Replies

1. GBA Editor
   Martin Holladay | Jan 11, 2018 05:08am | #1

   MZGBS2,
   First of all, can you tell us your name?

   In the old days, the color of any particular type of roofing was a fairly good indicator of whether or not the roofing would be "cool." (A cool roof has high solar reflectance -- that is, the ability to reflect sunlight, measured on a scale of 0 to 1 -- and high thermal emittance -- that is, the ability to emit thermal radiation, also measured on a scale of 0 to 1).

   SRI is defined as a measure of the constructed surface’s ability to reflect solar heat as shown by a small temperature rise. It is defined so that a standard black with reflectance of 0.05 and emittance of 0.90 is zero (0) while the SRI for a standard white (reflectance of 0.80 and emittance 0.90) is 100.

   White roofing is cool; black roofing isn't.

   That was in the old days. In recent decades, coating manufacturers have developed special reflective pigments that reflect sunlight. These newly developed pigments allow even darker colored roofs to reflect solar heat. So the color of the roofing is no longer a good indication of its reflectance or emittance.

   It's certainly true that the gauge of the metal roofing -- for example, whether it is 24 gauge or 26 gauge -- has nothing to do with its SRI.

   If two brands of metal roofing that appear similar have very different SRI ratings, it's because their coatings are different.

   One final point: If your roof assembly has adequate insulation, the SRI value of the roofing doesn't matter very much (as long as you don't have any ductwork in your attic).