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MEEF - Middle East Economic Engineering - Articles  - Air Barriers  previous page

Water-Resistive Barriers/Housewraps

What is a Housewrap?

A housewrap, or water-resistive barrier, is a thin membrane layer typically 5 to 15 mils thick (.005 to 0.015 in. or 0.13 to .38 mm) which is intended to resist liquid water that has penetrated behind the exterior cladding from further intruding into the exterior wall assembly. Housewraps come in a variety of widths and lengths as well as a variety of materials. These materials consist of polyolefin-based sheets (plastic) and can include spun-bonded (fibers), cross-woven (tapes), film (sheets) or a combination thereof.

A housewrap is different than Grade D building paper (paper-based). Most commercially available polymeric housewraps are highly moisture-resistant and therefore, do not absorb water like typical Grade D or felt building papers. Polyolefin housewraps are typically stronger than Grade D or felt building papers, thus they tend to have better durability than conventional building papers.

What Does A Housewrap Do?

The primary function of a housewrap is water resistance. Housewraps are hydrophobic (designed to resist bulk water penetration). A housewrap deflects water and channels it down the surface of the exterior wall. In addition, housewraps allow moisture vapor to move freely, thus reducing the potential for condensation build-up, which reduces the likelihood of rot/degradation.

Housewraps function as air retarders. Some Housewraps may also meet the requirements for an Air Barrier Material (0.02 L/s·m˛ @ 75 Pa or 0.004 cfm/ft 2 @ 1.57 psf ) reducing air permeance when evaluated in accordance with

ASTM E 2178. Housewrap materials that qualify as air barrier materials must be installed in strict accordance with the manufacturer’s instructions in order to perform as required.

How Do Housewraps Work?

A Housewrap decreases the air infiltration into the wall cavity, hence allowing the insulation in the wall cavity to function as it was intended, by trapping air and creating a dead air space.

Housewraps also resist bulk water penetration resulting from wind-driven rain that penetrates the exterior cladding, ultimately migrating into a well-built wall cavity. Housewraps enable walls to dry out by allowing moisture vapor to escape when liquid water evaporates.

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