The fundamental purpose of a roof assembly is to keep water from entering a building through the roof. Low-slope roof assemblies, when properly designed and constructed, perform this function well. However, moist air within a building can enter a roof assembly and condense into water. The climate in which a building is located significantly will affect the type, direction of flow and degree of moisture migration and vapor drive that will occur into and out of a building.
Vapor drive from a building's interior to exterior is likely to be strongest when the exterior temperature and relative humidity are low and the interior temperature and relative humidity are high. These conditions will occur most often during winter months in cold climate regions.
Warmer interior air exerts a higher vapor pressure than cooler outside air. Roof assemblies create a barrier between these areas of differing vapor pressures. If a roof assembly is not sufficiently insulated, warm, moist air will rise into the roof assembly and may cool to its dew-point temperature, causing condensation to occur.
There are three primary methods used to prevent moisture from accumulating in low-slope roof assemblies: use of a vapor retarder, ventilation of interior space and self-drying roof assembly design. This article will focus specifically on vapor retarders.