There are more benefits than just using less energy, however. UV
and heat exposure together are the primary cause of breakdown
and deterioration of roofing material, so in many cases keeping the
roof cooler allows the material to last longer. And the same reflectivity that keeps buildings from overheating and roof membranes
from prematurely aging also battles the urban heat island effect.
But how can you determine which of the hundreds of cool roof
products fits your facility best – or whether you need a cool roof
at all? This guide to cool roof specification will help you decipher
performance data and understand where to use a cool roof.
What Performance Metrics Mean
There are three key metrics that can help you evaluate the
performance of different materials and slopes when it comes to
keeping your roof cool.
Solar Reflectance Index (SRI) rating: A measure of the
roof’s ability to reject solar heat. Ratings are assigned based on
testing by the Cool Roof Rating Council – high-performance
roofs can reach over 100. To put this in context, the Lawrence
Berkeley National Laboratory defines a “perfect” SRI as roughly
122, but that score would require your roof to have very low
emissivity and be a perfect mirror that absorbs zero sunlight.
Some products also provide an aged SRI value to show how the
product will perform after a few years of weathering.
Solar reflectance (albedo): The fraction of the incident solar
energy reflected by the surface in question. A higher number
means more energy is reflected rather than absorbed.
Thermal emittance: The relative ability of a surface to radiate
absorbed heat. The higher the number, the faster the surface
sheds the heat it has absorbed. Thermal emittance and solar
reflectance are both measured in values of 0-1.
Of the three, comparing the SRI value is the easiest way to
see how one roofing material stacks up against another. For
example, white TPO single-ply membranes can have SRI values
as high as 101, while the SRI of granulated SBS or modified bitumen cap sheets that aren’t cool roof rated averages around 26.
One black EPDM roof I found actually has an SRI of -1.
However, SRI may not tell the whole story, especially when
you’re trying to compare like materials. If you need additional
performance information, bring reflectance and emittance ratings into the mix. For example, the white TPO specimen with a
101 SRI carries a reflectance value of 0.77 and an emittance value
of 0.87. Many materials have a similar emittance value, so reflectance is usually a better indicator of which roofs qualify as cool.
The SBS cap sheet in this example has the same emittance
value, but its reflectance is 0.26 – it sheds absorbed heat at the
same rate as the TPO, but doesn’t reflect nearly as much solar
energy. The black EPDM product has a reflectance of 0.06 and
an emittance of 0.88, showing that it barely reflects any of the
solar energy hitting it.
What should you look for when you compare these values?
The roofing industry generally defines cool roofing materials as
having reflectance and emittance ratings that are at least 0.65
to 0.70. Areas where cool roofs are mandatory may set a certain
minimum standard for roof ratings – for example, California’s
Title 24 requires at least 0.70 reflectance and 0.75 emittance.
Chicago takes a slightly different approach and allows low-slope roofs to have either an initial reflectance of 0.72 or a
three-year aged reflectance of 0.5.
HOW TO EVALUATE
BY THOMAS M. GERNETZKE
The commonsense benefits
of cool roofing are numerous.
A roof that reflects extra UV and
heat instead of absorbing it keeps
the building underneath cooler,
leading to a reduction in cooling
costs and the opportunity to
downsize HVAC equipment.
Those of us who have visited
facilities of questionable construc-
tion over the years can vouch for
the comfort difference – in build-
ings where the roof absorbs too
much heat, you can practically
feel the heat radiating down from