Replies

1. 
   Jon_Harrod | May 27, 2017 11:31pm | #1

   Oak Ridge National Lab has a "Cool Roofing Calculator" that does exactly this.

   http://web.ornl.gov/sci/buildings/tools/cool-roof/

   The better the roof insulation, of course, the smaller the magnitude of the effect.

   Cool roofs have "whole system" benefits that aren't reflected in these calculations, reducing urban heat island effects and summer demand peaks.

2. GBA Editor
   Martin Holladay | May 28, 2017 05:54am | #2

   David,
   In general, as Jon points out, a cool roof won't affect the energy use of a well-insulated building (with HVAC equipment located within the home's thermal envelope). It may affect the energy use of a poorly insulated building or a building that has HVAC equipment in an unconditioned attic.

   Note also that cool roofing lowers the temperature of the roof sheathing. With some roof assembly types, this can make the sheathing more susceptible to moisture accumulation. For more information, see Night Sky Radiation.

3. 
   Anon3 | May 28, 2017 09:26am | #3

   Yes run a BEopt simulation, it's a free download.

   Run the simulation with light vs dark roof also try out the radiant barrier decking option.

   Generally darker roof increase cooling but decrease heating .

4. 
   greenhouse437 | May 28, 2017 11:18am | #4

   Thanks for the replies. In my case I just installed closed cell foam under my roof decking and on gable walls, so I'm not concerned with my 5 year old light-colored roof raising my heat load. But I was discussing this with a friend who is thinking twice about putting in a cool roof--this is for an area just above NYC, climate 4A. And in my case in particular a light roof is also beneficial for the life of the shingles, as with insulation right up to inside sheathing, there will be some temperature rise from trapped heat. And @Jon, to your point on whole system benefits, I'd gather that the summer demand peaks are much higher than winter's.

5. 
   Anon3 | May 28, 2017 02:42pm | #5

   Light roof stains (natural or biological) more, also lighter roof has smaller heat rise so the smell is more noticeable. There's a reason almost all new constructions are darker roof. Plus they look bad.

6. 
   Jon_Harrod | May 29, 2017 10:09am | #6

   David, right now the summer demand peak in NY is quite a bit higher than in winter, though I could imagine that changing as heat pumps increase in popularity.