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Building Science

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

A dark roof gets hot, and that heat conducts downward into the attic. The best way to keep that heat out of the attic is to prevent it from being absorbed by the roof.
Image Credit: Cameron Taylor

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 barrier 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…

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5 Comments

  1. User avater
    Dana Dorsett | | #1

    A problem many have with light colored roofs...
    ... 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. Kevin Dickson, MSME | | #2

    How to make an asphalt shingle roof last 50-100 years
    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. Dan Kolbert | | #3

    Coatings
    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. George Zarifian | | #4

    Water between rigid foam and zip panel
    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. User avater GBA Editor
    Martin Holladay | | #5

    Response to George Zarifian
    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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