Permeability

The permeability of plywood is different from solid wood in several ways. The veneers from which plywood is made generally contain lathe checks from the manufacturing process. These small cracks provide pathways for materials to pass into when entering through the panel edge. When permeability is measured through the panel thickness, a number of variables will affect the actual flow rate. The anatomy of the species, consistency of the glue line, number of void spaces, and growth characteristics all affect permeability. Exterior-type plywood, and especially High Density Overlaid plywood, is a relatively efficient barrier.

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 Water Vapor Permeance table below presents results for various species of 3/8-inch Exterior plywood.

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 Water Vapor Permeance table below presents similar results for OSB panels tested in 1983.