This pamphlet provides guidance on protecting moisture sensitive materials such as wood exterior sheathing and framing against decay caused by exposure to moisture. This checklist and guide is not intended to be exhaustive and all-inclusive, but merely a guide to installation techniques and practices builders can use to create a water-resistant barrier around the structural system. As such a guide, this information is not intended to be a standard or minimum requirement, and the techniques expressed in this pamphlet are not to be considered the only way to prevent moisture from penetrating past the exterior envelope. As materials and conditions vary from project to project, the builder needs to examine all areas of the home's exterior and take positive actions to eliminate moisture from entering the structure, whether shown in this pamphlet or not. This pamphlet concerns primarily siding applications with wood sheathing. Exterior systems, such as brick veneer or EIFS, should utilize a drainage path between the exterior system and the sheathing. Although there are similarities in details, brick veneer and EIFS have special details to lead the water out of the bottom of the cavity.
Wood exterior wall sheathing, whether plywood or Oriented Strand Board (OSB), is an integral part of the wood-frame structural system. The sheathing provides lateral strength against wind and other forces and a surface supporting the exterior siding. Moisture trapped between the exterior envelope and the sheathing will result in deterioration of the sheathing, resulting in loss of the structural attributes of the sheathing. Moisture in wood sheathing is normal, and it varies with season and climate. Normal moisture variation can be as much as 5 to 20%. Temperature and humidity causes water vapor movement through the walls' materials causing absorption and release of moisture. There is no concern of decay with moisture levels in this range. However, prolonged periods of elevated moisture beyond the fiber saturation level (approximately 25% to 30%) will promote fungal growth and structural decay. Decay is limted to the "wet" area.
The solution to this problem is twofold: 1. prevention of moisture penetrating past the exterior envelope and 2. removal of whatever moisture that could come in contact with the wood sheathing. The exterior envelope, a combination of roofing, siding, flashing, windows, and doors, is intended to totally shield the structure and building interior from the intrusion of moisture. Although this doesn't always happen, the envelope needs to be constructed with full intent to be weather resistant. Moisture, in whatever form, is to be repelled by the envelope and led away from the building. A second line of defense, the weather barrier, however, needs to be provided to lead whatever moisture enters through the envelope away from the sheathing and the structure.
NAHB Research Center
National Association of Home Builders