Why Do You Need House Wrap?
House wrap technically is a type of water-resistive barrier or WRB. The purpose of using house wrap is to prevent the entry of moisture into the wall cavity from outside. While house wrap is water-resistant, it typically is not water impermeable or waterproof, with good reason. Just as rain and moist air can enter a wall cavity from outside the house, moisture-laden air can also enter a wall from inside the house. If a house wrap is impermeable, it can trap this indoor moisture in the wall cavity, potentially leading to rot and mold growth. Most WRBs have a permeability rating between 5 and 60; a minimum of 5 is required by most building codes. Contrary to many claims, house wrap usually does not serve as an air barrier to prevent cold air from entering wall cavities. Only a few special types of WRBs are designed to perform this function, and they must be installed meticulously to create a reliable air barrier. House wrap is intended to be installed over the sheathing and behind the siding, no matter what siding you are using: wood, fiber cement, vinyl, brick, stucco, and others. Siding manufacturers may recommend specific types of WRB to use with their products.
House Wrap Types
The original WRB was asphalt-impregnated felt paper or “tar paper,” the same material commonly used as an underlayment under roof shingles. More recently, the term “house wrap” is usually used to describe plastic fabric WRBs such as Tyvek.
Asphalt felt (tar paper)—The standard requirement is Type 1 felt meeting ASTM D 226 standards.Grade D building paper—Made of asphalt-impregnated kraft paper, this house wrap is commonly used under stucco siding.Polyolefin fabric—“Plastic” house wraps typically are made of woven polyethylene or polypropylene fibers.Liquid WRB—These tar-like liquids are applied with a paint roller or spray equipment and can serve as an air barrier.Rigid foam—Some types of rigid foam insulation board can serve as a WRB if they are properly sealed.WRB sheathing—Sheathing panels made of oriented strandboard (OSB) with special coatings can serve as both sheathing and WRB; installation includes sealing all panel joints with the manufacturer’s tape.
House Wrap Installation Tips
Always follow the manufacturer’s installation instructions, and use only approved fasteners and sealers, as applicable. General recommendations that usually apply to fabric-type house wrap include:
Seal all seams with a special tape provided by the house wrap manufacturer. Work from the bottom up, overlapping the lower courses. Install house wrap before doors and windows. Wrap the fabric around the edges of windows and doors to the inside of the frame. Install the house wrap between the double top wall plates. Prevent chemicals from coming in contact with the fabric, as they can adversely affect the house wrap’s water-resistance. Extend the house wrap over the footing top at least 2 inches. Install a drainage provision at the bottom of the external siding material, if recommended. Use staples or nails specifically designed to hold down house wrap material. Nails should be at least 1 inch long and spaced 12 to 18 inches on-center. Overlap horizontal joints of wrap by at least 6 inches. Overlap vertical joints by at least 6 inches and no more than 12 inches, depending on the severity of the area’s weather conditions. Lap house wrap over the entire top of any step-flashed areas. Install a protective rain screen to control moisture movement when porous siding is being installed.
