Replies

1. GBA Editor
   Martin Holladay | Dec 29, 2013 07:07am | #1

   David,
   In some areas of the country, notably California, regulators are promoting the use of "cool roof" technology. A cool roof should have both high solar reflectance (ability to reflect sunlight, measured on a scale of 0 to 1) and high thermal emittance (ability to emit thermal radiation, also measured on a scale of 0 to 1).

   If you want a cool roof, you can buy new roofing products with these characteristics. It's also possible to buy a coating (paint) that, when dry, has a relatively high solar reflectance and thermal emittance. For more information, see the website of the Cool Roof Rating Council.

   Having said all that, it's worth noting that you don't necessarily need a cool roof. If you have an adequate layer of insulation between your hot roofing and the interior of your house, the solar reflectance and thermal emittance of your roofing are, for all intents and purposes, irrelevant. Moreover, if you live in a cold climate that requires you to heat your house in the winter, cool roofing can be counterproductive, slightly increasing your heating bills.

   The most common reasons that cool roofing is advocated are two design errors: (a) the builder didn't include enough insulation, or (b) the builder put the ductwork in the attic. (Ducts don't belong in attics.)

   If any GBA readers are interested in the article on insulating paint that David is referring to, here is the link: ‘Insulating’ Paint Merchants Dupe Gullible Homeowners.

2. 
   hankroberts | Feb 19, 2014 05:08pm | #2

   California -- maybe a bit over enthusiastic.

   We replaced an ancient tar-and-gravel roof and chose a cool (high emissivity) IBRoof a couple of years ago. I got three bids, asking advice from the Oakland Cool Roof Council and the Lawrence Livermore cool roof group informally about them. I specifically asked about adding insulation on the roof deck before the new roofing. None advised adding insulation under the new membrane because we already had insulated the attic floor (it's a low slope roof) saying it wasn't needed.

   The temperature in the house dropped dramatically on hot days -- 40 F. cooler in the attic, and much more livable in the house on hot days. (Heat the low-slope-roof attic space up all through a very hot day, and the insulation heats up, then the lath and plaster and that heats the living space!)

   So that worked. But the humidity in the attic went up, and up, and up. To where water was dripping off the screws where they penetrated the roof deck planks. Very bad.

   My neighbor, a greengrocer, explained the problem. High emissivity roof radiates heat away day and night. It's always the coolest surface around and condenses water -- both outside and inside. Yep, the downspouts drip water every morning, even in bone dry weather -- any day there's condensation at dawn on parked cars, there's condensation on the outside of the roof. And on the inside, to the extent there's any air circulating at all. And with 1920s California building structure, it's never going to be airtight.

   Oops.

   Big mistake. Called back, and the Cool Roof Council said they've never actually tested this stuff in our area and they don't evaluate actual building results, only the performance of the coating on the exterior side.

   I found belatedly that a Florida solar research group is applying this principle as a way of dehumidifying living space by collecting moisture in an attic under a cool white roof at night then venting it with fans in daytime.

3. GBA Editor
   Martin Holladay | Feb 19, 2014 06:11pm | #3

   Hank,
   Cool roofing caused a cluster of roof assembly failures in Arizona or New Mexico several years ago, due to the phenomenon you are describing. Night sky radiation was overcooling the roofs, and condensation was dripping on the insulation under low-slope roof assemblies. A soggy mess. I wrote about the problem for Energy Design Update.

   I'm not a fan of cool roofing.