Water-Resistive Barriers

Bird's Eye View

Water-resistive barriers protect exterior walls

A water-resistive barrier (also called a weather-resistant barrier or WRB) is installed between the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. or studs of an exterior wall and the siding as a second-line of defense against moisture. Most siding leaks, at least once in a while, and some types of claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. , such as cement-based stucco, are naturally porous. A WRB is designed to keep liquid water that gets around or through the siding from coming into contact with sheathing or framing.

A variety of materials can be used for WRBs. Although there are some exceptions, in general they are not designed to be air barriers. When an air barrier is created on the exterior side of the wall, the best strategy is usually to make it at the sheathing layer rather than with the WRB.


Key Materials

Builders have many choices for WRBs

Asphalt felt has a long history as a water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material.. A century ago, it was made from recycled cotton rags and was actually a type of felt. Today's building felt contains a mix of recycled corrugated paper and sawdust, and it's only one of several choices builders have.

In all, there are a half-dozen different materials used as WRBs, including rigid foam insulation, perforated and non-perforated housewrap, Zip System sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , liquid-applied materials and another type of building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt. called Grade D building paper.

WRBs can be vapor-permeable or impermeable. They key is that they must allow the wall assembly to dry once it's gotten wet.


Design Notes

Flashing and other details are still important

Most water-resistive barriers will help protect walls from water for a while, but not indefinitely. In fact, all of the building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt. WRBs will allow water to pass through in a single day, so a WRB can't take the place of flashing and other details that keep bulk water out of the wall.

Different types of WRBs show various levels of water resistance. Liquid-applied barriers and a unique product called Delta-Dry seem to be the most waterproof. Grade D building paper and perforated housewrap are at the bottom of the list. The others fall somewhere in the middle.


Builder Tips

Rainscreens offer additional protection

Given that most types of water-resistive barriers will not keep water out forever, adding a ventilated rainscreen is an option that builders also may want to consider. A rainscreen is essentially an air gap between the back of the siding and the WRB, typically created by attaching furring strips to the wall before the siding is added. The strips create an air space that promotes air circulation and helps moisture drain away or evaporate, thus speeding up the drying process. Some types of WRBs do the same thing. (For more information on rainscreen installations, see All About Rainscreens.)

What builders should avoid is creating a wall assembly that can't dry out. Trapped water inevitably leads to trouble.

Although WRBs are typically a separate building product, builders looking for an all-in-one approach can choose the Zip System manufactured by Huber Engineered Woods. When seams between the OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. panels are taped, they function as a WRB and don't require another layer of houserwrap, building felt or anything else.


The Code

The code

Section R703.2 of the International Residential Code requires builders to add a layer of number 15 asphalt felt or paperbacked stucco lath over sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. or studs. This is a little odd since manufacturers of asphalt felt specifically intend for it to be used on roofs, not walls. Regardless of this anomaly, asphalt felt makes an effective WRB.

The IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. also allows alternatives to asphalt felt. Several products have passed tests by the International Code Council Evaluation Service (ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large.), including Grade D building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt., plastic housewrap, liquid-applied WRBs and certain building assemblies that include rigid foam. Rather than being forced to use any particular product, builders have a lot of options in conforming to current code.

OTHER CONSIDERATIONS

Delta-Dry is unlike other WRBs
Delta-Dry, made from 22-mil high-density polyethylene, is different than other WRBs. Although it's vapor-impermeable, it still allows drying to the exterior because of the way it's formed. And because it's vapor impermeable, it stops inward solar vapor drive, the push of moisture from the outside from the power of the sun.

The plastic is formed into a 1/2-in. thick, egg-carton pattern, which creates an air space on both the siding and sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. sides. Air moving behind Delta-Dry carries any moisture away from the sheathing. Moisture that's driven through the siding can drain away or evaporate.

Delta-Dry can be cut with scissors or a utility knife. It comes in a 39 in. by 50 ft. roll and is fastened to the sheathing with either roofing nails or staples. It's important that both the top and bottom edges are not blocked or sealed with caulk so air has a clear passage.

In tests, it appeared that wind and sunlight were enough to drive ventilation air through Delta-Dry's channels and remove significant amounts of moisture from wood sheathing. Building scientist John Straube said it seemed to work better than asphalt felt, especially behind absorbent claddings like brick, stucco or cultured stone. "The only caveat is that you need to let air get behind it," he said. "There could be problems if someone were to try to seal the air gap at the bottom of the wall."

For all of its apparent benefits, Delta-Dry has not yet won approval by the ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large. for use as a weather-resistant barrier. Builders who use it also will need asphalt felt or an approved WRB alternative.

GREEN POINTS

LEED for HomesLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Up to 3 points in Sections 3.2 and 3.3 for reducing envelope air leakage.
NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. 10 points for incorporating air sealing package to reduce infiltration in Section 3.3.1.

ABOUT WATER-RESISTIVE BARRIERS

Keeping bulk water away from vulnerable building components

A water-resistive barrier (WRB) is a material installed between the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. (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 intallations 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 paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt., asphalt felt and Grade D building paper; plastic housewraps such as Tyvek and Typar; rigid foam insulation; liquid-applied sealers; Zip System sheathing; and a unique product called Delta-Dry.

Most WRBs are vapor-permeable. Whatever type is used, it's essential that wall assemblies have the ability to dry out.

WRBs are required by code

The International 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-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large.) for use as a substitute, including plastic housewraps, liquid-applied WRBs, Grade D building paper, and certain wall assemblies incorporating rigid foam insulation.

WRBs are not actually waterproof

Common WRBs, including most housewraps, asphalt felt, and Grade D paper, offer only so much protection against water. Eventually, water will get through. George Tsongas, a former professor of mechanical engineering at Portland State University, says, "In fact, they are not moisture barriers. If you get any significant amount of water behind the siding, the building paper will not hold back water — not even 15-pound felt. All the papers will allow liquid water to go through them in one day.”

This means builders can't rely on a moisture-resistive barrier as a substitute for correctly installed flashing at doors and windows and other construction details that shed water. Builders also may want to build in an air gap between the siding and sheathing, a detail called a ventilated rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , to promote drying and water drainage. (For more information on rainscreen installations, see All About Rainscreens.)

MORE ABOUT WATER-RESISTIVE BARRIERS

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 ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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 WRB is Grade D building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt., common in the West but not widely used in the East. 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-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large. performance tests can be used as weather-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 (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) as well as Pactiv's GreenGuardThird-party certification program that identifies building products and materials which produce relatively low levels of emissions. GreenGuard is administered by the nonprofit GreenGuard Environmental Institute (GEI). Other GEI programs include the Children & Schools standard, which addresses emission standards for educational facilities, and the GreenGuard for Building Construction Program, a mold risk-reduction program that certifies the design, construction, and ongoing operations of new multifamily and commercial properties. XPS. Expanded polystyrene made by Insulfoam (R-Tech EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) 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. (For more information on these details, see Using Rigid Foam As a Water-Resistive Barrier).

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. Thee 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. layer could consider these products as an alternative to Huber's Zip System sheathing.

This type of WRB 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.

Zip System sheathing doesn't need a separate WRB

The Zip System, manufactured by Huber Engineered Woods, consists of 7/16-in. thick OSB panels and a 3 3/4-in. wide polyolefin tape that's used to seal the seams between panels. Huber maintains that no additional WRB is needed, and the ICC-ES has, in fact, issued a report recognizing the Zip System as an acceptable alternative to asphalt felt.

One of the chief advantages of the Zip System is that it saves a step by eliminating the need for a separate WRB, and it allows builders to establish an air barrier at the sheathing level.

There are, however, a couple of caveats. One is that if it's used beneath wood siding, the siding should be back-primed or back-painted, and if it's used under cedar shingles, Huber suggests a layer of Cedar Breather or Home Slicker. Zip System beneath stucco must be protected by at least one layer of Grade D building paper.

Window flashing details also are a little different because there's no separate WRB to lap over the top window flange. Instead, the watertightness of that connection depends on the durability of the tape.

Although the tape appears of excellent quality, building scientist John Straube sums up concerns this way: "At the end of the day, we don't know how it will adhere in the long run. It is an unanswerable question."

FURTHER READING

All About Water-Resistive Barriers

Using Rigid Foam As a Water-Resistive Barrier

Housewrap in a Can: Liquid-Applied WRBs

Product Guide: Water-Resistive Barriers

What’s New with Water-Resistive Barriers


Image Credits:

  1. Fine Homebuilding
  2. DuPont Tyvek
Tags: , ,
8.
Mar 20, 2013 3:42 PM ET

Response to Martin Holladay
by ken levenson

There is no "tent effect" with the SOLITEX. It is a different animal. With the typical housewraps the surface tension of the water is broken when you touch it with finger or other contaminents and it tents through the micro-tears. The SOLITEX is a solid membrane - the vapor permeability is at the molecular level.
Agreed that penetrations need to be self-sealing/under positive pressure to maintain waterproofness - if you have an exposed tear it is going to leak.

Please take a look at this brochure: http://www.foursevenfive.com/spec/SOLITEX%20MENTO%20System%20brochure%20...
I'll also try to track down further documentation and will follow-up.


7.
Mar 19, 2013 2:00 PM ET

Response to Ken Levenson
by user-756436

Ken,
I think that some plastic housewraps (and 30-pound asphalt felt) would probably pass your "Christmas present on the roof of a car" test.

The issue seems to be fasteners and sandwiching. Once you staple or fasten a membrane to a wall, and pinch it between the siding and the sheathing, most membranes leak. The solution is to include an air gap between the WRB and the back of the siding.

It's a little like an old-fashioned single-wall tent. The tent won't leak until you touch it from the inside with your finger. Then your finger gets wet.


6.
Mar 19, 2013 1:29 PM ET

Edited Mar 19, 2013 1:29 PM ET.

Response to Martin Holladay
by ken levenson

Hi Martin,
I'll admit to not having done that particular test - however, my own anecdotal test was driving on the highway for 4 hours at 70mph in a driving hurricane like rainstorm (the whole way) with a large package wrapped in SOLITEX Mento 1000 WRB on the roof of the car. Upon arriving in Washington DC, we nervously removed the WRB, finding much to our relief, a bone dry back to the WRB and bone dry contents within. The membrane is waterproof. (And tannins don't degrade it.)


5.
Mar 19, 2013 6:53 AM ET

Edited Mar 19, 2013 6:55 AM ET.

Response to Ken Levenson
by user-756436

Ken,
The link to the spec sheet doesn't really tell me how Solitex Mento behaves in a wall without a rainscreen gap. If the membrane is pinched between lap siding and sheathing -- or perhaps panel siding and sheathing -- without a drainage gap, and if I subjected the wall to several hours of exposure to a water hose, would the sheathing get wet?

I don't know the answer. But I know that if the test is performed with a plastic housewrap or asphalt felt, the sheathing will get wet.


4.
Mar 18, 2013 8:49 PM ET

Thanks for the link Martin
by albertrooks

I read the short instructions. Since this is a whole wall WRB, and Huber requires an 1/8" gap where 4' edges meet, the WRB system is heavily dependent on the tape joint, and the tapes bond in order to stop the weather. For every panel there is a 12' tape joint that needs to last the life of the building. These joints don't even have the benefit of gravity lapping (is that the right term?).

It does make me appreciate a good lapped membrane since they can be laid in a way that even if a tape joint failed, proper lapping would keep the water out.

Not that I'd even admit that tape fails :) (Just joking... Everything fails at some point.)


3.
Mar 18, 2013 10:50 AM ET

Edited Mar 18, 2013 11:09 AM ET.

Some updated information is needed
by ken levenson

SOLITEX Mento 1000 and Mento Plus WRBs can be fairly described as "waterproof". Please see spec sheet here: http://www.foursevenfive.com/spec/SOLITEX%20MENTO%201000%20Spec.pdf
With taped seams they can also provide Passive House level airtightness - while being quite vapor open. See a description of the membrane here: http://www.foursevenfive.com/index.php?main_page=page&id=15&p=533

The SOLITEX Mento membranes' waterproof performance are also not degraded by building site contaminants or wood siding tannins.

The SOlTEX Mento membranes offer an affordable high-performance approach. And they are available across the US from www.foursevenfive.com.

I hope you can update the information provided in this WRB posting. Let us know if you have any further questions.
Sincerely,
Ken


2.
Mar 18, 2013 8:42 AM ET

Response to Albert Rooks
by user-756436

Albert,
According to the installation instructions provided by Huber, there is no special treatment required for panel edges that have been cut. Here is a link to the web page with installation instructions:
http://www.zipsystem.com/installation/wall-sheathing.aspx


1.
Mar 18, 2013 8:09 AM ET

Treating cut edges on Zip?
by albertrooks

Since, as I understand it, the Huber Zip product is a lamination of a woven WRB onto a specialized OSB panel. As that makes it a "topical treatment" in my mind, what is recommended treatment for cut edges of panels? Such as a lap joint at the building corner, window & door RO's, service penetrations. Is the standard practice to tape and cut edge to seal it?


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