MIL-HDBK-1001/5A
of insulation and/or incorporating specific methods for controlling vapor flow. Refer to ASHRAE 1993 Handbook - Fundamentals, Chapter 21.
4.1.3 Influence on Roof. Moisture condensation in the roof can result in reductions in insulation thermal resistance and dripping of condensed water from the underside of the roof. The dew point analysis should consider all potential condensation possibilities in order to provide proper control measures to preclude condensation. Generally, these control measures include insulation, ventilation, and vapor retarders. The analysis
should consider not only design conditions of temperature and vapor pressure, but also daily and seasonal variations.
4.2 Vapor Control Methods. A variety of methods exist for controlling vapor in buildings and are generally set forth in ASHRAE 1993 Handbook - Fundamentals, Chapter 20.
4.2.1 Vapor Resistance. All building materials have some degree of resistance to vapor flow. A table of values for most building materials is included in ASHRAE 1993 Handbook - Fundamentals, Chapter 21, along with calculation methods to determine the magnitude of vapor flow and location of condensation planes.
4.2.1.1 Materials. Any material to be used as a vapor retarder should have a maximum rating of 0.5 perm (28.75 perm). Selection of the perm rating required for any given conditions should be based on the analyses described above. Water vapor retarders may be classified as rigid, flexible, or coating types of materials. Materials chosen should provide long term performance and be compatible with the other roofing materials. Frequently used vapor retarders are the flexible types, including plastic films and coated krafts and felts. These materials are available in a wide variety of perm ratings.
4.2.1.2 Calculations. Calculations for vapor flow should be made in accordance with ASHRAE 1993 Handbook - Fundamentals, Chapter 21.
4.2.1.3 Location. Where vapor flow is out of the building, locate vapor retarder as close as possible to the inside surface of the building envelope, usually on top of the roof deck. Where vapor flow is into the building, most often, the roofing membrane provides adequate vapor resistance to the vapor flow. Where
vapor flow is both into and out of the building, the general
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