The best way to control mold is to control moisture, according to the EPA (Environmental Protection Agency). A study – published in May 2008 – demonstrates how white roofs have moisture issues in areas with very cold ambient temperatures – like Chicago, IL. White roofs accumulate moisture in colder regions, due to the reduced surface temperatures during the day. Furthermore, the long-wave radiation can lead to overcooling of the surface below ambient temperature. Such low temperatures during the night can cause the temperature to drop beneath the dew-point, followed by condensation of moisture below the membrane.
The U.S. Department of Energy precautions: Although cool roofs have been used successfully for many years, their use is growing and cool roofs are now being installed in a wider range of climates. There are some important questions about the durability of cool roof systems in certain applications. Cool roofs maintain lower temperatures than dark roofs, and so they may provide less heat to dry out moisture. Potentially, this could make a cool roof more susceptible to moisture accumulation when used in colder climates.