Download Paint and Gypsum Board

Number 2 10/03
Paint and Gypsum Board
Q. How do typical latex paints, with permeance in the range of 5 to 9 perms, impact
MemBrain’s ability to allow systems to dry toward the interior?
A. Latex-painted gypsum board, combined with MemBrain™ as an interior vapor retarder, will
not impact the ability of the wall system to dry towards the interior during periods of high
relative humidity in the insulated cavity. The amount of moisture allowed to diffuse through
MemBrain under high relative humidity conditions is relatively small and manageable by the
gypsum board. This buffering action, along with the permeance of paint, allows for system
drying toward the interior.
Vapor retarders are defined as having a water vapor permeance of 1 perm or less, when tested in
accordance with the ASTM E 96 standard, dry cup method. Historical data from ASHRAE on
water vapor permeance of typically applied latex paint is between 5 and 9 perms under dry
conditions.
Semi-permeable latex paints, as described above, are recommended without vapor retarders in
warm-humid and mixed-humid cooling dominated climates to allow insulated wall cavity
systems to dry toward the interior. The application is described in the ASHRAE Handbook of
Fundamentals and the Energy & Environmental Building Association’s Builder’s Guide.
Q. What is the permeance of gypsum board?
A. The water vapor permeance of unpainted ½ inch gypsum board (drywall) varies between 20
perms, under very dry conditions, and 90 perms, under very wet conditions. Results will be
similar for 5/8-inch thick drywall. The data is published in the 2001 ASTM Technical
Publication MNL40, Moisture Analysis and Condensation Control in Building Envelopes. The
permeance of 3/8 inch unpainted drywall, published in the ASHRAE Handbook of Fundamentals,
is 50 perms.