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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.

Source: ASTM International. Click to enlarge.

Here are the key points from the table.

  1. 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.
  2. Tensile strength or breaking force: There are three different ASTM test methods from which to choose; all evaluate the strength of the material.
  3. 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.
  4. Pliability: The pliability test ensures WRBs are pliable even when they are cold (32?F).
  5. 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.

Posted by Peter Yost on January 19, 2012

Comments

Choosing the Best Housewrap: A New Standard for Weather Barriers

While standards are all very good for those of us concerned with doing things right, one major problem is that most installers of WRB's are carpenters with little or no understanding of what the product they are using is supposed to do, or how to properly install it for it to work. Likewise manufacturers know that their product is not being properly installed (because they do not sell as much tape as they know needs to be used) and they really don't care. I confront their reps at trade show all the time about how little tape they sell, and their response is that they know they sell too little tape and that they will educate anyone who asks for help. The problem is that the carpenters do not know enough to know that they are not installing the product improperly, so they do not know to ask. The inspectors miss so much of these details that I have to wonder how they become qualified t be inspectors. In my experience at least 95% of contractors, architects and suppliers are clueless about the building science involved and as a result they improperly sell, specify and install the product. Having taught about green building since 2004 I think that we need to start requiring evidence of understanding of the designers and builders of our buildings. Most of them are clueless about what it is that they don't know. Though I make a fair amount of money correcting the mistakes of those who built the houses ( I am just finishing up $25,000 of work on an eighteen year old building that leaked so badly that the furnace rested about five minutes out of an hour. It now goes almost an hour between firing), I would much prefer to find a way to educate the trades and designers about how to do the projects right in the first place. I had one builders ask me after a talk I did for the local NAHB Chapter what I though of the "new" advanced framing techniques. I informed him that these "new" advanced framing techniques were first introduced in the 60's and adopted by the National NAHB in the 70's, and that it just showed how far behind the builders are that in 2007 they considered these techniques to be new.

It's great that there is a un

It's great that there is a uniform test standard for WRBs, but there are other considerations that effect performance that are not included in it: such as ease of installation; interfacing with doors, windows and flashings; reliance on tape rather than lapping; and hygroscopicity (ability to absorb and diffuse condensate on the backside). It is not without reason that the IRC 2012 still recognizes ASTM D226 felt as the WRB against which all others must be compared. It has enough, without too much, vapor permeance (too much allows exterior water vapor to penetrate inward); allows hygroscopic diffusion of backside condensate to be released outward, which no polymeric material does; and the shingle-lapped installation not only makes it far easier to integrate with architectural penetrations and flashings, but also uses the much more reliable physics of gravity drainage rather than the questionable integrity and durability of chemically-bonded tapes

While I was considering

While I was considering installing a new housewrap since I think my current housrwrapping is leaking too much, I became kinda afraid of not having a qualified contractor to install it after I read Bruce Wilson's comment on how unqualified many contractors can be at the point of having no knowledge on what the product is supposed to do. I feel very confortable of being able to choose a housewrap that will meet my needs from the GreenSpec weather barriers list, but still how can I find a contractor that is fully qualified to install it on my house? It would be a great addition to the site if GreenSpec had a list of qualified contractors to install and replace housewrappings.

Thanks in advanced.

Alfred,http://actodc-converter.info/

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