OSB Frequently Asked Questions

How vapor permeable is OSB? Should it be used with a special vapor barrier?

The permeability of a wood panel is the rate that moisture passes through the panel under stated conditions of moisture vapor pressure. It is influenced by the density, degree of orientation, and thickness of the panel.

OSB compares favorably with other structural panels. Panels with a permeability of 1.0 perm (60 ng/Pa.s.m2) or less are considered to act as vapor barriers and panels with a permeability of 2.0 perms (120 ng/Pa.s.m2) or more are considered to pass sufficient water vapor that a wall cavity will dry out when constructed with green lumber.

For example, 5/8" (15.5mm) panels can be installed as a floor over unheated well ventilated spaces without the need of a vapor barrier, while 7/16" (11 mm), when installed as wall sheathing, will allow a wall cavity containing saturated stud lumber and glass fiber insulation to reach an equilibrium moisture content below 19 percent in approximately 60 days.

Sheathing membranes, although not always specifically mandated in some building codes under some types of claddings are recommended over OSB wall sheathing.

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