Risk of Condensation in Mechanically Attached Roof Systems in Cold U.S. Climate Zones

Abstract

Flat roofs have generally a high potential of nightly overcooling and therefore an increased risk of condensation within the construction, particularly in cold climate zones, depending on their specific assembly. A white exterior surface, so-called “cool roof”, applied to decrease cooling loads and to save energy, increases the condensation risk due to lower energy gains at day time. The nightly overcooling phenomena will be intensified and leads to much lower temperature at the exterior surface compared to a standard roof. Observations (Energy Design Update® 2006) show that the increased condensation risk may lead to moisture damage. There have been questions raised about the sustainability of using cool roof membranes in Northern US climate zones due to the potential of moisture accumulation below the membrane. Transient hygrothermal simulation using real climate data are state of the art today and can help to study different effects. Variations of several input parameter, such as short-wave absorptivity of the solar radiation, ventilation underneath the membrane with interior or exterior air, specific climate data, etc., show their influences on the moisture accumulation underneath the membrane of a typical mechanically attached roof systems for commercial buildings. Consequently, the most significant input parameter can be determined and used as additional criteria for a better design.