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Choosing the Best Housewrap: A New Standard for Weather Barriers

by LiveModern Webmaster last modified Jan 20, 2012 10:49 AM
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by Peter Yost last modified Jan 19, 2012

There are more than 20 different standardized tests manufacturers can invoke to "qualify" as a code-accepted weather-resistive barrier (WRB); with our GreenSpec section on WRBs, we've picked just one that we think does the job. It's not easy being... Source: ASTM International. Click to enlarge. Here are the key points from the table. Two types of WRBs: Type I WRBs have what is described as a "base" level of water resistance. Type II WRBs have what is described as an "enhanced" level of water resistance. This difference is reflected only in the water-resistance requirements. GreenSpec requires Type II compliance. Tensile strength or breaking force: There are three different ASTM test methods from which to choose; all evaluate the strength of the material. Vapor permeability: All WRBs must be a minimum of 5 perms, considered to be vapor semi-permeable (Joe Lstiburek, Ph.D., P.E., of Building Science Corporation classifies materials in the range of 1 to 10 perms as Class III vapor retarders, based in part on the Canadian General Standards Board approach). This is ideal because WRBs should keep water out but also allow drying. Pliability: The pliability test ensures WRBs are pliable even when they are cold (32?F). Aged testing: The tests for tensile strength and water resistance must be conducted for materials "as received" and "aged." Aged testing involves cycles of wetting and drying as well as ultraviolet (UV) light exposure. Find out if your favorite housewrap qualifies Most high-quality, well-known spun-bonded polyolefin housewraps (such as Tyvek and Typar) comply with the new ASTM standard Type II requirements; the same is true for quality building papers (Fortifiber, for example). You might not find explicit compliance to this new standard on a manufacturer's website; if it's not clear, ask them. Or just use a building paper or housewrap listed in GreenSpec . You still have to install it right! Although the new proposed ASTM standard for flexible, mechanically fastened WRBs does not cover installation, one can hope that its requirements for tensile strength and pliability will help prevent situations like this. Does this new standard solve all of our building-assembly problems? Not by a long shot; you still need to marry the WRB to all flashing details at penetrations and transitions. But it sure makes a lot of sense to start with the right materials as you design, spec, and build high-performance building assemblies. What are your experiences with WRBs, and questions? Please post your comments below.






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