A recent article on a Huffington Post Green blog, "White Roofs, Green Myth" (preceded by a more detailed paper by the same author at the 2009 Symposium on Building Envelope Technology) grabbed my attention for its brazen denouncements of cool roofing. Cool roofing reflects the heat from the sun, and has repeatedly been shown to reduce cooling loads. The energy savings are greatest in warm climates, and on buildings with less-than-optimal levels of roof insulation (which describes most existing buildings in most U.S. cities). However, the "Green Myth" author, Samir Ibrahim, feels cool roof energy savings are overstated, and this may often be the case: In heat-dominated climates, the energy savings add up to pennies per square foot. Yet, rarely is a cool roof a detriment to energy savings, as Mr. Ibrahim implies, unless there is little to no roof insulation. (It is surprising that he argues code-level roof insulation reduces the savings on cooling, but he ignores this when discussing the loss of solar heat gains in hot climates). Also, the heating penalty of cool roofs in cold climates isn’t nearly as severe as it could be, argues Greg Zimmerman in a blog for facilities managers, owing to lower angle of the sun during winter. Indeed, the biggest risk of a cool roof in a cold climate is with condensation, due to reduced surface temperatures during the day, which can later cause the temperature to drop beneath the dew point during the night.
But this problem only occurs in very cold climates; Anchorage, Alaska is given as the North American example in this explanation presented at the 2008 International Conference on Durability of Building Materials and Components. For more detailed discussion of selecting a cool roof, and determining if one is appropriate for a given project, see the U.S. Dept. of Energy's Guidelines for Selecting Cool Roofs.
