Controlling Moisture: Vapor Bariers and Building Paper
Vapor Bariers and Building Paper 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 and damage to sidings and finishes. Vapor barriers installed on the warm side of the wall
largely prevent the migration of moisture vapor but they are not perfect. Residual vapor must be allowed
to escape to the outside. Housewrap installed on the outside face of the sheathing helps prevent rain and
snow from penetrating the walls but allows the escape of moisture vapor. The WRCLA recommends the use
of rainscreen wall construction (see diagram and related section later in this publication) especially when installing
over rigid foam.
Proper wall construction includes the use of housewrap (such as DuPont Tyvek’s Drainwrap) or gas permeable
building paper and interior vapor barriers. These are extremely effective in helping to prevent moisture problems.