Especially in the wet Pacific Northwest climate, understanding how moisture moves through building materials will help trade allies determine the best work scope for a home. Using correct materials mitigates moisture-related problems like mold and rot.

Moisture transmission through building materials, also known as permeability, is rated in perms. A perm describes the ability of a material to diffuse vapor in a similar way that a U-Value describes the ability of a material to transfer heat. Vapor pressure results where humidity is present on a surface. Moisture moves from areas of higher vapor pressure to lower vapor pressure, and this movement is called vapor diffusion. A high perm rating of a building material indicates it is very permeable and allows more vapor diffusion. Building materials are separated into four general classes based on perm ratings:

• Vapor impermeable vapor barriers have a perm rating less than 0.1. They also serve as effective air barriers. Common impermeable materials include foil-faced insulating and non-insulating sheathings, polyethylene films, rubber membranes and sheet metal.
• Vapor semi-impermeable vapor barriers have a perm rating of 0.1 to 1.0. Common semi-impermeable materials include unfaced extruded polystyrene, XPS, at least one inch thick, vinyl wall coverings and oil-based paints.
• Vapor semi-permeable vapor retarders have a perm rating of 1.1 to 10. Common semi-permeable materials include plywood; oriented strand board, OSB; un-faced expanded polystyrene, EPS; XPS less than one inch thick; latex-based paints and heavy asphalt building papers.
• Vapor permeable vapor retarders have a perm rating greater than 10. Common permeable materials include unpainted drywall and gypsum board, Tyvek™, Typar™ and other house wrap materials.

The vapor permeability requirements of an assembly, and the location of materials with varying permeability within that assembly, depend largely on the local climate. The mixed climate conditions found throughout Oregon and southwest Washington can make it difficult to pick the right combination of materials. Smart vapor retarder materials have been developed to address these climatic demands. Responsive to changes in humidity, smart vapor retarders become more permeable when relative humidity is high and less permeable when humidity is low.

Accurately rated perm materials to control moisture movement and awareness of the local climate will improve the quality of your work and decrease long-term moisture-related problems for your customers.