Condensation, Moisture, and Ice

Designing a roof that can withstand and control moisture is essential since uncontrolled moisture could cause damage to the roof or the building. The following considerations illustrate how cool roofs handle moisture differently than dark roofs.

Ponding occurs when water, typically from rain, accumulates in pools on the roof. This happens when a roof has insufficient slope (caused by poor design or damage) or drain blockage. It takes longer for ponded water to evaporate from a cool roof due to its lower temperature. If your cool roof cannot tolerate ponding, it may be necessary to inspect the roof more frequently to prevent damage or leaks.

Moisture from the indoor air can also condense within roof materials. If allowed to accumulate over months or years, moisture could damage those materials. Ordinarily, heat from the sun dries out building materials during the daytime and throughout the summer. In consistently hot climates, like Phoenix, AZ (Climate Zones 1-3 in Figure 8), there is little risk for this kind of moisture build-up.7 In colder climates, like Chicago or Alaska (Climate Zones 5-8 in Figure 8), there is less heat available to dry out the roof and more opportunities for condensation to occur. Without proper design, both dark and cool roofs can accumulate moisture in colder climates. 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. While this issue has been observed in both cool and dark roofs in cold climates,8 the authors are not aware of any data that clearly demonstrate a higher occurrence in cool roofs. The potential for persistent moisture levels to arise in different roof designs and climates is the subject of ongoing research.