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More About Permeance

Water-Vapor Permeance

Water vapor permeance of structural panels refers to the rate of moisture transmission through the panel as a function of the water vapor pressure gradient that can exist between the two faces. Water vapor transmission is measured using ASTM Method E96. This uses a controlled environment in combination with either a desiccant (dry-cup) or water (wet-cup) to create a vapor pressure gradient. In either method, the weight change over a specific time is used to calculate the permeance. Values are reported in perms (grains per ft²-hr-in. HG vapor pressure). A grain is 1/7000 lb. (0.065 g).

Research at the National Institute of Science and Technology has shown that the water vapor permeance is very sensitive to the relative humidity gradients. For example, at 50% humidity the water vapor permeance of plywood is approximately 1 perm but the water vapor permeance may be increased by a factor of 10 when the humidity is increased to 90%. Similar results are reported for an OSB siding product which had been coated with a latex paint.

Water vapor permeance of structural panels has been evaluated by APA using the dry cup method. Plywood species selected to be representative of the industry were evaluated in the 1970's. The table below presents results for various species of 3/8-inch Exterior plywood.

Water Vapor Permeance
	Perms	(g/hr/m2/mmHg)
3/8" Plywood Doug-fir, coast Doug-fir, north interior   Southern pine Western larch Western hemlock True fir, western Western white pine	0.78 0.53 1.43 0.63 0.89 0.88 0.45	.021 .015 .039 .017 .024 .024 .012
3/8" MDO plywood One side MDO Two side MDO	0.3 0.2	.008 .006

When adjusted for the relative volume use of the various species, a water vapor permeance value of 0.8 perm is appropriate for 3/8-inch Exterior type plywood or plywood with exterior glue (Exposure 1). Use of overlays significantly affects the water vapor permeance.

The following table presents similar results for OSB panels tested in 1983.

Water Vapor Permeance
	Perms	(g/hr/m2/mmHg)
7/16" OSB 15/32", 1/2" OSB    19/32", 5/8" OSB 23/32", 3/4" OSB	0.91 0.70 0.72 0.49	.025 .019 .020 .013

Related Publications

The following publications from APA - The Engineered Wood Association may provide more information:

• HDO/MDO Plywood, Form B360
• Technical Note: Water Vapor Permeance of Wood Structural Panels and Wood Wall Construction, Form J450