Designing for Building Maintainability - Effective Building Maintenance: Protection of Capital Assets - page 146

c. A vapor retarder is a disadvantage in summer, when vapor mi-
gration is generally downward through the roof. Hot, humid
air can infiltrate the roofing sandwich through the vents, or by
diffusion through the membrane. It may also condense on the
vapor retarder itself.
d. A vapor retarder may be the weakest horizontal shear plane in
the roofing sandwich. Failure at the vapor retarder-insulation
interface can split the membrane. The vapor retarder intro-
duces an additional component where shear resistance may be
critical to the membrane integrity.
When should a vapor retarder be used? The old school of
thought was: “If in doubt, include a vapor retarder.” However,
modern policy is: “If in doubt, omit the vapor retarder.” ASTM
Manual 18, MoistureControlinBuildings , recommends, based on
research by the Army's Civil Engineering Research Laboratories,
consideration of a vapor retarder on any roof where the indoor
winter relative humidity at 68°F exceeds approximately 60% RH
(ranging from 55% RH in the western counties to 70% RH in the
extreme southeastern counties). At 72°F indoor temperature, a
more typical winter condition, this criteria is equivalent to a 70%
RH. Facilities with such high indoor winter humidity levels may
include some research facilities, laundries, cafeteria/kitchens, na-
tatoriums, athletic facilities, “process” plants, dorm shower areas,
etc.
Whenavaporretarderisnotneeded,itshouldnotbeused since it is
expensive and, more importantly, will allow “cancers” of wet insu-
lation to grow within a compact roof system having membrane or
flashing flaws. Flawed roofs without vapor retarders tend to leak
water into the building sooner, which often reduces the lateral ex-
tent of wet insulation. And, typically, little guidance is provided to
designers as to how to seal vapor retarders at flashing and penetra-
tions. If a vapor retarder is required, the designer must research
and carefully detail these conditions.
Research shows that roofing insulation, once wet, takes a year
or more to dry if there is no vapor retarder. This insulation never
dries if there is a vapor retarder to trap the moisture.
Other vapor retarder design considerations should be incorpo-
rated: