©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
When it’s time to cover wall 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. with 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. (WRB), most residential builders choose plastic housewrap, asphalt felt, 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., or rigid foam sheathing. Some commercial builders, however, choose a fifth option: a liquid-applied building wrap.
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. Once cured, these rubbery coatings have a major advantage over housewrap or asphalt felt: they provide a very high level of airtightness. While the cured films are highly resistant to water penetration, they are fairly permeable to water vapor. In other words, a wall coated with a liquid-applied WRB can still dry to the exterior.
For builders who prefer to establish a home’s air barrier at the sheathing layer , liquid-applied WRBs are a good alternative to Zip System sheathing.
The Sto Corporation was the first manufacturer to develop a liquid-applied WRB (StoGuard) that is vapor-permeable. Released in 2000, StoGuard remains the market leader. Sto’s competitors include W. R. Grace (the manufacturer of Perm-A-Barrier VP), Henry Company (the manufacturer of Air Bloc 31), DuPont Tyvek (the manufacturer of the Fluid Applied WB System), and Tremco (the manufacturer of Enviro-Dri). GBA's review of Enviro-Dri  was published in July 2012.
All About Wall Rot 
I'll take an in-depth look at two of these products — the ones manufactured by Sto and DuPont.
According to the ICC Evaluation Service , StoGuard is an acceptable alternative to water-resistive barriers specified in the International codes. (Manufacturers interesting in getting code approval for a liquid-applied WRB must submit evidence to the ICC-ES that their product complies with AC212, Acceptance Criteria for Water-Resistive Coatings Used as Water-Resistive Barriers Over Exterior Sheathing .)
While StoGuard is usually used under stucco, brick, or exterior insulation and finish systems (EIFS), residential builders aiming for improved airtightness have learned that it can be used under any type of siding.
Sto makes two separate lines of StoGuard systems. “StoGuard with Gold Coat” is designed for use under EIFS, while “StoGuard with EmeraldCoat” is designed for use under cementitious stucco. “Stucco requires a different product, because you don’t want stucco to bond with the waterproof barrier underneath,” explained Lisa Petsko, Sto’s product manager for StoGuard.
Under brick, fiber-cement, or wood lap siding, either the Gold Coat or EmeraldCoat system can be used; however, Sto usually advises builders to choose StoGuard with EmeraldCoat for all siding types except EIFS.
Gold Coat and EmeraldCoat can be installed over plywood, OSB, or gypsum sheathing. For builders using the Gold Coat system, wall sheathing joints must first covered with 4-inch-wide self-adhesive fiberglass tape called Sto Mesh. Wider (9-inch-wide) fiberglass tape is then installed around the perimeter of all rough openings, around outside corners, and at inside corner joints.
The lengths of fiberglass tape are covered with a liquid product called Gold Fill, applied with a trowel or a texture sprayer. Sheathing fastener heads must also be spot-coated with Sto Gold Fill.
Gold Fill cures in about four hours. At that point the entire surface of the building’s wall sheathing can be treated with Sto Gold Coat, which is applied with a roller or sprayer. (Gold Coat has a thinner consistency than Gold Fill.)
The EmeraldCoat system is similar to the Gold Coat system. Most EmeraldCoat installers use StoGuard Fabric tape — a polyester tape — instead of the fiberglass Sto Mesh tape.
Contractors familiar with StoGuard praise the product’s durability. “It’s a thick coating, and once it is applied, there is no way to take it off,” said Ernesto Medina, owner of Stucco and Masonry Renovators in Dallas, Georgia. “It will not peel away.” The product’s tenacity will frustrate contractors who get it in their hair.
StoGuard provides some ability for a wall to dry to the exterior. Since it is a Class III vapor retarder (according to definitions enshrined in 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. and IECC International Energy Conservation Code.), it falls into the same permeance category as latex paint. One coat of GoldCoat has a permeance of 7.7 perms, while two coats have a rating of 3.1 perms. The corresponding figures for EmeraldCoat are 8.5 perms and 3.5 perms.
Sto promotes the StoGuard system for use under any type of siding. According to Petsko, there is no reason that a 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. application — using either vertical strapping or a three-dimensional plastic mesh product — can’t be used over StoGuard.
At the Waldsee BioHaus  project in Minnesota — the first certified Passivhaus building in the U.S. — StoGuard was credited with helping the building achieve an air leakage rate under 0.6 ACH at 50 Pascals.
StoGuard costs more than WRBs that come in a roll. “To do StoGuard right is not difficult, but it is tedious,” said Medina. “It takes at least 20 minutes for each window. If I wanted to just spray it on, I could be done with a complete house in one day. That would just be like installing a conventional housewrap by running it over the window openings, cutting an X, and folding back the flaps, and then calling it done. Most guys charge about 15 or 20 cents a square foot for a job like that. But a careful contractor would charge maybe 40 cents a square foot to do housewrap properly. I charge about 60 or 65 cents for StoGuard.”
Sales of StoGuard materials are not limited to licensed EIFS dealers; Sto Corporation will sell the materials to any contractor.
DuPont’s new WRB, called the DuPont Tyvek Fluid Applied WB (Weather Barrier) System, was just launched this month. Currently available in Texas, the product will be introduced in other areas of the U.S. over the next few months.
The DuPont system is very similar to the StoGuard system. Each gallon of DuPont’s liquid WRB covers between 56 and 64 square feet. The cured product is considerably more permeable than StoGuard; a 25-mil coat has a permeance of 25 perms.
The DuPont system (like the Sto system) includes a crack-covering and seam-bridging product (Fluid Applied Flashing and Joint Compound) designed for use with fiberglass tape. Another product, Sealant for Fluid Applied System, is used to seal around windows, doors, and penetrations.
DuPont’s liquid-applied WRB is suitable for use on concrete block, OSB, or plywood. The temperature of the wall should be between 25°F and 100°F. According to DuPont, the product can be applied to a damp wall. “It is extremely elastic,” said Kerry Shea, a marketing manager for Tyvek. “It stretches and recovers, so it is able to move with your building.”
The liquid is applied to a wall with a sprayer, a trowel, or a roller. (The manufacturer advises using a Graco Pressure Roller — a roller connected to a compressor that delivers the liquid coating directly to the roller.)
If you are unable to order one of these products through your local building supply yard, you should contact the manufacturer to obtain the name of a local distributor.
DuPont Building Innovations 
Henry Co. 
Sto Corporation 
W. R. Grace Co. 
Liquid-applied WRBs are not goof-proof. Perhaps the trickiest aspect of the application is obtaining the perfect product thickness. The idea is to install a single coat at a thickness of 25 mils. Achieving a “Goldilocks” application — not too thick and not too thin — is a matter of skill and experience. The manufacturer advises, “Thickness should be controlled by applying the appropriate volume over a marked area and by spot-checking with a wet mil gauge.”
When the product is used behind stucco or stone veneer, DuPont notes that the WRB “should be separated from the stucco [or stone veneer] by an intervening layer. Tyvek Fluid Applied WB serves as the wall system’s weather barrier and is integrated with window and door flashings, the weep screed at the bottom of the wall and any through-wall flashings or expansion joints. Lath shall be installed over the intervening layer.”
Cleanup requires a citrus-based cleaner or mineral spirits.
After the WRB is applied, contractors are advised to wait at least 24 hours (but no more than 9 months) before installing siding.
According to DuPont, its Fluid Applied WB System has been tested to several standards, with the following results:
Liquid-applied WRBs perform better than other available housewraps, and they do a great job of air sealing. Their only disadvantage is that they cost more than more conventional WRBs. If air tightness is important to you, and your construction budget can handle the cost, a liquid-applied WRB makes a lot of sense.
Last week’s blog: “Testing a Thirty-Year-Old Photovoltaic Module”