House wrap is the current term that defines all synthetic materials that have become replacement materials for asphalt-treated paper, or asphalt saturated felt. 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 weather-resistant barrier, preventing rain from getting into the wall assembly while allowing water vapor to pass 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.
House wrap must be both water shedding and have a high moisture vapor transmission rate (MVTR) 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 siding 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.
House wrap is installed over the sheathing and behind the exterior siding. Siding can be vinyl, 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.
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