Replies

1. GBA Editor
   Martin Holladay | Apr 13, 2015 07:20am | #1

   Peter,
   A "cool roof" saves energy when installed on a poorly designed home. It won't really save any energy when installed on a well-designed home.

   The classic situation that might call for a "cool roof" is a hot-climate house with ductwork in an unconditioned attic. If you have such a house, you might consider installing a "cool roof."

   If you follow GBA recommendations -- and all of your ductwork is located inside your home's conditioned envelope, and you have ceiling insulation that at least meets minimum code requirements -- you will see hardly any benefit from a cool roof.

2. 
   charlie_sullivan | Apr 13, 2015 08:09am | #2

   As an addendum to Martin's correct answer, a cool roof also make a contribution to lowering the urban heat island effect. If you live in a hot climate urban area, using a cool roof is an important contribution to mitigation of that problem.

3. 
   Dana1 | Apr 13, 2015 08:42am | #3

   A cool roof also has the intended consequence of wintertime lower roof deck temps, which becomes a (very) modest increase in space heating load, but a more significant increase in high wintertime moisture accumulation in the roof decks for unvented insulated roof assemblies.

   With very low pitch roofs there is still a cooling season energy use advantage to having a cool roof even at an IRC code minimums, but from 3:12 or higher that advantage fades very quickly. On large flat roof commercial buildings it's still "worth it", even in cold climates.

4. 
   jackofalltrades777 | Apr 13, 2015 03:27pm | #4

   Would a cool roof with a high SRI help with RADIANT HEAT? For example, say there was a fire nearby and the heat from the flames creates radiant heat. Would a cool metal roof with a high SRI help to mitigate that radiant heat from penetrating into the roof sheathing/building?

5. Expert Member
   Dana Dorsett | Apr 13, 2015 03:44pm | #5

   The cool roof absorption/emissivity profiles are in the deep infra-red- the same spectrum as say, a forest fire. Only about half the solar spectrum energy is in the deep infra-red, the rest is in the visible and UV spectrum. Cool roof's work by re-radiating the deep infra-red back into the cold radiant-temp sky, while reflecting a good chunk of the near-infra-red, UV, & visible spectra.

   So the short answer is, no, a cool roof with an SRI of only 10 won't reflect very much of the heat radiating from a nearby fire. It is not a low-E coating with broad infra-red reflectance- it is designed to be a deep infra-red absorber & emitter. A titanium-white high SRI finish would reflect a good portion of forest or house fire spectrum though.

   But any metal roof has a lot to offer for keeping burning embers falling from that fire from igniting your roof!

   http://www.intechopen.com/source/html/38093/media/image6.jpeg

   http://science.nasa.gov/media/medialibrary/2000/09/05/sunspectrum_resources/spectrumgif.gif