Choosing the Best Housewrap: A New Standard for Weather Barriers
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 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.
Finally, water-tight standards
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.
- 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!
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.
Posted by Peter Yost on January 19, 2012
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