Attic needs more ventilation

[jimbo48236]

Originally posted by newhomeboston
Maybe I'm just thick, but why won't the solar gain warm up the attic air hotter on the inside causing the snow to melt even if its just below 30 outside where the melt will freeze down?

Also, isn't my ac going to work a lot harder than it should in the summer as I can envision 150 degree days once it hits 100 around here if it gets to 130 on sunny 80 degree days. That's OK???

Thanks guys.

Radiation coming from our sun is less during the winter months. As the earth tilts away from the sun we go from summer to winter and get shorter days. The atmosphere actually acts as a barrier reducing the amount of radiation reaching your roof. Your attic shouldn’t get much warmer than the outside in the winter because of its heat gain is coming from your home. If it is properly sealed up you might get a 10 degree gain, but that would be rare.

Snow is almost a perfect “black body”, except in the visible light spectrum in which snow reflects the energy. A black body absorbs radiation, I don’t think you will have a winter ice problem with you roof it your house and attic are properly insulated and ventilated. I think you have the insulation covered and now need a bit more ventilation.

You can find out more on heat transfer at how things work, or search Google using heat transfer.

[uktra]

The temperature of the sheathing depends on amount of rdaiant energy absorbed, and the type/color of material on the roof deck surface. Rdaiant barriers don't nullify radiant transfer, they lower it by reflecting radiant energy to some degree. As far as how much savings are generated by radiant barriers, the big difference is if the ducts and air handler are in the attic. In terms of insulation storing radiant energy, the flow is constant depending on several factors. Heat will dissapate from different materials at different rates.

[newhomeboston]

Thanks...so

a. I will try to work on adding more ventilation one way or another and would still love ideas on which methods could complement the current but poor ventilation from the soffits to ridge vent.

b. Which is a better rb, installed foil or the metal paints that act as such?

c. just an fyi for those southerners...snow often blows off the top of a roof and collects on lower areas or valleys as I have in my house. When the house was under construction, snow stayed in the valleys for quite a while while the rest of the roof was clear which is why I am concerned about the roof heating up from sun hitting the shingles exposed not necessarily going through the snow.

More tests tomorrow to see if I'm getting any outflow from the ridge vent up there and crawling around on the moonscape of foam carefully to look down those baffles and see if they are blocked or not.

[cem-bsee]

uktra: very LITTLE heat is transfered through a ceiling via radiation -- maybe 2%!!

[uktra]

Cem-bsee--I don't know how you came up with that idea--but I can tell you for sure its wrong.

[pstu]

>>...poor ventilation from the soffits to ridge vent.

It seems to me there is evidence some of those soffit vents are blocked off. If there is any reasonable way to unblock those, I would consider that an important repair.

>>which is a better rb, installed foil or the metal paints that act as such?

Clearly the foil is higher performance, blocking about 97% of the radiation. If you use it in new construction, there is no added labor cost when you have foil backed OSB or plywood roof decking. However stapling up foil as a retrofit to an existing attic, is very labor intensive. Look into the costs and you will probably confirm this is out of sight for your house.

The spray-on radiant barrier paint from a good manufacturer, will block 75% or more of radiation. Combined with much lower labor costs for installation, this is today's conventional wisdom for existing homes. Do not be surprised if you can get a contractor to quote a job with the spray method, and have trouble getting a quote for stapling up foil. So in your shoes, I would accept the trade-offs with the paint method.

If you do things just exactly right, and under the right conditions, a power attic fan has helped some people. But for the majority, the item consumes more power than it saves. Conventional wisdom says this is more likely to be a placebo purchase rather than a solution to a problem.

If you can get any consulting advice from Building Sciences Corporation, which I believe is headquartered close to you, I would trust their opinion fully.

Hope this helps -- P.Student

[cem-bsee]

again: solar radiation depends upon the 4th power of absolute temperature -- so not much difference between 155F and 105F, so not much affect of radiation either from 155F to 75F in thru the ceiling next to the attic.

this was affirmed by my best friend, a retired NASA rocket engineer, who was charged with determining the best paint for the fuel tanks for mitigating solar effects upon the fuel tanks during fueling -- Tom helped put man on the moon -- using slide rule & Merchant calculators -- BTW, the chosen paint was such that color had such little influence that the tanks could be of any color, or have any logo desired! Tom knew every flow system of the vehicle, and the heat transfers involved -- such that they could predict the preformance of new device designs.

[uktra]

Cembsee--We are not talking about solar radiation in space. We are talking about radiant heat transfer in attic assemblies. The radiant heat transfer is from the hot roof sheathing to the cooler attic floor below. this can be calculated by the formula Q=s x (t1 to the 4th) - (t2 to the 4th) T1 being the roof sheathing. t2 being the temperature of the attic floor before insulation is put in place. If roof sheathing is 180 degress, the radiant heat transfer from the sheathing down to the floor (at 75 degrees) is: 147.1 Btu/h/sq.ft. In a 1500 sq. ft. attic that is about 222,500 btu/h! Of course that does not account for insulation which would lower that number. But 2%--no way.

[old yeller]

Uktra- I have an honest- no arguement question about attic heat. Please don't get defensive if this sounds like an arguement question, because it isn't. I am really hoping for some good information on how radiant heat effects my current home (no PV's).

Here tis': I have noticed that as the sun is about 1/2 (noon) to 3/4 down, when I would THINK the heat load to be the highest of the day, my AC does not run longer. BUT, when the sun is 3/4 or more, down to sunset and past, the heat load in the house seems to increase and the AC has a much harder time keepng a set temperature. This happens until about 10pm. IT CAN do it, it runs great, but I notice it runs more.

Some obvious points that should be known:

1. The later-sun facing part of the house has 2 medium-small solar-screen covered windows and a gable-end of the roof. The are lots of trees to block all sun about 1 hour before sunset.

2. System has been checked and is running optimally.

3. Roof is brown composite.

4. 1 Whirly-bird with gable end vents and some soffit vents (new).

My theory is that at sunset any wind stops and the latent heat buildup does not move much at all, just sitting in the attic with no place to go. Also, without the roof surface continuous heating from the sun, I lose any inherent stack effect to help keep the air moving at a decent pace.

It seems that once the sun is off the roof, the radinat effect would decrease dramatically as the roof could give it's heat off to the air air relatively quickly.

You are welcome to tell me that what I am seeing is not really happening, but I am hoping there really is something to this that I may be able to cure with more soffit vents or something.

Let me know what you think-

[uktra]

What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down. If you have ducts in the attic, you may want to look at a radiant barrier. If you can do it yourself, it would be cost effect. Of course sealing ducts and sealing holes in attic floor is the first priority in lowering heat ransfer to the living areas.
Hope this helps

[Edited by uktra on 06-20-2005 at 04:36 PM]

[old yeller]

Originally posted by uktra
What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down.

So the guts of the house are absorbing heat on a linear scale...and they take longer to "add" to the heat issue?

I would agree with that. Is radiated heat an issue at this point? Wouldn't it help to "flush" out this latent heat? I just seems that the house materials would cool off much faster, and since the problem is no longer radiation- at least not as much- that this could be a good point to try to get rid of the heat. That would help bring the temperature of the insulation down a bit faster.
I beleive there are tests that have shown that after outside temps go down, the core of fiberglass insulation can stay very hot for a while and this can radiate into the house for some time.

If there is no longer a heat source to continue heating the roof surface, it seems some sort of ventilation would be beneficial at this point? NOT a PV, just "something".

I have been intersted int he RB for a while, and if I replace my roof someday I am definitely going to use it. BUT, concerning the RB, if there is no longer a radiant roof surface, it won't do much good for this time of day, true? Again- I know it works, that is not my point.

[Edited by old yeller on 06-20-2005 at 04:48 PM]

[old yeller]

Originally posted by uktra
What you are seeing is the stored up heat in the building products of the home. The roof sheathing and other wood framing members takes time to cool down. If you have ducts in the attic, you may want to look at a radiant barrier. If you can do it yourself, it would be cost effect. Of course sealing ducts and sealing holes in attic floor is the first priority in lowering heat ransfer to the living areas.
Hope this helps

[Edited by uktra on 06-20-2005 at 04:36 PM]

IF my ducts are sealed, is there anything I can do to protect them from radiant heat directly? RB on my entire roof is close to impossible right now. MAybe cover them with radiant barrier material? They are much easier to get to than my roof decking.

[robnjr]

Old Yeller:

You can do what I did: spray a lot of loose fill cellulose over the attic ducts, and bury them...you won't get all of the duct-runs covered, but a majority of the ducts can be protected from "heat" (radiant or convection, chose your poison)....only costs about $200 at Home Depot for the insulation, and they give you the blowing machine free for 24 hours if you buy more than 20 bags (about $7 per bag)....just be prepared for a lot of mess when you spray this stuff, and leave your AC system OFF....

PS: Lots of debates about cellulose vs fiber glass, but I'm not touching that one here...

[jimbo48236]

Originally posted by uktra
Cembsee--We are not talking about solar radiation in space. We are talking about radiant heat transfer in attic assemblies. The radiant heat transfer is from the hot roof sheathing to the cooler attic floor below. this can be calculated by the formula Q=s x (t1 to the 4th) - (t2 to the 4th) T1 being the roof sheathing. t2 being the temperature of the attic floor before insulation is put in place. If roof sheathing is 180 degress, the radiant heat transfer from the sheathing down to the floor (at 75 degrees) is: 147.1 Btu/h/sq.ft. In a 1500 sq. ft. attic that is about 222,500 btu/h! Of course that does not account for insulation which would lower that number. But 2%--no way.

Uktra. I am talking about the Suns radiation. Radiation is heat transfer by the emission of electromagnetic waves which carry energy away from the emitting object. For ordinary temperatures (less than red hot"), the radiation is in the infrared region of the electromagnetic spectrum.

The relationship governing radiation from hot objects is called the Stefan-Boltzmann law. The roof of the house could never exceed the tempurature of the surounding air without heat grain from the Sun's radiation. Reducing the amount of radiation that makes it into the attic will result in much lower temperatures in the attic.

If the man had a steel roof, good insulation (reducing both conduction and convection heat transfer) his attic could be around 20 degrees warmer then his second floor. Adding that gable fan, and his attic could be the same temp as the ambient air outside.

[old yeller]

Originally posted by robnjr
Old Yeller:

You can do what I did: spray a lot of loose fill cellulose over the attic ducts, and bury them...you won't get all of the duct-runs covered, but a majority of the ducts can be protected from "heat" (radiant or convection, chose your poison)....only costs about $200 at Home Depot for the insulation, and they give you the blowing machine free for 24 hours if you buy more than 20 bags (about $7 per bag)....just be prepared for a lot of mess when you spray this stuff, and leave your AC system OFF....

PS: Lots of debates about cellulose vs fiber glass, but I'm not touching that one here...

I agree with burying them, but the radiation factor does still remain. I am actually looking at doing the blown in like you are talking about, then going over the entire attic with Radiant Barrier Chips. These are supposedly way better than trying to line the rafters or floor with solid sheets and WAAAY easier to do without tearing off the roof.

This seems like a big heat double-whammy. I'll keep the majority of radiation off the insulation, which can actually make your problem worse in some situations by holding retained heat against the ducts. Then the insulation will help retard the ambient heat in the attic away from the flex duct.

What do you think? What did you do with the air handler? Did you "build up" some insulation around the non-maintenance sections of the air handler?

[uktra]

jimbo48236--I understand source of heat is the sun by radiant transfer. I agree with everything you say up to the point where you say insulation will reduce convective transfer. This depends on the type of insulation. The real problem is why put a power vent in, when the small reduction in attic air temperature, and the smaller amount of heat transfer from the hot air to rooms below will be offset by the energy to power the fan?

[cem-bsee]

1974 CollegePhysics by Sears& Zemansky Young:
temps to be in Kelvin
the quoted formula indicates temp to 4th power only, (t-sub1 & t-sub2)
the quoted formula is for heat transfer from small hot body to surrounding walls --

[uktra]

cem-bsee--Actually the formula is in degrees R (Rankin--F+460). This formula calculates the radiant heat transfer from a higher heat source to a lower heat source.

[tuccillo]

Originally posted by uktra
cem-bsee--Actually the formula is in degrees R (Rankin--F+460). This formula calculates the radiant heat transfer from a higher heat source to a lower heat source.

Kelvin is the more typical temperature scale for science. Whatever you use, make sure you have the correct Stefen-Boltzmann constant.

[cem-bsee]

my understanding of the Physics book is that the formula was only applicable for a small hot heat source inside a very large room --

That does not the description of a large source next to one of simular size, with not much temp difference.