of insulation and/or incorporating specific methods for
controlling vapor flow. Refer to ASHRAE 1993 Handbook -
Fundamentals, Chapter 21.
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.
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.
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
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.
Calculations. Calculations for vapor flow should be
made in accordance with ASHRAE 1993 Handbook - Fundamentals,
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