What Qualifies as a Cool Roof

Typical minimum cool roof requirements are shown in Table 1, and this is what we mean by “cool roof” throughout this document. A roof can qualify as cool in one of two ways. The first way is by meeting or exceeding both the minimum solar reflectance and thermal emittance values. The alternative way is to meet or exceed the minimum SRI requirement. This allows some roofs that have a low thermal emittance and a high solar reflectance (or vice versa) to still qualify as a cool roof.

Cool roof requirements depend on the roof’s slope. Low sloped roofs have a pitch of 9.5° or less (2:12 rise over run), while steep sloped roofs have a pitch greater than this. Requirements are usually less stringent for steep sloped roofs. Some heavier roofs – such as those with concrete pavers, ballast, or vegetation – also have less stringent cool roof standards. The weight of these roofs causes them to heat up more slowly, and during the night some of that stored heat is returned to the outdoor environment.

Others use different cool roof definitions. For example, the US Green Building Council’s (USGBC) Leadership in Energy and Environmental Design (LEED) program currently uses minimum SRI values of 78 and 29 for low and steep sloped cool roofs, respectivelyiii. The U.S. Department of Energy (DOE) has decided to implement cool roofs on all its buildings whenever practicable3. The DOE uses the low sloped cool roof definition from Table 1 and defines steep sloped cool roofs as those with a 3-year aged SRI of 29 or greater. The ENERGY STAR® program specifies minimum solar reflectance (low slope: 0.65 initial, 0.50 aged; steep slope:

0.25 initial, 0.15 aged) and does not consider thermal emittance. To satisfy local building codes or to meet rebate program requirements, be sure to find and use the appropriate cool roof definition.