Because the shell is so important, manufacturers are developing a variety of products to help keep houses dry; some are more effective than others. Pfeiffer says products such as Home Slicker from Horsham, Pa.-based Benjamin Obdyke, for example, effectively promote draining in the exterior wall, while radiant barriers block summer heat gain and prevent winter heat loss. But one-step sheathing products that have housewrap already attached are less reliable. Indeed, it's hard to keep track of the number of housewraps on the market. Product lines such as Tyvek from Wilmington, Del.-based DuPont; GreenGuard from Pactiv Corp. in Lake Forest, Ill.; and Typar from Old Hickory, Tenn.-based Fiberweb are now standard.
This “drainable/dryable” wall assembly from Georgia-Pacific is designed to air out after flooding. Substituting closed-cell foam for rigid foam board, Louisiana State University’s AgCenter used a version of the wall for Crescent House—a project that’s par
“Housewrap performs two basic functions in a wall system,” says Laura Dwyer, residential market manager for DuPont's building envelope business. “It prevents water from soaking the wood system, and it's important for energy efficiency.” But not all housewraps qualify as air barriers, she warns. “A lot of products don't prevent air from passing through.”
“Tyvek makes a lot of sense—especially up north,” Pfeiffer adds. “If there's any moisture vapor that migrates into the wall from inside the house, it can eventually evaporate, because it has that breathability.” In a hot, humid climate such as Houston or Austin, however, you want to keep the humidity out of the building and out of the wall system, so their breathability goes both ways. In his Austin climate, Pfeiffer opts for commercial-grade Tyvek, which he says is much less vapor-permeable. But the point could be moot anyway. “Really, I'm looking for the housewrap to be a bulk water-shedder,” he explains. “It's a raincoat. I'm really going to rely on something like spray foam insulation to keep the infiltration and the vapor transmission to a minimum.”
Some architects eschew housewrap, saying old-fashioned methods work just fine. Ohmart's firm, for example, uses felt instead. “Despite what people say, the felt won't leak water, and it allows moisture to move out of the siding,” he says. Unfortunately, clients care less about the walls than they do the countertops or drapes. “Walls are usually the place where you put lots of money, but you don't see it and your clients don't see it,” Dean says. “But when we take the time to explain the benefits, they take the extra step and go with the better wall.”