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It's not easy being a weather-resistive barrier (WRB): it has to stop liquid water, be tough and not tear, but also be flexible to wrap around building elements. And it often needs to be vapor-permeable to promote drying.
In the past, manufacturers could cherry-pick the standardized test to use to "qualify." That's how we ended up with industry acceptance of perforated and cross-woven housewrap that literally leaks like a sieve.
Now we have a new ASTM "Standard Specification for Vapor Permeable Flexible Sheet Water-Resistive Barriers Intended for Mechanical Attachment." This standard also aligns with the latest version of the ICC-ES Acceptable Criteria (AC) 38–"Acceptance Criteria for Water-Resistive Barriers (PDF)." The table below presents the requirements for WRBs used in the new standard and now by GreenSpec.
Here are the key points from the table.
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.
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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