Composite Products and Moisture Management.

Composite wood products posses different characteristics than lumber or timbers. Whereas lumber and timbers are solid wood, engineered composites use adhesives to bond various layers, wood chips, or particles together. In terms of moisture and durability, composite products deserve special consideration.

According to Build Green: Wood Can Last for Centuries, composite products are an increasingly common building material used in building construction. Products such as oriented strand-board, commonly called OSB, are composed of large wood chips compressed and held together with adhesive. Composites are an efficient way to use smaller pieces of wood that might otherwise go to waste. Products such as I-joists and rim boards make up a growing proportion of materials used in home construction.


I-joists are set above structural timbers to provide support for floors and roofing.

Knowing about the special needs of composite products can save homeowners from costly repairs. It is very important to minimize wetting and to maximize drying when composites are exposed to water.

Composite products for exterior applications, such as marine-grade plywood, composite siding, or exterior grade OSB, may be treated with preservative chemicals. Wax added to the adhesive provides additional protection from water. Composite products, such as OSB or plywood sheathing and subflooring, can absorb water more readily than solid wood, particularly from cut edges that are wetted repeatedly. Absorbing water causes these products to swell (or in the case of plywood, to delaminate); once these products have absorbed enough water to swell, they dry very slowly and are vulnerable to decay. When they eventually dry, they do not return to their original dimensions and consequently lose strength.


Close-up end view of OSB (left) and wetted OSB (right) that has absorbed water and swelled. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)