Example of a widely-used water-resistive barrier
Keeping Bulk Water Away from Vulnerable Building Components A water-resistive barrier (WRB) is a material installed between the sheathing (or studs if there is no sheathing) and the siding. It is designed to prevent water from reaching building components that could be damaged by moisture. Builders should assume that siding installations aren’t truly waterproof, and that some water will find its way through or around the siding (at least once in a while). Without a WRB, sheathing and other parts of the wall assembly would be much more susceptible to damage.
A WRB can be one of a variety of materials sold under dozens of trade names. There are two types of building paper (asphalt felt and Grade D building paper), plastic housewraps such as Tyvek and Typar, and fluid-applied barriers.
Most WRBs are vapor-permeable. This is important because, whatever type is used, it’s essential that wall assemblies have the ability to dry out.
WRBs are Required by Code The California Residential Code requires builders to install a layer of number 15 asphalt or paperbacked stucco lath over the wall sheathing or studs. However, builders can still meet this requirement with some “other approved water-resistive barrier.” A number of materials have met the requirements of the International Code Council Evaluation Service (ICC-ES) for use as a substitute, including plastic housewraps, liquid-applied WRBs, Grade D building paper, and certain wall assemblies incorporating rigid foam insulation.
WRBs Aren’t Literally Waterproof Common WRBs including most housewraps, asphalt felt, and Grade D paper, offer only so much protection against water. Given the perfect storm, water will get through. This means builders can’t rely on a water-resistive barrier as a substitute for correctly installed flashing at doors and windows and other construction details that shed water. Builders might also consider to build in an air gap between the siding and sheathing, a detail called a ventilated rainscreen, to promote drying and water drainage.
Asphalt Felt is an Old Standby Asphalt felt, which a century ago was actually made from recycled cotton, is a familiar WRB that’s still in use today. Now it’s made from recycled corrugated paper and sawdust and it’s a lot lighter than it used to be. It originally weighed 15 lb. per 100 sq. ft. (and was called “15-pound felt”). To save money, manufacturers have since reduced the weight to between 7 and 14 lb. per 100 sq. ft. and call it “number 15 felt.”
There are two ASTM standards for asphalt felt. In order to meet code, builders must use felt that complies with the ASTM D 226 standard, which weighs a minimum of 11.5 lb. per 100 sq. ft. A second ASTM standard requires felt weigh only 8 lb. per square.
Asphalt felt has a permeance of 5 perms when dry, but 60 perms when wet. Unlike plastic housewrap, asphalt felt soaks up water when it gets wet and slowly allows it to dry to the exterior.
Another type of paper water-resistive barrier is Grade D building paper. It’s often used under stucco but can be used under other types of siding as well. Grade D building paper is asphalt-impregnated kraft paper with a minimum water resistance rating of 10 minutes and a permeance rating of 5. Manufacturers also offer 20-minute, 30-minute and 60-minute papers. Overall, it’s lighter and less expensive than asphalt felt. Grade D building paper degrades if it’s exposed to too much water. Builders often use two layers, but even that may not be enough in areas that get a lot of rain.
Plastic Housewraps Are Widely Used Housewraps such as Tyvek and Typar are a familiar sight on residential job sites. Made from polyolefin fabrics of either polyethylene or polypropylene plastic, housewraps are lightweight and come in rolls 8 or 9 ft. wide, so they go up quickly. Housewraps have a vapor permeance range of between 6 and 59.
There are two types: perforated and non-perforated. Perforated housewrap (Barricade, PinkWrap, Weathermate) has been punched with tiny holes, which allow water vapor but not bulk water to pass through. With non-perforated housewrap (Tyvek, Typark, R-Wrap, Weathermate Plus), water vapor passes between the fibers of the fabric. Non-perforated housewraps are better at keeping bulk water out, but both types can be damaged by chemical extractives that leach out of wet cedar or redwood siding. This problem also affects asphalt felt, but not as severely.
There also are wrinkled housewraps (DuPont StuccoWrap, Home Slicker Plus Typar, Benjamin Obdyke HydroGap, Barricade WeatherTrek and others) which have small vertical corrugations to help water drain away. Experts do not agree on how deep corrugations must be in order to help water drain.
Housewrap is often installed with staples, but one study found that could produce lots of tears and holes. Switching to plastic-cap nails is one possible solution.
Some Brands of Rigid Foam Can Be WRBs Rigid foam insulation that passes certain ICC-ES performance tests can be used as water-resistive barriers. Tests include exposure to sun lamps and cycles of baking in an oven followed by soaking in a bucket of water. Wall assemblies sheathed in foam must also pass a water-spray test. A number of products have passed, including many of Dow’s polyisocyanurate and extruded polystyrene panels (XPS) as well as Pactiv’s GreenGuard XPS. Expanded polystyrene made by Insulfoam (R-Tech EPS) also is on the approved list.
In order to be used this way, however, the foam panels must be installed exactly like those that passed the tests. That could include a certain fastening schedule or a certain type of seam-sealing tape. In the case of Dow Styrofoam, the bottoms of window flanges must be set in caulk. Flashing the tops of windows is one tricky detail when foam is used without another layer of housewrap. Some experts are wary of relying on special tapes that are typically used to seal seams and window flanges. Shrinkage of foam panels is another concern, although manufacturers have tinkered with chemical formulations in an effort to minimize the problem.
Liquid-applied Products Form a Tough, Flexible Coating A very effective, although more expensive, option is one of several liquid-applied WRBs. These product are applied with a brush, trowel or sprayer to form a tough, flexible coating. Probably because of their higher cost, these products are more common in commercial construction than they are for residential projects.
Liquid-applied WRBs (StoGuard, Tyvek Fluid Applied WRB, Prosco, LaHabra Stucco Solutions, Enershield) are good at blocking bulk water but they’re also fairly permeable to water vapor.They make especially effective air barriers. Builders who like to establish an air barrier at the sheathing layer could consider these products as an alternative to Huber’s Zip System sheathing.
This type of water-resistive barrier may include companion materials that are used to cover cracks and seal seams between sheathing panels before the entire wall is treated. There also may be products specifically designed for use around windows and doors. Although they’re very effective, liquid-applied WRBs also can be tricky to apply correctly. It takes practice to get the correct thickness, and builders who get the stuff in their hair learn to be more careful next time.
For Further Reading Resources for further research into water-resistant barriers include: • The Construction Specifier (https://www.constructionspecifier.com/wrb/) • Energy Vanguard (https://www.energyvanguard.com/blog/does-your-water-resistive-barrier-really-resist-water) • Article by John Straube, Ph.D., P.Eng at the site of Dorken, Inc. (https://dorken.com/insights/air-barriers/water-resistive-barriers-when-and-why-does-vapor-permeance-matter/).
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