In a report, Guidelines for Selecting Cool Roofs, July 2010, the U.S. Department of Energy makes the following observations:
• Climate has the biggest impact on energy savings.
Cool roofs achieve the greatest cooling savings in
hot climates (Climate Zones 1-3) but can increase
energy costs in colder climates due to reduced
beneficial winter time heat gains.
• When cool roofs are optional yet encouraged, the
decision should be considered carefully as cool
roofs do not perform equally well everywhere.
• The energy cost savings you can realize from a
“cool”(white) depends on many factors, including
where you live.
• Although annual cleaning can restore up to 90% of
initial reflectance, the energy cost savings alone
does not warrant the cost.
• As a cool roof becomes dirty from pollution, foot
traffic, wind-deposited debris, ponded water, and
mold or algae growth, its reflectance will
decrease, leading to higher temperatures.
• There are some important questions about the
durability of cool roof systems in certain applications.
• In colder climates, like Chicago or Alaska (Climate
Zones 5-8), there is less heat available
to dry out the roof and more opportunities for
condensation to occur.