Vapor Retarder Precautions
If water penetrates a roof membrane that has a
vapor retarder, it may go undetected for an extended period of time because
the vapor retarder may prevent the water from actually entering the building.
This can result in damage occurring to other components, such as the rigid
Finding the source of water penetration into a roof
assembly that has a vapor retarder can be much more difficult because
water may not enter the building at the same location as it may enter
the BUR membrane. The vapor retarder can prevent the water from leaking
into the interior of the building where it can be noticed until it migrates
to a location where the vapor retarder can be penetrated. This may occur
a great distance from the location where the water penetrated the BUR
membrane and thereby complicate the effort to find the point of water
entry for repair.
Two Criteria for Specifying
a Vapor Retarder
Vapor retarders shall be specified for low-slope roof membranes when
both the following conditions exist:
||The outside average January temperature
is below 40° F.
||The expected interior winter relative
humidity is 45% or greater.
|Chart 1: Vapor drive map showing
the area of the United States with a mean average January temperature
below 40° F as colored gray.
Research conducted by the U.S. Army Corps of engineers
has shown that vapor retarders should be installed according to the Vapor
Drive Map shown as chart 1 and the following chart 2 and graph 1.