The exterior finish of a home, whether it is brick, stucco, vinyl, wood, or another material, is not waterproof. Rainwater, snowmelt, and wind-driven moisture wet the finish, penetrate it, and wet the drainage plane behind the exterior.
Water intrusion can occur through any air leaks in the wall assembly, and, over time, can also shorten the life of the exterior finish. Building rain screens can help extend the life of the sheathing and the entire wall assembly by enabling them to dry more quickly.
A rain screen is simply an intentional gap, typically 1-1 ½” wide, built between a home’s drainage plane and the exterior finish. This gap allows air to circulate freely behind the exterior finish, helping it to dry faster (via evaporation), and thus last longer.
Rain screens are especially beneficial in climates with medium to heavy rainfall amounts and all coastal areas, where the sheathing is wetted frequently. Homes in drier climates also can benefit from rain screens, although the benefit may not outweigh the additional costs in time and materials for building them.
The Best Practice for building rain screens is to make sure that they’re pressure-equalized. This means that air gaps are also left at the top and bottom of the wall to ensure that the space between the sheathing and the drainage plane maintains equalized air pressure (neither positive nor negative). If the air gap was to become pressurized, air and water would be pushed through any air leaks in the wall assembly to the home’s interior, wetting the framing, insulation, and drywall.
To create a pressure-equalized rain screen (PER), follow these 5 steps:
Install the drainage plane.
Flash all doors and windows.
Attach furring strips of pressure-treated lumber over the drainage plane by nailing into exterior wall studs (16” o.c.).
Install the exterior finish. Be sure to stop the material short from the top of the wall.
Install mesh material across the air gaps at the top and bottom of the walls to prevent insects and rodents from entering the space.
Rain screens are recommended for use with brick, stucco, wood siding, and cement board siding. They are not recommended for use with vinyl siding because lengths of vinyl siding are too short to span between exterior wall studs (16” o.c.), where the furring strips are attached.
For more information about rain screens, visit the ToolBase website. Additional Best Practices® for protecting your homes from water damage can be found in BuildIQ University’s Water Management Suite of online training courses.
