Nowadays, any approved weather-resistive barrier, from #15 felt to high-tech house — wrap, touts the dual benefit of being a weather-resistive drainage plane that also allows the passage of water vapor. But not every product balances these two features equally. To add to this confusion, house — wraps are now available in dozens of varieties, so how do you choose? Unfortunately, there’s no easy answer.
The American Society for Testing and Materials (ASTM) is working to standardize the tests used to evaluate weather-resistive barriers. For now, when trying to gain code approval, manufacturers can choose from at least two dozen different tests. Even if two manufacturers choose the same test, though, there is nothing to regulate the way in which the test materials are set up. This variability makes it nearly impossible to compare one product’s performance to another’s.
According to Paul Fisette, director of building materials and wood technology at the University of Massachusetts, one tested value that usually stands up to side-by-side comparison is a material’s permeance rating, but sometimes even that can be misleading.
1. Create a secondary weather barrier behind the siding, preventing wind-driven rain and other water from reaching the sheathing.
2. Serve as an air barrier
to prevent air infiltration, helping to reduce heating and cooling costs.
3. Provide a vapor — permeable membrane
that allows moisture in framing lumber or insulation to escape.
Perm Ratings Tell Part of the Story
Permeance ratings, or perms, reflect the measure of a material’s ability to transfer water vapor; the higher the perm number, the more permeable the material. For instance, 6-mil polyethylene sheeting has a very low perm rating of 0.06, which means that it prevents the passage of nearly all water vapor. Current building codes require a weather-resistive barrier to match or exceed grade-D building paper, which has a perm rating of about 5.0. To meet this requirement, perm ratings for commonly available brands of housewrap range from about 5.0 for Dow®’s Weathermate™ to
58.0 for Tyvek®’s HomeWrap®.
Materials with higher perm ratings speed the escape of trapped moisture. But higher ratings do not necessarily equal better housewraps, because the methods of achieving a high perm rating can be different.
For instance, some low-tech housewraps achieve their high perm ratings with mechanically punched perforations in the membrane. These perforations increase the passage of water vapor, but they also make the housewrap more susceptible to bulk — water leakage.
On the other hand, more-advanced nonperforated housewraps, such as HomeWrap and R-Wrap®, offer even greater moisture — vapor transmission (higher perms) than their perforated counterparts. They are also more effective at preventing the movement of bulk water.
