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
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...
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