Why Use a Spray-Applied Water-Resistive Barrier?
A liquid WRB won’t blow off, it goes on fast, and it's an economical way to make a house airtight and waterproof.
Synopsis: There are several ways to add a water-resistive barrier (WRB) to a house: attach housewrap to the sheathing, use sheathing panels with an integral WRB, or attach a fully adhered WRB to the sheathing. In this article, Andrew Hall describes the method his firm uses—a spray-applied product that not only provides water resistance but enables the builders he works with to achieve the airtightness requirements in the 2012 IRC with very little additional air-sealing. Hall provides an overview of how the product is applied first to seams and corners, and then to the field. Senior editor Patrick McCombe adds a brief analysis of the relative costs of the different methods of installing a WRB.
My company does a lot of work in and around Boston, an area that’s been undergoing a residential building boom for almost a decade. When I drive around, I routinely see single-family and multifamily homes under construction—with the housewrap flapping in the breeze. I often wonder if anybody will make repairs before the siding is installed. Even when the housewrap is securely fastened, I also regularly see reverse laps, missing sections, and sloppy corners that the siding subcontractor is unlikely to fix before he does his work. Even worse, the sider may simply cut off or tear off the flapping wrap so he can fit the siding, potentially dumping water on the bare sheathing until somebody notices years down the road.
My company started out doing concrete coatings in the late 1960s, and I’m the third generation to work in the family business. In addition to installing waterproofing and epoxy coatings on concrete and masonry, we’ve installed Tremco Barrier Solutions’ Enviro-Dri weatherproofing system since 2009. Enviro-Dri can only be installed by trained contractors. This vapor-permeable (12 perms) spray-applied water-resistive barrier (WRB) forms a continuous waterproof skin over the sheathing, greatly reducing air leakage and blocking water that gets through the siding. The asphalt-based coating will not blow off, it covers the sheathing nails to prevent water from following the nail holes into the framing, and it can be left exposed for up to 120 days. Although the system costs more than regular housewrap, our builders routinely meet the new airtightness requirements in the 2012 IRC with very little additional air-sealing. We can even return after the rough-ins to seal around penetrations, using flexible flashing that integrates into the spray-applied WRB. On many projects, we include a flashing that’s designed to go over the windows’ nailing fin. On this project, we will return to install the counterflashing above the windows after they’re installed because they are getting a special casing. Usually we install the counterflashing before the windows are installed. Other specialty flashings allow us to integrate brick ledges, intersecting roofs, and arches.
When my company first started offering this weather-barrier system, we tried applying it from buckets with extension rollers. Later we used pressure rollers fed from a small airless sprayer. Both methods are slow compared to the gas-powered Graco Gmax 5900 sprayer we use now. It’s also messier and more difficult to get the required thickness of 12 to 15 wet mils with a roller.
From Fine Homebuilding #280
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More about water-resistive barriers:
- UV-Resistant Housewrap
- Self-stick WRB: Renovating a 150-year-old house presents leaky challenges
- How to Install a Ventilated Rain Screen