The Best Way to Keep Your Attic Cooler is to Change Your Roof Color

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The Best Way to Keep Your Attic Cooler is to Change Your Roof Color

Reflective roofing, unlike powered attic ventilators, addresses the problem where it begins

Posted on Aug 26 2015 by Allison A. Bailes III, PhD
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The most contentious issue I’ve written about since I started blogging isn’t bad Manual Js. Nor is it endorsing government intervention by raising efficiency standards or improving energy codes. Incredibly, it’s not even whether or not naked people need building science. Nope. The topic that really gets readers hyperventilating is powered attic ventilators (PAVs).

Some people swear it’s the best way to keep their attic cool and reduce air conditioning costs. Apparently they haven’t seen the research about what works better than PAVs without the drawbacks.

Do you really need to cool your attic?

First off, let’s limit the discussion in this section to homes with unconditioned attics. Conditioned attics are great, but I want to focus on all the homes out there that have the insulation and air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. at the ceiling, not the roofline.

What about new homes? If you’re designing and building a new home, keeping an unconditioned attic cooler (in summer) is pretty much irrelevant. Just make sure the ceiling is airtight and fully insulated, and the temperature in the attic doesn’t matter much. Whether it’s 110°F or 130°F, there’s not much difference in the amount of heat flowing from the attic to the living space below.

Oh, you also have to make sure not to do something stupid, like putting your air handler and ducts up in the attic. When you’re designing and building a new home, these are choices you can and should make. If you don’t, worrying about how best to keep the attic cool is a band-aid on a self-inflicted wound.

In existing homes, the question is important. (Again, I’m assuming that you’re not considering a sealed attic.) Lowering the temperature of an attic that contains ducts can reduce your air conditioning costs. If the home doesn’t have ducts, there still may be reasons a homeowner would want to reduce the attic temperature. A lot of people like to store stuff up there, for example. I once had a homeowner who complained of the rubber handles on a wheelchair melting because it was so hot in the attic.

OK, let’s look at one more scenario: non-air-conditioned homes. Sometimes people say they want to install a PAV to cool the house because they don’t have AC. In that case, what they really need is a whole-house fan, not a powered attic ventilator. And they’d still want to have the ceiling air-sealed and fully insulated to minimize heat transfer from the attic to the house. As with new homes, those without air conditioning shouldn’t need to worry about keeping the attic cooler.

So, sometimes people do have good reasons for wanting to keep an attic cool. What are the options? Powered attic ventilators seem to be the one a lot of people want. It’s relatively inexpensive. It’s quick to install. And it does lower attic temperatures. But I know of two other methods that both are better than PAVs. One is a radiant barrier. The other is a reflective roof. And one of those is better than the other. (Of course, there's a fourth option: You could install an air conditioner in the attic, but that's the stuff of April Fool's Day jokes.)

The research on reflective roofs

There’s been some good work done on how the type of roofing affects the attic temperatures and cooling loads in the house. The Florida Solar Energy Center (FSEC) has done quite a bit of work in this area and even built a test building called the Flexible Roof Facility to study different types of roof and attic assemblies. They’ve also done research on a number of real homes in Florida.

One of the earliest studies they did was to look at the difference between black shingles and white shingles. The first graph below shows the results, and you can see that it’s about what you might expect. White shingles reflect more of the incident solar radiation and stay cooler. They don’t show the attic temperatures here, but if the shingles are cooler, the attic is cooler. (You can read more about this in FSEC’s literature review of attic ventilation. The graph is from page 30.)

In another study, they changed the color of a home’s tile roofing from dark grey to white. (See Image #3,. below.) Here they did measure the attic temperatures, and you can see them in the next graph (Image #4). The white roof lowered the attic temperature by about 20 F°. (These images came from a presentation Danny Parker gave on FSEC's cool roof research.)

The third graph shows attic temperatures from the middle of the attic for several different types of roofing as well as a sealed attic. In this case, the most reflective roofing material (ivory IR selective metal tile) is 23.4 F° cooler than attic covered with dark shingles. (Read more about this in Danny Parker’s 2004 Home Energy magazine article, Improving Attic Thermal Performance.)

But it’s not just FSEC that’s been studying what’s going on with roofs and attics, of course. Jeff Gordon gave a great presentation on attic venting at the 2011 Affordable Comfort conference and included some data they’ve taken at the Buildings Research Council lab in Illinois. They found that dark shingles are 27% hotter than white shingles.

My friend Cameron Taylor has done a less formal study on his own home in Fort Worth, Texas. The design temperature there is 98° F, and they've been experiencing quite a few days where it gets above 100° F in recent years. He replaced his roof with a reflective metal roof (shown below) and wrote this to me in an email:

My experience with my own reflective roof is going on three years, now. Each summer has been consistent with air temperatures in attic tracking below outdoor ambient until mid afternoon, where it then tracks with outdoor ambient.

Why reflective roofs beat PAVs and radiant barriers

As you might expect, it’s not hard to show that reflective materials keep things cooler. When solar radiation hits a surface, three things can happen. It can be reflected; it can be absorbed; or it can be transmitted. The part of the solar spectrum that heats things up doesn’t get transmitted through roofs, so the incident solar radiation is either reflected or absorbed there. The more of it you can reflect, the less gets absorbed. Reflective roofing attacks the problem right there where it starts.

Once the solar radiation is absorbed, some of the heat still gets ejected to the outside but much of it conducts downward through the roofing materials and roof deck. There it radiates down into the attic, heating up the insulation, the framing, the ductwork, and the boxes of Christmas tree ornaments. A radiant barrier can help by reducing the amount of that heat that radiates downward. But still, the heat is already in the roof. Radiant barriers are fine, but why not stop the heat on the side of the roof where it first enters?

Powered attic ventilators are the worst way to try to keep your attic cool. They’ve been well-discussed here and other places, but briefly they’re not cost-effective, don’t deal with the problem where it starts, are using convection to solve a radiant heat problem, and can even be dangerous by backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. combustion appliances and putting carbon monoxide in the home.

Reflective roofs are the winner, first because they deal with the problem where it begins. If we’re talking about existing homes, they beat radiant barriers because it’s not something extra. You gotta have a roof. Changing out a roof before it’s due to be changed would rarely be cost-effective, but if you need a new roof, going with something reflective would be the way to go. Still, don’t forget this doesn’t absolve you from fully air-sealing and insulating the ceiling.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.


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Image Credits:

  1. Cameron Taylor
  2. FSEC

1.
Aug 26, 2015 2:36 PM ET

A problem many have with light colored roofs...
by Dana Dorsett

... is that they look pretty grungy/dirty from algae & dirt after only a few years of use. The accumulated crud cuts into the long term solar reflectance index (SRI) a bit too, but that's the least of the objections.

To get around that problem there are moderate to high SRI roofing that is not so highly reflective in the visible spectrum, but highly reflective in the non-visible portion of the solar spectrum. In cooling dominated climates it's worth researching the CRRC product listings for a not-so-light but still decent SRI roofing products.

http://coolroofs.org/

http://coolroofs.org/products

In climate zones 3-8 lower peak & average roof deck temps will also lead to higher average moisture content in the roof decking, sometimes to the level of rot potential. That problem that doesn't occur with radiant barriers stapled to the underside of the rafters, while achieving comparable energy use goals. With moderate and high pitched roofs in a heating dominated climate zones 3+, a cool-roofing products can also lead to modestly higher heating energy use as well (though that increase is pretty small relative to the total heating season's use, a slow single-digit percentage, or even less.)


2.
Aug 26, 2015 6:51 PM ET

How to make an asphalt shingle roof last 50-100 years
by Kevin Dickson, MSME

Denver had another multi-billion dollar hailstorm on June 4th. There are literally thousands of crews tearing off and replacing roofs all over the city.

Since I have several rental houses, I'm embroiled in several spats with several different insurance companies.

What if: Instead of replacing all those roofs, you could prevent further deterioration of an asphalt shingle roof by applying a coating? The material cost is about one tenth that of shingle replacement.

When it starts looking grungy after several years, another coat can be applied. What is the failure mode of such a roof, and why couldn't you keep it going indefinitely?

It's difficult to find a coating that is guaranteed in this application, so the market is ripe for disruptive innovation. The shingle manufacturing industry association has always said, "shingles are not designed to accept or require field-applied surfacing,” No surprise there.

It's a win-win: you get a reflective renewed roof for cheap. I found this quote from a roof coatings guy: " A quality acrylic elastomer, properly applied over a suitable asphalt shingle substrate (which would be one that is sound and fully adhered) is most definitely effective in extending the life of the shingles and providing additional reflectivity values in lighter colors. B. Mann, United Coatings"


3.
Aug 26, 2015 8:18 PM ET

Coatings
by Dan Kolbert

Great idea, Kevin - I've been tempted to try Gaco on shingle roofs but have always been scared off by their tech guys. One day I'll work up the courage to do it.


4.
Aug 27, 2015 8:43 AM ET

Water between rigid foam and zip panel
by George Zarifian

During installation of 2 inch xps rigid foam over zip panel should I be worried of rain water getting in between? The foam will be so snug against the sheathing I wonder if it will dry. On the gable end of the house where the foam won't be installed to the roof line do you tape the top edge of the foam to the sheathing to prevent any water from getting in between the sheathing and the foam? I've seen your backyard tape test and I'm thinking I should use the 3m tape for the seams of the GreenGuard xps foam instead of the $12 GreenGuard tape. What would you do? Also, at the bottom of the furring strips which will be installed over the foam should I use metal bug screen? I've seen drawing details where metal flashing is used to protect the foam from rodents and other details where a fabric bug screen is used in the space between the foam and the furring strips, why not attach a galvanized bug sceen to the sheathing and wrap it under the bottom edge of the foam around to the face of the furring strips? Sorry to bombard you with all of these questions. I've been reading alot about this and I am uncertain about these things and I'm coming close to putting the foam on my house. Thank you, George [CT]


5.
Aug 27, 2015 10:05 AM ET

Edited Aug 27, 2015 10:15 AM ET.

Response to George Zarifian
by Martin Holladay

George,
Q. "During installation of 2 inch XPS rigid foam over Zip panels, should I be worried of rain water getting in between?"

A. Yes. Keeping things dry is always better than letting things get wet. If you are worried about damp Zip sheathing, choose vapor-permeable materials on the interior side of the Zip sheathing so that inward drying is possible.

Q. "On the gable end of the house where the foam won't be installed to the roof line, do you tape the top edge of the foam to the sheathing to prevent any water from getting in between the sheathing and the foam?"

A. It can't hurt. But the real answer to your question depends on what you are using for a water-resistive barrier (WRB). The upper part of your gable wall needs a WRB, and so does the lower portion with the rigid foam. This WRB must be properly lapped and without any interruptions.

Q. "I've seen your backyard tape test and I'm thinking I should use the 3m tape for the seams of the GreenGuard XPS foam instead of the $12 GreenGuard tape. What would you do?"

A. I would use Siga Sicrall tape for XPS. I have never tested GreenGuard tape, though -- it may be fine. More info here: Return to the Backyard Tape Test.

Q. "At the bottom of the furring strips which will be installed over the foam should I use metal bug screen?"

A. Yes. Here are links to two articles to help you with these details:

How to Install Rigid Foam Sheathing

All About Rainscreens

Q. "Why not attach a galvanized bug screen to the sheathing and wrap it under the bottom edge of the foam around to the face of the furring strips?"

A. You could do that if you want. Most people bend some metal flashing on a brake to protect the bottom of the foam, and use the Cor-A-Vent SV-3 Siding Vent as screening between the furring strips.


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