and air space are on the same side, or opposite sides.

[dn0121]

I work in a lot of attics in Texas, and can tell you from experience that radiant barrier roof decking makes an amazing difference in the amount of heat entering an attic.

I'm with you there. It's one of the cheapest ways I know to save money. The only other one that maybe cheaper is sealing air leaks.

If you have a good system, an air tight house, and the right amount of insulation, then add a radiant barrier and feel the difference yourself.

[blackfeather]

I must concede, then:

The voice of multiple professionals with RB on their roof-decking used in hot climates speaks much louder than my complete lack of experience.

Perhaps the foil on the roof decking helps pull the heat out of the deck, and radiates it into the air-gap where convection carries that heat to the ridge. Without that foil, the excess heat in the decking may have been conducted through the rafters into the attic.

Whatever the science behind it, thanks for setting me straight.

- Blackfeather

[jzeeff]

I've heard that a radiant barrier on a horizontal surface can be equivalent to R8. On a wall, R3. But you can't add multiple layers to get more.

On another subject, I've found that a radiant thermometer is wildly inaccurate when used on reflective surfaces.

[mark beiser]

On another subject, I've found that a radiant thermometer is wildly inaccurate when used on reflective surfaces.

Yeah, they are, and if you use a contact thermometer, heat will be conducted to it, so the reading is still not accurate, lol.

[glewis29]

Blackfeather,

The science of it is this... A surface's reflectivity of energy is inversly proportional to it's emissivity (how much energy it gives off). You'll see RB rated at 97% reflective, 3% emissive for example (Interestingly, this is also why home radiators are often painted black.... black has a low reflectivity and a high emissivity). However, if you don't have an airspace, conduction will allow for a path of thermal transfer and make any gains from RB worse (if not nil). This is what Mark meant when he said it doesn't matter whether the reflective side is up or down as long as there is an air space.

If you're trying to keep heat out of the house and the shiny side is up, the reflective properties of the RB will reflect the energy back toward the roof. If the reflectve side is down, then as the RB warms up, the low emissivity will prevent it from radiating energy down into the attic/house. It's a matter of perspective and either way will work. Personally I think applying the RB to the roof decking would be best to keep the attic from getting warm. This does raise the roof temp slightly, but not enough that any warranties are voided by it.

The RB would work in the opposite direction for keeping a house warm. If facing down, it will reflect the energy back into the house. If facing up it will prevent emission of the energy from the house.

Also, someone recently posted that dust will make a RB ineffective... this may be more myth than fact http://www.radiantguard.com/index.asp?PageAction=Custom&ID=28

Hope this helps.

[OOC]

I knew I saw that TVA report somewhere. Thanks for posting it.

[pstu]

A couple questions were asked of me, but I am glad to see that others have given better answers than I could have. Deep thanks to all.

Best of luck -- Pstu

[mark beiser]

Also, someone recently posted that dust will make a RB ineffective... this may be more myth than fact http://www.radiantguard.com/index.asp?PageAction=Custom&ID=28

The dust making RB less effective was in the DoE/Oak Ridge National Labs fact sheet.

Even if dust isn't much of an issue, there are still huge negatives to putting the RB on the attic floor in many applications.

A RB on the attic floor does nothing to reduce heat gain in ductwork and HVAC equipment located in attics. Worse yet, it makes the heat gain worse because the attic temperature actually goes up.

RB foil on the attic floor makes it difficult, even dangerous, to get around in the attic if any work needs to be performed, and any work performed is more time consuming.

It may compress the insulation a little when installed, and it causes problems with adding insulation to the attic in the future.

If the RB isn't going to be protecting the ductwork and equipment, it would be cheaper and more effective to just blow in more insulation than to install a RB on top of the existing insulation.

[skodicious]

I've heard that a foil radiant barrier works somewhat more efficiently.

These guys say a spray on radiant barrier by Lo/MIT blocks 77% of radiant heat:

http://www.stophighenergybills.com/sprayrb.htm

[pstu]

I've heard that a foil radiant barrier works somewhat more efficiently.

These guys say a spray on radiant barrier by Lo/MIT blocks 77% of radiant heat:

http://www.stophighenergybills.com/sprayrb.htm
I believe you are right Skodicious. The labor savings from spray-on, are where the savings come from. Whenever someone says "efficient" they are thinking of a ratio of two numbers, it pays to deliberately ask yourself what are *their* two numbers. It could well be "solar heat reflected / solar heat passed", which is what the 77% means. But to a businessman, it could well be "price of job / total cost", and man-hour savings might mean spray-on is all he thinks worth doing. I cannot blame him, both are legitimate measures of efficiency.

Regards -- Pstu

[DFW RBS]

Hey guys....Great forum!

I'm Chris with StopHighEnergyBills.com. I am the operations manager and part owner. I would like to respond to a few things on here but want you to know that I will do so in complete honesty and hopefully give a little insight into the business. I welcome any and all questions and comments and will answer them with complete honesty....mostly because it seems like no one else in the business will!

PSTU, You are right when you commented about the term "efficiency" and what each person means when they say it. When I speak with customers, it means exactly 77% efficient. In other words, the E-Value of the spray on product we use is .23, which means it only allows 23% of the heat that the surface has, to actually emit or radiate away from itself. It blocks the other 77% (inverse of its E-value).

The business owner in me wants to sell nothing but the spray because I can make more money per hour installing it. Meaning, I can install about 2 spray jobs in the time it takes me to install 1 foil job. However, I also know that if I get you more savings then I estimated for your home, or a quicker return on your money then you thought, I will ideally receive more referrals and good feedback then if I was out selling only what netted me the highest profit per hour.

Sadly, we find a LOT of contractors are getting into this business and not bringing much ethics with them. Or at least not learning enough about the technology and products to do an ethical job with them. This really is the number one issue I have with our industry right now. We have phenomenal products, but people who would rather make a quick buck then apply good products the right way.

As for Dust accumulation and how it affects radiant barriers....

http://txspace.tamu.edu/bitstream/handle/1969.1/6461/ESL-TH-91-12-01.pdf?sequence=1

This is a very good Master's Thesis done in 1991 at Texas A&M. To summarize quite a bit and hopefully without error, Dust does indeed effect the E-value of radiant barriers. The more surface area the dust consumes, the higher the E-value rises. This means that dust has a higher E-value then the foil does and as it takes up more surface area on the foil, the overall effect is the reduction in reflectivity and increase in emissivity.

The only study I have EVER seen (and I have looked a lot and will continually search for more research) was done in the 1980's and sited that "dust does not have the negative effect once assumed". The next few lines also indicate that more research should be done on this subject. Since that one page essay by the Tennessee Valley Authority, a lot more research has been done.

Hope this wasn't too long but there are a lot of incredibly good uses for radiant barriers. Just like anything else, it takes educated and qualified people to use them correctly.

-Chris

[dn0121]

So the key is to vacuum up the dust once in a while.

[cem-bsee]

so, yesterday I saw a 1964 vintage radiation barrier under & next to the plywd sheeting of a 'flat' roof [does have slope to south & back of hse] with LOTS of discoloration on both sides of the aluminum!
Like a dozen water spots in 3 sq inches --

Hsv Util gets their pwr from TVA.

maybe I will get pic tomorrow --

anyway, I doubt it is doing much good now, nor for several years. --
BUT, what I saw was 'inside' the 3+ft overhang over the front porch.

given our high RH% here, I am not surprised --

BTW, we did have 2 nights last week with <40% RH --
but, we are back to normal this weekend = 28F dry, 26wet bulb at 6am.