Housewrap

Housewrap (or house wrap), also known by the genericized trademark homewrap (or home wrap), generally denotes a synthetic material used to protect buildings. 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 may also serve as an air barrier if it is sealed carefully at seams.[1]

A building wrapped with Tyvek during construction

Housewrap is a replacement for the older 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.

Major types
  • Asphalt-impregnated paper or fiberglass (the original material, still widely used, also known as asphalt felt paper)
  • Micro-perforated, cross-lapped films
  • Films laminated to spunbond nonwovens (Typar or CertaWrap)
  • Films laminated or coated to polypropylene wovens
  • Supercalendered, wetlaid polyethylene fibril nonwoven ("Tyvek")
  • Drainable housewraps

Installation

Housewrap must be both water shedding and have a high moisture vapor transmission rate (MVTR) to be effective.[2] 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.

Housewrap is installed over the sheathing and behind the exterior siding. Siding can be vinyl, wood (i.e., cedar) shingles or shakes, brick facade, or numerous other acceptable building materials. 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.

Properties
  • Typical MVTR ~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

References
  1. Krigger, John; Chris Dorsi (2004). Residential Energy: Cost Savings and Comfort for Existing Buildings. Helena, Montana: Saturn Resource Management. p. 110. ISBN 1-880120-12-7. OCLC 56315804.
  2. Reed, Ryan (May 2004). "Housewrap Felt or Paper: Comparing specs on weather barriers". BUILDERnews Magazine. Pacific NW Publishing, Inc. Archived from the original on 2004-08-20. Retrieved 2016-02-24.
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