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Horizontal exterior sidings like wood clapboards, vinyl, and fiber cement have traditionally been nailed directly to the underlying sheathing, typically over a water-resistant underlayment of asphalt felt or some sort of housewrap. That’s still common practice, but over the past few decades, a more labor-intensive technique known as rain-screen siding has been steadily gaining ground.

Reportedly first recommended by Canada’s National Research Council in the early 1960s, the rain-screen approach involves fastening the siding to vertical furring strips, which are usually spaced to correspond with the wall studs. The resulting vertical channels are usually covered with insect screening at top and bottom but are otherwise left open to permit free passage of air. As in a conventional siding application, the OSB or plywood beneath should first be covered with a properly flashed and lapped layer of felt or housewrap.

Draining and drying. According to rain-screen advocates, the open channels tend to reduce wind pressure against the housewrap or felt, making it less likely that wind-driven rain will penetrate imperfections in the housewrap. (In fact, rain-screen siding is sometimes described as “pressure-equalized siding.”)

Any moisture that does drive through the siding drains away quickly, while the air space behind the siding promotes rapid drying. That, in turn, may help prevent cosmetic problems — such as peeling paint — and mold and wood rot, which can develop when moisture works its way past the sheathing and into the wall cavity.

The rain-screen approach has long been embraced by engineers and building scientists. Among them is the Massachusetts-based building scientist Joe Lstiburek, who makes a strong case for a 3/8-inch vented space behind siding in a recent issue of ASHRAE Journal. (That article, “Mind the Gap, Eh” is available as a PDF at bookstore.ashrae.biz/journal/journal_s_article.php?articleID=987). Such support has helped push rain-screen siding into the mainstream; recently it became a requirement under some U.S. and Canadian building codes. Builders in coastal areas of British Columbia, for example, have been required to provide a vented 10-mm (3/8-inch) air space beneath exterior siding for several years now. And since this past January, Oregon has required contractors to provide a minimum 1/8-inch drainage space beneath siding, although the provision can be met without a true rain-screen installation.

Code-based solution. The impetus for Oregon’s drainable-siding provision dates back to a large number of moisture-induced building failures in the state during the late ’90s and early 2000s. Many resulted from water penetration of synthetic stucco, but various forms of coursed siding were also involved. When the scope of the problem began to make it difficult for contractors to find affordable liability insurance, says Oregon Building Codes Division official Richard Rogers, the state legislature pressed building officials to come up with a code-based solution.

At a series of meetings in 2008 and 2009, code officials and the state’s Residential Structures Board — a nine-member body that includes four residential contractors — hammered out the final version of an amendment to section R703.1 of the Oregon Residential Specialty Code. To comply, builders must “provide the building with a weather-resistant exterior wall envelope and a means of draining water that enters the assembly from the exterior.” That requirement can be met either by furring out the siding or by using an approved drainage material. To be permitted under the revised code, drainage wraps must pass an ASTM E2273 test (originally developed for water-resistant barriers used under synthetic stucco) that requires that they drain 75 percent of the water introduced to a test assembly within a specified time. Among the products that have so far demonstrated compliance with the ASTM test, Rogers told JLC, are Tyvek DrainWrap, Greenguard RainDrop, Valeron Vortec, HomeGuard HP Plus, Benjamin Obdyke Home Slicker, and HydroTex.

Building science vs. field experience. Because true rain-screen siding is labor-intensive and costly — it adds 25 percent to 30 percent to the cost of a typical siding job, according to several builders who have used the approach — code officials portray the stucco-wrap option as builder-friendly. “You have to cover the sheathing with some kind of water-resistant barrier anyway,” says Rogers, “so the added cost is only the difference between a standard housewrap and a drain wrap.”

In effect, the new code provision assumes that some drainage is always going to be better than no drainage. But because years of field experience have shown that siding applied with no provision for drainage can also perform well, the new requirement has generated quite a bit of controversy. Skeptics abound along the Oregon coast, where heavy rains are frequent and sustained winds can top 70 mph.