Making Your House Watertight
Here are all of the basics on commonly used drainage plane materials and their properties.
Despite a homebuilder's best efforts, some water will penetrate the first line of defense, the exterior finish (siding, brick, stone, or stucco), and may also enter through gaps around windows and doors. When wood framing gets wet and is unable to dry, over time, it can rot and cause structural damage, possibly causing the home to settle or shift. In addition, when wood framing is wet, mold can grow on it. What's more, visible damage from moisture (like peeling or bubbling paint) isn't pretty, and reduces the value of a home.
It's important in the framing stage to implement a drainage strategy to protect the vulnerable wood shell of the home by providing a barrier to water and draining the water away from the exterior of the home. The first part of this strategy includes a continuous roof and wall drainage plane. Drainage plane materials fall into four general categories: felt building paper, house wrap, thin structural sheathing, and rigid foam sheathing.
No matter which material a builder uses, the drainage plane must be continuous from the roof to the ground, which means that joints and corners should be covered, and special materials called flashing used around window and door openings.
The following are the commonly used types of drainage plane material and their basic properties.
- Building Paper -- Also know as felt paper, tarpaper, roofing paper or roofing underlayment. Building paper is an asphalt-impregnated paper that comes in different weights. For example, 15-lb. paper is used for most roofing and wall applications. For most builders, felt paper is the drainage plane of choice for roofing, and many builders use it to provide a drainage plane for the walls as well. Building paper resists air and water getting into the home, but allows moisture to diffuse through it. Microscopic pores in the paper allow moisture through but are so small that bulk water can't penetrate its surface.
- House Wrap -- House wrap is a thin plastic that's literally wrapped around a home over the wood or foam sheathing, cut out around windows and doors and taped at the seams.
Like building paper, house wrap resists air and water getting into the home, but allows moisture to diffuse through it.
- Thin Structural Sheathing -- Thin structural sheathing can provide both a sheathing and a drainage plane for the walls. It's approximately 1/8-inch thick, is constructed from recycled wood fibers and has a water-resistant surface that provides the drainage plane.
- Rigid Foam Sheathing -- Rigid foam sheathing provides insulation as well as a sheathing and a drainage plane for both framed walls above grade and foundation walls below grade. The R-value of rigid foam sheathing ranges from R-3 to R-5.