Home construction is a conservative business in which tradesmen are notoriously reluctant to try to new technology. No wonder so many pros toss around terms like “tried and true construction practices.” But some customs, such as siding installation, could use some updating.
The traditional installation—boards pressed tightly against the building to keep out moisture—has been around for many years, but we now know this practice isn’t always best. As building scientist Joseph Lstiburek has written, “People continue to put their faith in every kind of cladding material, but in the real world all claddings leak sooner or later. They always have, and they always will.” Rather than fight this inevitability, more architects are adopting a different approach: the rainscreen.
“Rainscreens shed most of the rain and manage the rest, preventing moisture intrusion and the resulting premature decay in homes,” according to ToolBase Services, the technical information arm of the NAHB Research Center in Upper Marlboro, Md. “Rather than attacking the symptoms of moisture intrusion, rainscreens tackle the source—the forces that drive water into the building shell. By neutralizing these forces, rainscreens can withstand extreme environments.”
A well-managed wall, Lstiburek frequently says, includes a drainage plane; air space between the wall cladding and the drainage plane; flashing at every opening or penetration to kick water out and down; and vented or porous cladding. This technology has been used on European buildings for many years and on commercial projects in the United States, and now its use is growing among residential practitioners.
Architect Jeff Jordan had detailed commercial projects with ready-made rainscreen systems, but he didn’t specify his first residential project until the Higgins Lake House in Michigan. “One of the reasons we used a rainscreen was the wood [siding] itself,” says Jordan, of Jeff Jordan Architects in Jersey City, N.J. “Our concern was that because the wood is so pricey, we didn’t want it to degrade over time.” The rainscreen allows the boards to dry faster, preventing premature failure, he says. “Separating the wood from the wall also has a thermal benefit. It gave us a thermal break, so the temperature differential helps from a cooling standpoint.”
Rainscreens are indeed effective, but knowing when to use them is key. They are more effective in regions that receive at least 20 inches of rain per year, and though they can be used with any cladding material, wood (especially painted wood) is an excellent candidate. “Clapboard rainscreens have been recommended for decades to mitigate moisture problems in wood walls,” ToolBase states on its website. “In this approach, siding is typically installed on vertical furring strips (commonly wooden 1x2s) attached to studs. Wood claddings are then back-sealed with a breathable, moisture-resistant, penetrating sealer.”
Architect Paul Masi, AIA, beautifully demonstrated this detail on the Roaman Residence in Amagasett, N.Y. The ground level of the home is wrapped in custom-milled rainscreen siding; the top edge of each board is beveled to promote drainage. “The air circulation helps the boards dry evenly, which helps hold paint and stain better,” says the principal of Sag Harbor, N.Y.–based Bates Masi + ARCHITECTS.
Though relatively straightforward, rainscreens require some thought. “The most important part is the weather barrier,” says Jordan, who specified products from Gig Harbor, Wash.–based VaproShield on the Higgins Lake House. “You need to do a really good job installing the tape and the membrane.” Jordan also specified 1x2 pressure-treated furring strips and Western red cedar boards with ¼-inch spacing.
Another concern is money. There are added material costs for furring strips and labor for painting or priming, but expense also varies based on the type of weather barrier, furring strips, and cladding. William T. Ruhl, AIA, of Ruhl Walker Architects in Boston—who typically details his rainscreens with VaproShield or Delta Rainscreen Systems from Cosella-Dorken Products—says, for example, that you can use AZEK PVC trim boards for battens, but they cost more than pressure-treated wood, which in turn costs more than regular wood.
Architects simply vary their assembly to meet budget. Regular housewrap is an option, but UV radiation could penetrate the spaces between the cladding and break down the product. Others say a double layer of 30-pound felt might be adequate. Another architect says Home Slicker from Horsham, Pa.–based Benjamin Obdyke might be enough to create an air space that vents the siding. “Sometimes the installation costs the same,” Jordan says. “Because there is space [between the cladding], you actually use less wood.”
Ultimately, architects must evaluate each project to determine if a rainscreen is appropriate. “There must be practical considerations,” Ruhl says. “There has to be a reason for doing things.”