Note, however, that even some of the dark-colored materials
are considered cool despite their lower SRI and reflectance ratings. Steep slope projects may also benefit from architectural
shingles, with one sample product boasting an SRI of 29, reflectance of 0.27 and emittance of 0.92. Shingles aren’t necessarily as
cool as other materials, but SRI and reflectance values like these
represent a significant improvement over traditional shingles that
aren’t cool roof rated.
The best resource to compare materials that I’ve found is the
Cool Roof Rating Council product directory at coolroofs.org – it
includes all products that have received the organization’s cool
roof designation and provides extra detail in addition to ratings.
Where Cool Roofs Don’t Work
Despite the benefits, cool roofs don’t make sense for every
facility. Building owners can find themselves thinking that they
need a cool roof on every property, but it’s not always appropriate to do that. Some roof assemblies need extra heat to dissipate
accumulated moisture, and a cool roof that maintains a lower
roof temperature won’t help with that.
In one project I worked on, a wooden building with a white
membrane had moisture draining from the fire alarms and light
fixtures. We reviewed the assembly – which included a vapor
retarder, rafters with fiberglass insulation and roof sheathing –
and were stumped as to why this was happening until the owner
mentioned that other buildings in the area had the exact same
construction but with black membranes on top. We performed
a WUFI hygrothermal analysis on both membrane colors and it
became clear that the black membrane generated enough heat
to dissipate that moisture but the white did not.
In some areas of the country, you may be confronted with a
heating season penalty where you spend more heating a “cool”
building in the winter than its light-colored roof saves you on
air conditioning in the summer. Most buildings’ weather-related
energy costs are spent on air conditioning, however, so unless
you already know heating is the bigger problem for you, this
probably doesn’t apply.
Also note that even if your climate is perfect for cool roofs,
your specific location might not be. A pet peeve of mine is owners who are adamant about specifying a cool roof but know it
won’t remain clean and have no interest in keeping it white. If
it won’t have a chance to remain reflective or cool, why bother?
Understand that scrubbing the roof will lower the life expectancy, so if you know your building will attract a lot of grime
and you let it build up instead of just addressing minor dirt
deposits during regular maintenance, a cool roof may not be for
you. If you do decide to clean the roof, take into consideration
what the cleaning process will do to the membrane and make
sure you’re using safety precautions for people who are cleaning
a slippery roof.
Finally, it’s crucial to take the buildings around you into
consideration when you’re weighing cool roof implementation.
Reflective roofs can cause serious glare for adjacent buildings,
but you may be able to mitigate the issues by opting for a lower
reflectance rating or using a gray or tan product instead of a
white one. B
Thomas M. Gernetzke is a Project Manager for Facility Engineering,
Inc., in Madison, WI. He specializes in roofing and waterproofing
systems. Gernetzke is a Past President and Fellow of RCI.
METAL is a popular cool roof material for steep slope roofs.
The metal is coated with reflective finishes or light colors.
Current metal cool roof products vary widely in reflectance and
SRI values, with light-colored ones achieving higher scores.
BALLAST AND AGGREGATE SURFACING typically has a
reflectance value of about 0.30. However, pay close attention
to emittance ratings – the layers of surfacing may take longer
to release accumulated heat.
LOW SLOPE ROOFS have a wealth of options. If you’re not
planning to re-roof or re-cover your roof in the near future, a
fluid-applied or retrofit coating can help you reap the benefits
of cool roofing and prolong the life of the roof underneath.
WHEN YOU SPECIFY COOL ROOFING, make sure your
application and location won’t cause maintenance issues.
This warehouse likely doesn’t emit much in the way of grime
or pollutants, so the membrane will stay clean.