Housewrap
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Housewrap defines all synthetic replacement materials for sheathing tar paper or asphalt paper. These materials are all lighter in weight and usually wider than asphalt designs, so contractors can apply the material much faster to a house shell. Housewrap functions as a moisture barrier, preventing rain from getting into the stud wall construction while allowing moisture vapor to pass out from the house's interior living space to the exterior. If moisture from either direction is allowed to build up within stud or cavity walls, mold and rot can set in and fiberglass or cellulose insulation will lose its R-value due to heat-conducting moisture.
[edit] Major types of housewrap
- asphalt-impregnated paper or fiberglass (the original material, still widely used)
- micro-perforated, cross-lapped films
- films laminated to spunbond nonwovens
- films laminated or coated to polypropylene wovens
- supercalendered, wetlaid polyethylene fibril nonwoven ("Tyvek")
[edit] Installation
Housewrap must be both waterproof and have a high water vapor transmission rate (WVTR) to be effective. It must also take handling abuse during installation and be resistant to UV. Housewrap is often left exposed for some time after construction, awaiting exterior sheathing installation. The original asphalt paper design, while heavy and slow to install, is still a contender. It can be nailed and abused during installation and still function. Some new designs must be installed carefully or they will slightly rip or tear during installation, possibly allowing for water infiltration at the damaged areas. Most newer designs do not "seal" well against nails or staples like asphalt products.
Housewrap is installed over the stud wall and behind the exterior sheathing. Sheathing can be vinyl siding, wood clapboard, cedar shingles or brick facade. In all cases, the housewrap is the last line of defense in stopping incoming water or exterior water condensation from getting into the wooden stud wall.
[edit] Properties
- typical WVTR ~200 grams/100 square-inches/24hours (or greater, i.e., Tyvek is ~400))
- typical 2 ounces/square-yard (varies greatly with manufacturer)
- typical width 90" on a 3" core
