Designing for Building Maintainability - Effective Building Maintenance: Protection of Capital Assets - page 145

migration of water vapor molecules from areas of high pressure to
areas of low pressure. In a cold climate, this vapor migration is in
an upward direction. In a hot, humid climate, the vapor migration
is reversed and is in a downward direction, i.e., into the building.
For a material to qualify as a roof vapor retarder, its vapor per-
meance rating should not exceed 0.1 perms. Materials qualify as
vapor retarders include polyethylene film, PVC (polyvinyl chlo-
ride) sheets, aluminum foil, coated Kraft paper or laminated Kraft
paper with bitumen filler, saturated felts (2-plies) with two or three
asphalt moppings, and coated base sheet with one or two asphalt
moppings.
There are advantages to having a vapor retarder:
a. A vapor retarder can ensure the continual thermal resistance
of insulation sandwiched between the vapor barrier and built-
up membrane.
b. A vapor retarder is a good safeguard against vapor migration
if a building's use changes from “dry” to a “wet” use. (Typi-
cally “dry” is defined as an interior winter relative humidity
under 20 percent, which almost never happens in hot, humid
climates, while “wet” is defined as an interior winter relative
humidity over 45 percent, which is common in hot, humid cli-
mates.)
c. A vapor retarder is advisable over wet decks (poured gypsum
and lightweight insulating concrete) to prevent vapor flow up-
ward into the insulation. For this purpose, the vapor retarder
should be a venting base sheet, with mineral granule under-
side surfacing, mechanically fastened to the decks.
And, there are disadvantages:
a. The vapor retarder, together with a roofing membrane, in-
evitably seals trapped moisture within the roof “sandwich,”
which can eventually destroy the insulation, split or wrinkle
the built-up membrane, or in gaseous form, blister it.
b. In the event of a roof leak through the membrane, the vapor
retarder traps the water below the insulation and releases it
through punctures that may be some lateral distance from the
leak, making the leak difficult to locate. A large area of insula-
tion may be saturated before the punctured membrane can be
repaired.