White Roof

[Carnak]

I was at a place recently, that used the white metal roofing.

His roof sheathing is pressure treated ply wood. He has ice and water shield on top of it, then foil bubble wrap, then covered with white metal standing seam roofing.

He did not use any vents in his attic, but you can still see a lot of daylight when you look around the attic perimeter, unsealed gaps where the roof line meets the exterior walls.

Anyways at 2 in the afternoon, an infrared thermometer was saying the underside of the sheathing was 93F. The air temperature was not that high, mid 80s. He had fibreglass insulation below at the ceiling plane.

I have a data logger in the attic and a few in the house below. Taking two weeks readings every minute. Should be interesting to see the effect that basically just having a white roof will make. Maybe the bubble wrap without an air space is R3

My money says it is the ultimate radiant barrier.

[tedkidd]

I have a commercial customer with a white roof. This summer before the drainage was put in there was a small lake up there. I happened to put my hand in the water towards the end of a hot fairly clear day - brrrrr.

I'd say there's something to that. (I hope there is - we only put 8 tons of cooling in)

[Carnak]

My gut feeling tells me this attic will be cooler than any of the well ventilated ones, including those that use fans, and those with a radiant barrier inside the attic

[tedkidd]

A well ventilated attic is little cooler than an unventilated one. It's like trying to cool a pot of boiling water with an eye dropper.

Much like natural draft is an energy intensive way to remove products of combustion, roof ventilation strategies are an energy intensive strategy for removing moisture.

Also, they are treating a disease that may or may not exist.

If symptoms of moisture exist in an attic, curing the disease seems a much wiser approach then treating the symptoms. (STOP TAKING SHOWERS IN YOUR ATTIC!)

[sparksandfan]

There's no doubt that white metal roofs work in hot climates. A number of folks have taken data including Florida Solar.

There is a combination of things going on which includes heat absorbed, and the way heat is re-radiated. White is not necessarily white in infrared wavelengths.

A good combination is white metal on the top and radiant barrier on the bottom of the roof deck.

[tinner73]

the hospital we just finished has a white rubber roof...BLINDING in the glaring sun.

[tedkidd]

Be good to understand this better.

Any residential applications?

[sparksandfan]

Check out the residential applications at Florida Solar Energy Center:

FSEC-PF-337-98
FSEC-CR-1475-04

Different roofs measured in Cocoa, Florida.

[Carnak]

There's no doubt that white metal roofs work in hot climates. A number of folks have taken data including Florida Solar.

There is a combination of things going on which includes heat absorbed, and the way heat is re-radiated. White is not necessarily white in infrared wavelengths.

A good combination is white metal on the top and radiant barrier on the bottom of the roof deck.

I think the white metal roof is the ultimate radiant barrier.

I have one at my own home, the white metal roofing is above insulation that is on top of the deck. My sealed attic is remarkably cool. Prior to conditioning the attic space, it managed to average in the top right hand corner of ASHRAE's comfort zone

I think a radiant barrier inside would be dumb in this case. The decking was only 93F, so will radiate similar to infloor heating. With dark shingles the underside of the deck could be 160, that radiates a lot more. Ambient air was about 90, no intentional venting, so white roof seems to be doing the trick.

The house I am looking at now will hopefully point out how the roofing alone will perform, I think the bubble wrap on top of the decking will be insignificant

I think a white roof works in any climate. You can have a well insualted and ventialted attic in Canada, and snow stays on your roof and does melt in the dead of winter. So it is white in the winter time also. :), Once you are into the cooling season does not matter where you are, the white roof helps.

Sure is better than letting the sun superheat a roof covered with dark shingles

[Carnak]

A well ventilated attic is little cooler than an unventilated one. It's like trying to cool a pot of boiling water with an eye dropper.

Much like natural draft is an energy intensive way to remove products of combustion, roof ventilation strategies are an energy intensive strategy for removing moisture.

Also, they are treating a disease that may or may not exist.

If symptoms of moisture exist in an attic, curing the disease seems a much wiser approach then treating the symptoms. (STOP TAKING SHOWERS IN YOUR ATTIC!)

it is different when you insualte the deck, my attic is significantly cooler than any vented attic I have been in.

A vented attic here is easily 130F, this attic was in the 80s. A lot of heat is being kept out in the first place, different concept than having a dark solar collector superheat a space, then trying to cool it off by ventialting it with humid air.

That is like getting LEED points for an eco friendlt sauna above your living space-- freee heat and free water.

the strategy for removing moisture is a cold climate one. Ventialte with cold air to keep the attic space dry and cold. Stops condensation and stops melting the snow on the roof which form icycles at the eaves

[tedkidd]

Right. Ill append my original. Stop the moisture AND the heat from going up there.

Stop showering in your attic AND stop heating your attic

[Carnak]

You keep speaking from a winter perspective.

[Carnak]

you get a lot of posts in here, from people in southern states who do not have to worry about icycles.

They are doing stuff like painting an RB paint on the underside of the roof deck, adding RB foil - or they go the ventilation route and follow Northern codes with a ridge vent and then the 'correct area' of soffit vents, or they power ventilate.

The problem I have with all of these strategies, and neglecting the humidity problem, is that they all try to deal with the solar heat after it has made its way inside of the attic.

I figure it is a no brainer, to deal with solar heat before it makes it into the attic.

So wear a condom or take valtrex for the rest of your life and be careful when you have an outbreak

[teaysvalley]

A flat white roof will more than likely become dirty in a city or farming area thus reducing the reflective aspects. I can see a sloping white metal roof staying cleaner.

3M has developed reflective granuals for asphalt type roofing that looks promising and they come incolors besides white.

http://solutions.3m.com/wps/portal/3M/en_US/IMPD/Roofing-Solutions/Products/Cool-Roofing-Granules/

http://coolcolors.lbl.gov/assets/docs/PAC-2005-03-03/3M.pdf

[Carnak]

rain keeps my roof clean

Take an infrared thermometer and go for a walk on the sidewalk.

Probably that side walk is far from clean, but I bet if you take its surface temperature its going to be 100F.

Then take the temperature of the asphalt road surface a few feet away. 130F

[paul42]

For my house,

White metal roof = $7,500 additional up front costs
Radiant Barrier = $1,500 additional up front costs

With R49 insulation in attic, payback on white metal roof exceeds lifetime of roof.

However, for rain water collection reasons, I would still like to have the metal roof.

[Carnak]

eliminate the radiant barrier and the white metal roof is 6K extra over POS shingles

The R49 probably fliters the hot humid air was it infiltrates through and short circuits the insulation.

They make lighter coloured shingles, however with the textured surface stuff will get lodged in it.

[paul42]

According to HVAC Calc, the heat gain through my ceiling is less than 1,500 btu/h. I know from real world experience that the total heat gain that HVAC Calc came up with is correct, so I have to assume that the number for the ceiling is close.

Spending $6K to decrease that 1,500 btu/h is just not cost effective.

I suspect that the $1,500 for the radiant barrier wasn't cost effective either. HVAC Calc did not ask if I had installed a radiant barrier.

[Shophound]

So we're talking about a sloped version of Dr. Joe's "perfect wall", save the cladding is standing seam white metal vs. asphalt shinging (which I agree is a POS).

If I had a fat remodel budget, my terra cotta colored asphalt shingles would be gone and a light colored standing seam roof installed in a heartbeat, with 2" foam insulation between it and the roof decking. Kiss big time heat gain to the attic ducts goodbye!

[Carnak]

According to HVAC Calc, the heat gain through my ceiling is less than 1,500 btu/h. I know from real world experience that the total heat gain that HVAC Calc came up with is correct, so I have to assume that the number for the ceiling is close.

Spending $6K to decrease that 1,500 btu/h is just not cost effective.

I suspect that the $1,500 for the radiant barrier wasn't cost effective either. HVAC Calc did not ask if I had installed a radiant barrier.

HVAC CALC assumes your ceiling plane is air tight, it isn't

It also assumes that infiltrating air is at ambient conditions and not superheated to 130F.

It also assumes the insualtion never settles.

HVAC CALC would also say that my roof with R7 insulation means I need a bigger AC than I have. But for some reason R7 on top of the deck and below white standing seam works better than R30 at the ceiling plane of a vented attic.

[Shophound]

HVAC CALC would also say that my roof with R7 insulation means I need a bigger AC than I have. But for some reason R7 on top of the deck and below white standing seam works better than R30 at the ceiling plane of a vented attic.

In your OP you said the underside of the roof deck with the white standing seam and bubble wrap was 93 degrees. If you doubt the bubble wrap is doing much to slow heat transfer, that means the standing seam metal exposed to the weather isn't getting very hot, unlike asphalt POS shake n' bake.

When I was a kid, every tract home built in my area had white asphalt shingling (the higher end ones had wood cedar shakes, but spring high winds and fire proved that roofing approach to be folly and they've since been banned). No such thing as colored asphalt shingling back in the day, except perhaps a light puke green. Even with the shingles white, I remember attics were hot. The material itself likely has a lot to do with heat retention and transfer as does the color. Maybe it's in my Handbook of Fundamentals...the heat content/u factor of asphalt shingling vs. standing seam metal. I'll need to look that up at some point.

It seems simple. If the roof deck doesn't get cooked it won't in turn cook the attic. Underside radiant barrier is applied for that reason, some more effective than others. If HOA's won't let certain subdivisions have standing seam (or the price point of the house), then go with roof decking that has radiant barrier already on the underside of the decking. Or foam the roof deck with open cell foam. Pay a bit more up front, save a lot more over the long haul.

[paul42]

I looked at using 4x8 sheets of polyiso foam on my roof deck with a white standing seam roof. It was my prefered approach. However, city code required that I use at least 5" thick panels which would have required two layers. Total cost of the foam would have been $12k just for the foam ($.50 per board ft.) Installation would have driven the price much higher. Total installed cost for the cellulose was approximately $1k.

I do not know what assumptions HVAC-Calc makes concerning an air-tight ceiling. Based on the work I did making it air-tight, I know that it is.

Based on long term readings from my CO2 monitor, I know that my whole house is very tight and that most of the air leaks that do exist are not in the ceiling. When the exhaust vents are on the leeward, the infiltration doubles. In any wind direction, the fresh air intake on my furnace is marginal. Any air sealing that I might do will also require adding a blower to the fresh air supply.

The Federal Trade Commission R-Value rule and accepted industry standards all require cellulose insulation to be specified and sold at settled density.

My total energy costs for air conditiong are running about $450 a year. According to HVAC-Calc, approximately 10% of that is due to heat gain through the ceiling. Because my duct work is not in my attic, any improvements to the roof are never going to have any pay back in energy dollars.

[Shophound]

I looked at using 4x8 sheets of polyiso foam on my roof deck with a white standing seam roof. It was my prefered approach. However, city code required that I use at least 5" thick panels which would have required two layers. Total cost of the foam would have been $12k just for the foam ($.50 per board ft.) Installation would have driven the price much higher. Total installed cost for the cellulose was approximately $1k.

I do not know what assumptions HVAC-Calc makes concerning an air-tight ceiling. Based on the work I did making it air-tight, I know that it is.

Based on long term readings from my CO2 monitor, I know that my whole house is very tight and that most of the air leaks that do exist are not in the ceiling. When the exhaust vents are on the leeward, the infiltration doubles. In any wind direction, the fresh air intake on my furnace is marginal. Any air sealing that I might do will also require adding a blower to the fresh air supply.

The Federal Trade Commission R-Value rule and accepted industry standards all require cellulose insulation to be specified and sold at settled density.

My total energy costs for air conditiong are running about $450 a year. According to HVAC-Calc, approximately 10% of that is due to heat gain through the ceiling. Because my duct work is not in my attic, any improvements to the roof are never going to have any pay back in energy dollars.

For many homes, windows and sliding glass doors are the worst offenders pertaining to heat gain, alongside infiltration. Where we North Texans shoot ourselves in the foot in regard to piling the attic floor sky high with blown in insulation is doing nothing about ducts in the attic that have at best R8 wrap on them, and air handlers that likely perform worse than that. While we reduce heat gain through the ceiling with a blizzard of insulation, we leave the ducts to bake under an overheated roof deck. That's an a/c capacity robber.

In your case you have ducts in the crawl space, so I wouldn't worry much about what's going on overhead, based on what you've reported here. < $500 annual cooling cost for a home your size is something many North Texans can only dream about.

[Carnak]

In your OP you said the underside of the roof deck with the white standing seam and bubble wrap was 93 degrees. If you doubt the bubble wrap is doing much to slow heat transfer, that means the standing seam metal exposed to the weather isn't getting very hot, unlike asphalt POS shake n' bake.

When I was a kid, every tract home built in my area had white asphalt shingling (the higher end ones had wood cedar shakes, but spring high winds and fire proved that roofing approach to be folly and they've since been banned). No such thing as colored asphalt shingling back in the day, except perhaps a light puke green. Even with the shingles white, I remember attics were hot. The material itself likely has a lot to do with heat retention and transfer as does the color. Maybe it's in my Handbook of Fundamentals...the heat content/u factor of asphalt shingling vs. standing seam metal. I'll need to look that up at some point.

It seems simple. If the roof deck doesn't get cooked it won't in turn cook the attic. Underside radiant barrier is applied for that reason, some more effective than others. If HOA's won't let certain subdivisions have standing seam (or the price point of the house), then go with roof decking that has radiant barrier already on the underside of the decking. Or foam the roof deck with open cell foam. Pay a bit more up front, save a lot more over the long haul.
I was in another place here that had to have its cathedral ceilings ripped out, was a problem similar to having a vapour barrier on the wrong side of the insulation.

Anyways with an infrared the under side of the wood decking with just ice and water shield and white metal roofing on top, was not much higher than what I observed in the house that this thread is based on.

Now in this other home, they already had it up over 100 degrees inside the small space as they were drying it out with a dehumidifer.

We had to open the doors for a bit to let some heat out. There was actually sections with bubble wrap on the underside of the decking still up, and other spaces where the bubble wrap was gone. The bubble wrap seemed to cool off quickly to the elevated room temperature, the wood decking started dropping in temperature as well. It was initially around 105 F shortly after we entered while bubble wrap was low 90s. In the course of say 1/2 hour, the wood decking was down to 98F and temp was dropping as the sun was getting higher in the sky.

I say the elevated temperatures were from running a dehumidifier in there steady, the bubble wrap was just cooling off faster than the wood.

[Carnak]

shophound

Some years back there was a primary school project, single story, soffit vent only attic, insualtion at the ceiling plane. Cannot say if it was felt or ice/water shield on top of the plywood sheathing, but it was a green standing seam roof.

It did not take much later than 9 AM Standard Time for it to be smoking hot in there, too bad I never measured it.

I find the attics under concrete tiles to not be too bad.

Even darker tiles, I think the thermal mass buffers the heat.

In my own attic, when it was just the concrete roof exposed to the sun during construction, the underside of the concrete would hit 100 and then some. It depended if I was measureing the solid rafter beams, or a block infill under the poured concrete, The blocks with the cores were cooler.

We were hanging the ductwork right around when I had the wood nailers on top of the deck with styrofoam in between and the black felt on top. It was brutal up by the deck hanging duct. I was second guessing myself and wondering if I was going to have to fork out for icynene.

But when the roofing went on, and before we started up the AC, I never measured it being more than one degree hotter in that attic than it was outside.

[Carnak]

This post was about a weekend home, so I ran a two week test, first week he just shuts off his AC for the week and lets the place get roasting hot, then goes out there friday AC runs 14 hours to cool the concrete down. Second week he left it set back to 82F when he was not there.

So weather was high 80s to just over 90 the whole time, its humid here.

So the first week with the AC off, the attic actually hit 97 degrees once, but you could see it building up every day as the AC was off the entire time and the inside temperature of the home rose up.

The second week with it being set back to keep 82F, the attic never got above 91 degrees.

This attic has no vents put you can see daylight around the perimeter and the week when the AC was off, the attic dew point pretty much seem to follow what the outside dewpoint was.

The week where he left the AC running, the dewpoints were significantly lower up there. So for sure I am telling him he needs to seal it up at the very least, the lower mositure level means it was transfering down to his home through the ceiling.

When they were actually there over the weekend running the AC at normal temps that attic never broke 90 degrees. House was cooling it off.

What kind of temps are you guys seeing with the interior radiant barriers and paint?

[Shophound]

What kind of temps are you guys seeing with the interior radiant barriers and paint?

Nice report on that house, Carnak. Thanks for posting it.

My own attic with radiant barrier paint on the roof deck underside peaked at 135°F this summer with outdoor ambient temps over 100 degrees F. Not great, but lower than what similar attics may peak at, say 150 or higher, depending on sun exposure and amount of ventilation.

Nevertheless I had no problem meeting my interior occupied setpoint targets during hot weather. No problem with humidity control, either. Right now, unfortunately, I have a gaping hole in the ceiling adjacent to my fireplace thanks to a pesky water leak from the stone chimney. Although I've covered this hole with plastic to stem airflow, it keeps wanting to peel off. When my wife ran the dryer today I could smell that hot attic odor seeping in, along with a localized spike in humidity. Nevertheless, with outdoor temps around 95, the a/c still cycled to hold 75 in the house. I got up on the roof this morning and applied an elastomeric coating to the mortar dome atop the chimney. If we get rain later this week as forecasted, and the chimney passes muster with no leak, I can then patch the ceiling, foaming the voids around the chimney beforehand (there's no insulation in this area at all) to reduce thermal bridging and slow air movement.

I'm building a freestanding storage and workspace structure adjacent to my house right now. Although the enclosed storage area won't be insulated, I used roof deck with radiant barrier on it anyway. It will be interesting to see how this little storage room performs when really hot weather returns next year. The workspace area has no door, but with the radiant barrier overhead I expect it to be around ambient air temp when I'm working out of it.

[timtrain35]

Hey guys, somewhat new here and not an AC pro. However, I am a rep for a line of spray polyurethane foam insulation (spf) and some roof coating products as well. The thinking from Shophound and Carnak is pretty much dead on. Cool roofing basically takes two attributes of any particular roof into account; Reflectance - the ability to reflect the sun's rays and prevent them from turning into heat. And Emissivity - the ability of said material to hold heat. When properly tested, these two values are mathematically combined into one number, called the Solar Reflectance Index (SRI). The place to go to find out all about this stuff, is coolroofs.org. That's the cool roof rating council, where all the manufacturers have to go to submit their products for testing. It's divided into two broad categories, low slope and steep slope. And also split into initial and aged values. Without going into too much more detail, the bottom line is that cool (white) roofs work in greatly reducing the heat transfer from the roof, into the living space. Thus, reducing the amount of energy required to cool off the living space. However, the savings are pale in comparison to what spf can do.

To use spf the way it should be used has been eluded to in this post. Seal up the house, don't ventilate the attic, keep outside air outside and inside air inside. Average savings on spf homes are in the 30-50% range when compared to conventional insulation. Sure, if it's super tight, a fresh air supply will need to be added, but in the long run, it's worth it.

Anyway,I could go on and on about this stuff and there's plenty of good info on it on the web. If anyone has any questions, I'll do my best to answer them.

[Carnak]

is your product permeable timtrain?

[Carnak]

thanks for the coolroof link, looks like bright wihite metal roofing reflects away about 70%

I have bright white standing seam above 1.5 inches of styrofoam, above a sloping concrete deck.

I find it performs well in a year round cooling environment and a latitude south of the tropic of cancer

[timtrain35]

Carnak,

The coating or the foam?

[Carnak]

the foam

[timtrain35]

I'll give you the abbreviated answer. Under normal building conditions and normal application thicknesses, yes it is vapor permeable while being a very effective air barrier as well. The two are not necessarily synonymous in all insulation or "air barrier" products.

[Carnak]

with ice and water shield above the deck, I would like to see something permeable below the deck