UPDATED on September 18, 2013
By now, almost all builders know the importance of installing a water-resistive barrier (WRB) behind siding. Most types of siding leak, so it’s a good idea (and a code requirement) to install a WRB to protect your wall sheathing from any wind-driven rain that gets past the siding.
A WRB can be vapor-permeable, like Tyvek, or vapor-impermeable, like foil-faced polyisocyanurate. As long as the wall assembly is designed to dry out when it gets wet, either vapor-permeable or vapor-impermeable WRBs work well.
In most cases, WRBs are not air barriers. (The notable exceptions are Zip System sheathing and liquid-applied WRBs.) If you want to create an air barrier on the exterior side of your wall, it usually makes more sense to create the air barrier at the sheathing layer, not at the WRB.
Although there are at least six categories of WRB — asphalt felt, Grade D building paper, plastic housewrap, liquid-applied WRBs, rigid foam, and Zip System sheathing — building codes calls for just one: asphalt felt. (One other category — three-dimensional vapor-impermeable barriers — is represented by a single product, Delta-Dry. Although it is often used as a WRB, Delta-Dry has not received code approval as a substitute for code-required WRBs.)
According to the IRC, you need asphalt felt
In section R703.2, the International Residential Code requires builders to install a layer of number 15 asphalt felt or paperbacked stucco lath over the wall sheathing or studs of every new home. The requirement includes a qualification: if you don’t want to use number 15 asphalt felt, you can use some “other approved water-resistive barrier.”
The code requirement calling for walls to be covered with asphalt felt is rather odd, because every asphalt felt manufacturer declares unequivocally that the product is intended for roofs, not walls. When builders install asphalt felt on walls as required by code, the manufacturer of the felt considers the application to be outside of the product’s intended use. In spite of this curious anomaly, asphalt felt performs well when used over wall sheathing as a WRB.
According to tests performed by the International Code Council Evaluation Service (ICC-ES), several products have been approved as substitutes for number 15 asphalt felt, including Grade D building paper, plastic housewraps like Tyvek and Typar, liquid-applied WRBs like StoGuard or Tyvek Fluid Applied WB, and certain building assemblies that include rigid foam.
(Just to make matters more confusing, the International Residential Code has a second reference to WRBs in addition to the one in section R703.2 mentioned above. Section R703.6.3 requires that “Water-resistive barriers shall be installed as required in Section R703.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper.” Although section R703.2 bases the performance of a WRB on the performance of #15 asphalt felt, and section R703.6.3 bases the performance of a WRB on a different product, Grade D paper, the two provisions of the code are not contradictory, since most approved WRBs meet both provisions. The different language in these two sections is a historical accident as well as a glaring example of bad code writing.)
One hundred years ago, asphalt felt was a true cloth felt made from recycled cotton rags. Since asphalt felt is now made from a mixture of recycled corrugated paper and sawdust, the term “felt” is a misnomer.
Over the years, asphalt felt keeps getting lighter. “In the old days, it used to weigh 15 pounds per 100 square feet, but not anymore,” explained Allen Snyder, a product engineer at CertainTeed, a manufacturer of asphalt felt.
The main reason that manufacturers have reduced the weight of asphalt felts is to lower their manufacturing costs. “The whole issue comes down to price,” said Ed Todd, technical manager at Atlas Roofing, an asphalt felt manufacturer in Atlanta. Since today’s asphalt felts weight between 7 and 14 pounds per square, manufacturers no longer call the product 15-pound felt; instead, it has been renamed “number 15” felt.
ASTM has established two standards for asphalt felt. The less stringent standard, ASTM D 4869, requires #15 felt to weigh at least 8 pounds per 100 square feet. The more rigorous standard, ASTM D 226, requires a minimum weight of 11.5 pounds per square. According to the IRC, any asphalt felt used as a WRB on walls must be Type 1 felt complying with ASTM D 226.
Because enforcement of the code requirement for ASTM D 226 felt is relatively lax, most lumberyards stock only lightweight asphalt felt with no ASTM rating; such felt typically weighs only 7.6 pounds per square. In areas of the country with more stringent code enforcement — notably Florida — ASTM-rated felt is widely available.
Asphalt felt is also available in a heavier version, commonly called 30-pound felt. This #30 felt is available in both unrated grades (15.7 to 19.9 lbs. per square) and ASTM-rated grades (26.4 to 27.3 lbs. per square). Since the lightest unrated #30 asphalt felt is still heavier than the heaviest ASTM-rated #15 felt, it a logical choice for builders in search of a felt that approximates the asphalt felt used by their grandparents.
Asphalt felt has a permeance of only 5 perms when dry, but a much higher rating of 60 perms when wet. Fans of felt note one of its advantages over housewrap: if water gets behind felt — either due to a flashing leak or condensation from solar-driven moisture — the felt can soak up the liquid water and gradually dry to the exterior. Plastic housewrap is not absorbent; any water trapped on the wrong side of plastic housewrap can only pass through to the exterior as vapor.
Grade D building paper
Builders in the western U.S., especially in areas where stucco is common, are familiar with Grade D building paper. In much of the rest of the country, however, including most of the East Coast, Grade D building paper is virtually unknown. Although Grade D paper is most often used under stucco, manufacturers point out that it can be used under any kind of siding.
Grade D building paper is an asphalt-impregnated kraft paper that looks like a lightweight asphalt felt. The term “Grade D” originated with federal specification (UU-B-790) which dates back to 1968. The specification requires that Grade D paper have a minimum water-resistance rating of 10 minutes and a minimum water vapor transmission rate that corresponds to a permeance rating of about 5 perms.
The water-resistance rating is based on a test, ASTM D 779, usually called the “boat test.” If a piece of building paper or housewrap folded into the shape of a toy boat can float in a dish of water and withstand soaking for at least 10 minutes, it meets the water-resistance requirements of the Grade D specification.
Many Grade D manufacturers choose to exceed the minimum water-resistance rating, producing 20-minute, 30-minute, or 60-minute papers. Although the UBC referred to these Grade D papers as “waterproof” papers, even a 60-minute paper is, at best, only water-resistant.
Like asphalt felt, Grade D building paper is an asphalt-saturated paper. It differs from asphalt felt in two ways: it uses a lighter-weight paper, and the paper is made from new paper pulp rather than recycled cardboard. Since Grade D paper weighs less than asphalt felt, it also costs less.
Manufacturers of Grade D paper like to point out that because it is lighter, it is easier to crease and install in inside corners. Some experts note, however, that the added thickness of #15 felt is one of its virtues. If Grade D paper gets wet and stays wet, it can rot. Although asphalt felt can also rot, its heavier weight helps it hold up better in extreme conditions.
Because a single layer of Grade D paper may not withstand repeated wetting, the use of two layers of building paper has become a standard detail under many types of siding. But in areas that get a lot of rain, even two layers of building paper can be overcome by regular soakings. “I’ve seen building paper rot, even if you have two layers,” building scientist Joe Lstiburek told me. “Grade D paper rots faster than roofing felt. The best paper for a wall is a roofing felt.”
According to Wesley Page, a retired waterproofing consultant from Novato, Calif., Grade D paper cannot withstand repeated wetting. “Grade D building paper will fail completely if it gets wet,” said Page. “It just disintegrates and disappears.” All experts agree that any paper or felt will be less likely to rot if it is installed behind an air space that permits drainage and speeds drying.
Plastic housewraps are made from one of several polyolefin fabrics, generally either polyethylene or polypropylene. Different brands of housewrap have different levels of vapor permeance; the range is from 6 to 59 perms.
Most building science experts downplay the importance of a housewrap’s vapor permeance. In theory, if a wall cavity gets wet, a WRB should be vapor permeable enough to allow the wall to dry to the exterior. Yet the ideal WRB would also prevent water vapor from being driven into a wall by inward solar vapor drive. Unfortunately, no one has yet developed a material that has “one-way permeance,” allowing vapor out but not in. High vapor permeance is probably a desirable feature in a WRB during cold winter weather, but may be undesirable during hot, humid weather, when the action of the sun on saturated siding can cause vapor to be driven into a wall.
According to Lstiburek, the permeance of a WRB “matters very little.” Most types of sheathing, including OSB and plywood, are not very vapor-permeable, at least when dry. “Asphalt felt is rated at 5 perms, and the housewraps have ratings that range from 5 to 50,” said Brad Allshouse, vice-president of marketing for Simplex Products, a housewrap manufacturer. “But commonly used sheathings have a permeance rating of less than 1. So the permeance of the housewrap is a moot point. A housewrap with a perm rating more than the code minimum of 5 is overkill.”
Housewraps can be divided into two categories: perforated and non-perforated. Non-perforated housewraps allow water vapor to pass between the fibers of the plastic fabric, while perforated housewraps are made from vapor-tight plastic films that are needle-punched with small holes to allow the passage of water vapor. Laboratory tests have shown that the non-perforated housewraps resist liquid water better than the perforated housewraps.
Some surfactants, which are chemical extractives that can leach out of wet cedar or redwood siding, have the potential to degrade the water resistance of plastic housewrap. However, surfactants can also degrade asphalt felt. “There have been problems with cedar and redwood sidings leaching wood sugars or surfactants,” said Lstiburek. “This has occurred with all the plastic housewraps and the felts. Everything is affected. But the plastics seem to have more of a problem than the felts.”
Plastic housewraps are rarely used under stucco. “You can’t stick stucco to any plastic housewrap, because if the stucco is in direct contact with a housewrap, the housewrap loses its water repellency,” said Lstiburek. Frank Nunes, an officer with the International Institute of Lath and Plaster, has also seen problems using housewrap behind stucco. “Some housewraps are very reactive to surfactants in the cement plaster,” says Nunes. “In one case I observed, the resins of the housewrap dissolved, leaving the fibers. It looked like a silkscreen — there was no material left.” (Because many areas of the country have suffered clusters of wet-wall failures behind stucco, most experts now recommend that stucco should always be installed over a ventilated air space. To learn more, read “To Install Stucco Right, Include an Air Gap.”)
If a wall is well designed and well flashed, any of the plastic housewraps will do the job. “The design of the wall is more important than the choice of housewrap,” said John Straube. Since the most important function of a WRB is to resist liquid water, you should probably choose a non-perforated housewrap (for example, Tyvek, Typar, R-Wrap, or Weathermate Plus) over a perforated one (for example, Barricade, PinkWrap, or Weathermate). “If I want to keep the water out, maybe I wouldn’t choose a housewrap with a whole bunch of holes punched in it,” says Straube. Not all experts agree, however. “All housewraps are perforated, because they are stapled or nailed,” said Joe Lstiburek. “Whether or not they come from the factory perforated is irrelevant.”
In general, housewraps cost more than building paper or asphalt felt. Most builders find that housewrap is easier to install than paper or felt, because it comes in wide rolls (usually 9 or 10 feet wide) and it weighs less. On the other hand, builders working alone or working on a very high building may find a wide roll of housewrap more awkward than a narrow roll of paper or felt.
Housewraps stay more flexible in cold temperatures than paper or felt, and they resist tearing better. However, asphalt felt is better able to seal around fastener holes than housewrap.
On the average job site, housewrap is almost never installed as carefully as it is when tested in a laboratory. After performing a field survey of installed housewrap, researchers from the Pennsylvania Housing Research Center reported, “In the majority of the houses where staples have been installed with an automatic staple gun, tears and holes in the housewrap were common.” One possible solution to this problem is to switch to plastic-cap nails, which provide much better sealing than staples.
Although most WRBs are able to protect sheathing from occasional wind-driven rain, a WRB can’t act as a drainage plane unless there is an air gap between the WRB and the siding to create a rainscreen. This can be done by installing vertical 1×4 strapping or by using a three-dimensional plastic mesh product like Cedar Breather or .
A third approach is to use wrinkled housewrap — a type of housewrap that has small vertical corrugations. Examples of such products include , , , , , , , , and .
In order to evaluate these products, one question arises: How thick does a corrugation need to be in order for water to drain? Unfortunately, building experts disagree on this issue, which awaits further research. “An air gap does not have be 3/4 inch,” says Mark Bomberg, editor of Journal of Thermal Envelope and Building Science. “I am quite happy with an air gap that is less than 1/8 inch. It does not ventilate, but it allows local drainage.”
StuccoWrap’s wrinkles are much shallower than 1/8 inch; in fact, they are less than 20 thousands of an inch high. Products that marry a three-dimensional plastic drainage mat with a conventional WRB — for example, EnkaBarrier and Home Slicker Plus Typar — are likely to provide better drainage than wrinkled housewraps like StuccoWrap, RainDrop, or Vortec.
It is possible to use foam sheathing as a water-resistive barrier (WRB). However, not all brands of rigid foam have been approved for this purpose.
Even if you choose a code-approved foam, you can run afoul of your local building inspector if you don’t follow strict fastening and seam-sealing details. Moreover, some building experts note that rigid foam can shrink, leading to worries that even excellent installations of rigid foam (using an approved foam and approved seam-sealing details) may not be durable enough to create a dependable WRB.
To learn more about the use of rigid foam as a WRB, see Using Rigid Foam As a Water-Resistive Barrier.
Liquid-applied WRBs come in a bucket and are applied to wall sheathing or concrete blocks with a roller or a spray rig. These products cure to form a tenacious, flexible coating that seals small cracks and penetrations.
Although liquid-applied WRBs cost more than housewrap, they also perform better. If air tightness is important to you, and your construction budget can handle the cost, a liquid-applied WRB may make sense.
For more information on liquid-applied WRBs, see Housewrap in a Can: Liquid-Applied WRBs.
Zip System sheathing
is a proprietary 7/16-inch thick OSB panel manufactured by Huber Engineered Woods. Huber advises builders using its Zip System sheathing to tape the seams between the panels with a special tape (Zip System tape), a 3¾-inch-wide polyolefin tape with an acrylic adhesive.
According to Huber, walls sheathed with properly taped Zip System panels do not require any housewrap; the taped panels function as a WRB. The International Code Council Evaluation Service has issued recognizing Zip System wall sheathing as an acceptable alternate to asphalt felt. The main advantage of Zip System sheathing, according to the manufacturer, is ease of installation: “Simply install the panels, tape the seams, and you have a complete structural wall system and a water-resistive barrier all-in-one.”
According to Michael Pyle, a Huber manager, Zip System panels are more water-resistant than ordinary OSB or AdvanTech OSB because they contain a “medium-density, phenolic-impregnated, kraft paper overlay” — in other words, an outer layer of plastic-impregnated paper.
Zip System wall sheathing can be used under most types of siding, including brick veneer. It can be used under wood siding, but only if the siding is back-primed or back-painted before installation. If used under cedar shingles, the manufacturer advises that the Zip System sheathing should first be covered with a layer of Cedar Breather or Home Slicker. If used under stucco, the Zip System sheathing must be covered by at least one layer of Grade D paper. Zip System tape cannot be installed in rainy weather, nor in weather colder than 20°F. Once the tape has been installed, it must be protected by siding within 180 days.
When it was first introduced to the market, Huber offered a warranty on their Zip System panels but refused to offer a warranty on the tape. However, Huber has recently improved their warranty, which now covers the tape as well as the sheathing panels for 30 years. There is a strange wrinkle in the warranty, however: the warranty specifically excludes coverage in a home that has “deterioration of the exterior roof or wall.” This exclusion is odd; taken literally (the way lawyers usually interpret such documents), it is a circular provision that should prevent Huber from honoring any warranty claims. Presumably, the only homeowners or builders who might need a warranty are those with deterioration of the exterior roof or wall. (Homeowner to warranty rep: “If my wall wasn’t deteriorated, I wouldn’t be calling you!”)
Builders who switch to Zip System wall sheathing will have to change some of their flashing details. Those who usually install conventional housewrap are accustomed to lapping the housewrap over window Z-flashing and other penetration flashings, so that gravity helps keep their walls dry. With Zip System wall sheathing, on the other hand, keeping the wall dry depends on chemistry — that is, on the adhesive component of Zip System tape and AC148 flashing tape.
John Straube, a professor of Building Envelope Science at the University of Waterloo in Ontario, is familiar with the Zip System components. “It’s a high quality tape,” Straube told me. “It’s better than any other tape I’ve seen before.” Yet Straube is reluctant to speculate on the longevity of the tape’s bond. “The quality of the installation depends on the adhesion and on workmanship,” he says. “At the end of the day, we don’t know how it will adhere in the long run. It is an unanswerable question.”
According to Straube, one of the Zip System’s best features is that “it is a wonderful air barrier.” As most builders of energy-efficient homes realize, that’s nothing to sneeze at. Many builders who appreciate this fact use Zip System sheathing as part of their air barrier — while still using housewrap, since housewrap can be lapped over window flashings and may be more dependable than Zip System tape.
One relatively new product, , differs in several important ways from most WRBs:
- Although Delta-Dry is vapor-impermeable, it still allows a wall to dry to the exterior.
- Delta-Dry is a barrier to inward solar vapor drive.
Although Delta-Dry can fulfill the role of a WRB, it is classified as a “rainscreen product” and has not obtained approval by the International Code Council Evaluation Service for use as a WRB. If you want to use Delta-Dry, you’ll probably have to use a housewrap behind it.
Delta-Dry is a membrane made of 22-mil high-density polyethylene. After a thermoforming process, the polyethylene is stiff enough to retain a three-dimensional egg-carton configuration. The total thickness of Delta-Dry is about ½ inch.
The manufacturer of Delta-Dry, Cosella-Dörken, has developed the product for installation over OSB or plywood sheathing. Because of Delta-Dry’s stiffness, the product creates two air spaces: a gap between the siding and the Delta-Dry, as well as a gap between the Delta-Dry and the wall sheathing.
While most housewraps depend on vapor diffusion to help walls dry, Delta-Dry depends upon air movement (ventilation) between the Delta-Dry and the wall sheathing. Although Delta-Dry is as vapor-impermeable as 6-mil poly, its grooves allow air to move behind it, carrying moisture away from the sheathing.
Like most WRBs, Delta-Dry protects wall sheathing from any wind-driven rain that sneaks past the siding. Unlike such conventional WRBs as asphalt felt or Tyvek, however, Delta-Dry has air channels that provide a capillary break and a rainscreen between the siding and the sheathing, facilitating the drainage of liquid water.
Delta-Dry also beats felt or Tyvek when it comes to handling solar-driven moisture; since Delta-Dry is vapor-impermeable, it is a total barrier to inward solar vapor drive.
Delta-Dry is easily cut with scissors or a utility knife. It must be fastened to the underlying sheathing every 12 to 16 inches in both directions, using ½-inch roofing nails or ¾-inch staples. Each roll of Delta-Dry measures 39 inches by 50 feet; horizontal seams should have a shingle-style overlap of 3 inches.
To be sure that damp sheathing has a chance to dry out, the top and bottom edges of the Delta-Dry must allow for air movement through the gaps behind the product. When Delta-Dry is installed behind stucco, ventilation holes at the bottom of the wall are assured by installing lengths of drainage track with weep holes. When installed behind brick, Delta-Dry is folded at the base of the wall so that it extends flush with the outer face of the brickwork.
There are several possible ways to detail the top of a wall. Perhaps the simplest detail is to extend the Delta-Dry membrane into the soffit. Alternatively, the siding installer can leave a ¾ -inch gap between the top of the siding and the bottom of the soffit. Of course, the ventilation gaps at the bottom and top of the wall must never be caulked.
Cosella-Dörken arranged for Delta-Dry to be tested by building scientist John Straube, who measured the performance of the new WRB in a lab, and by the Oak Ridge National Laboratory’s Achilles Karagiozis, who conducted computer modeling studies of the product. In both cases, the results look good. Straube and Karagiozis concluded that wind and sunlight can drive enough ventilation air through Delta-Dry’s channels to remove significant amounts of moisture from wood sheathing. Straube told me, “It does seem to work better than asphalt felt, especially behind highly absorbent claddings like stucco, brick, or cultured stone. It makes a lot of sense behind adhered veneers. The only caveat is that you need to let air get behind it — there could be problems if someone were to try to seal the air gap at the bottom of the wall.”
Delta-Dry costs about 70 cents per square foot ($114 per roll).
Are WRBs waterproof?
When it comes to resisting liquid water, liquid-applied WRBs and Delta-Dry appear to be the most waterproof, followed by non-perforated housewraps and asphalt felt. Grade D building paper and perforated housewraps trail far behind.
But most conventional WRBs — including most housewraps, asphalt felt, and Grade D paper — can’t keep water at bay for long. “Everyone seems to think of building paper as a moisture barrier,” said George Tsongas, a former professor of mechanical engineering at Portland State University. “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.”
Moisture problems in walls are best avoided by good wall design and proper flashing; the choice of WRB is a secondary concern. Regardless of which WRB you choose, including a ventilated rainscreen gap between your siding and your sheathing will go a long ways toward reducing any chance of wet-wall problems.
Portions of this article appeared in and in Energy Design Update.
Last week’s blog: “Questions and Answers About Air Barriers.”