• Housewrap has two functions: (1) prevent airflow through a wall and (2) stop (and drain) liquid water that has penetrated through the exterior finish.
• Housewrap is not a vapor retarder, it is designed to allow water vapor to pass through.
• The choice to use housewrap or building paper depends on the climate and on owner preference. Both materials can provide adequate protection.
• If housewrap is not installed properly it could be more detrimental than beneficial.

(*Proper installation, especially in lapping, is the key to successful housewrap use.)

Purpose of Housewrap
Housewrap serves as a dual-purpose weather barrier. It not only minimizes the flow of air in and out of a home, but also stops liquid water and acts as a drainage plane. The unique characteristic of housewrap is that it allows water vapor to pass through it, while blocking liquid water. This permits moist humid air to escape from the inside of the home, while preventing outside liquid water (rain) from entering the home.

When Should Housewrap Be Used?
Almost all exterior finishes allow at least some water penetration. If this water continually soaks the wall sheathing and framing members, problems such as dry rot and mold growth could occur. Housewrap stops water that passes through the siding and allows it to drain away from the structural members. In humid climates with heavy rainfall, housewrap is recommended to prevent water damage to the framing. Use in dryer climates may not be as critical, since materials are allowed to adequately dry, Housewrap also prevents air movement through the wall cavity, which is beneficial for insulating purposes.

Housewrap or Building Paper?
To answer: this question, it is important to consider what attributes are most important for a particular climate.
Five characteristics associated with secondary weather barriers are as follows:

• Air permeability - ability to allow air to pass through
• Vapor permeability - ability to allow water vapor (gaseous water) to pass through
• Water resistance - ability to prevent liquid water from passing through
• Repels moisture - ability to prevent moisture absorption
• Durability - resistance to tearing and deterioration