Permeable Building Paper and Vapor Barrier

High humidity output from washers, dryers, showers, kitchens and other sources, combined with today's energy-efficient construction, can result in a build up of interior moisture vapor. This vapor migrates through the fabric of a building from the warm interior to the cold exterior, and when it condenses to water can cause structural damage to sidings and finishes.

Vapor barriers installed on the warm side of the wall prevent most of the migration of moisture vapor but they have disadvantages. Residual vapor must be allowed to escape to the outside. Building paper installed on the outside face of the sheathing helps prevent rain and snow from penetrating the walls and also allows the escape of moisture vapor.

Proper wall construction includes the use of gas permeable paper and vapor barriers (see diagram above). These are extremely effective in helping to prevent moisture problems.

Ground Clearance
Keep the lowest edge of siding at least 6" off the ground and coat end grain at the bottom of vertical siding with water repellant.

Caulking
Thorough caulking of all joints is recommended including butt joints and where siding contacts openings or trim. Use only non-hardening caulks such as polyurethane, polysulfide or latex-silicone. Pure silicon caulks do not work well on cedar and are not recommended.

Do not reduce the siding overlap recommendations given in the installation sections of this publication. To do so could result in damage from wind-driven water.