February may seem like an odd time to bring up the subject of cool roofs and their AC-savings, but the reality is that in many northern cities (like here in frigid Milwaukee), the freezing winter months are when the argument about the merits of using cool roofs really heats up.
The question is whether the so-called heating penalty – in the winter months, the sun’s rays are reflected from a cool roof, whereas a dark roof would absorb them, help warm the building and reduce heating costs – trumps the savings achieved during the summer cooling months. Most experts say no, for a variety of reasons: The sun’s angle is lower in the winter so the energy hitting the roof is less intense; there are fewer purely sunny days in winter; snow accumulated on a roof reflects the sun’s rays as effectively as a white roof would, so it doesn’t matter whether the roof itself is absorbent or reflective.
Insulation is a huge factor, too. If a facility is already well-insulated, a cool roof may not have as much benefit. Oak Ridge National Laboratory has developed a calculator that can help facility executives determine how much, if any, energy benefit a cool roof might have for their specific location.
Another consideration is that cool roofs help mitigate the urban heat island effect, which is one of the reasons Chicago has used a cool roofing spec for low-slope non-residential buildings in its energy code (even though, until Dec. 31, 2008 when a higher standard kicks in, Chicago’s is much lower than most cool roofing standards: .25 reflectance for Chicago, .65 or .75 for some others – here’s a list.) The idea is that if everyone uses cool roofs, the overall temperature of a city will be reduced, and less air conditioning energy will be required for everyone. For most facility executives, this argument isn’t exactly a good justification for a major capital improvement, but if an organization is concerned about the environment, this may be a more of a factor than it seems at face value.