VAPOR BARRIER GUIDELINES

The decision on whether to use a separate vapor barrier must be made by the designer, based on the type of building, its end use, and its geographic location. A separate vapor barrier is sometimes omitted on a sandwich-type roof deck when the weather-shedding skin is not a membrane-type impervious to the transmission of water vapor. Although some times of rigid insulation have the properties of a vapor barrier, a layer of roofing felt is often placed between the deck and the insulation as an air check. Many specifiers still prefer to use a separate vapor barrier because it prevents vapor from condensing in the insulation, which reduces the overall efficiency.

Where a vapor barrier is used, care must be taken to ensure that the dew point is well to the outside in order to prevent condensation on the deck. Ideally, the vapor barrier should be as close as possible to the warm side of the roof, and the thickness of the insulation should be increased as the deck thickness increases to maintain the correct location of dew point. In unevenly heated buildings such as churches and halls, or buildings such as swimming pools where an unusually high level of moisture is generated, the excess humidity may have to be removed by mechanical means to prevent condensation on the deck. In air-conditioned buildings, use of the cold weather roof system allows a constant flow of air between the insulation and the roofing, helping to reduce the energy required for cooling. Full details on the cold weather roof system are given on page 20.

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Figure 17

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Figure 19

Figure 20

Please See More Details on Page 17