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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
insulation.
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 RetarderVapor retarders shall be specified for low-slope roof membranes when both the following conditions exist:
| 1. | The outside average January temperature is below 40° F. |
| 2. | The expected interior winter relative humidity is 45% or greater. |
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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.
