great walls

Home Slicker is a three-dimensional nylon matrix that provides a space for drainage behind cladding. It also creates a thermal break and pressure equalization, which helps eliminate trapped moisture.

Credit: Benjamin Obdyke

John Dennis Murphey, AIA, used to specify his exterior walls the way everyone else did—with 2x4 studs, fiberglass insulation, sheathing, and so on—but very little about his walls is the same anymore. Today he designs high-performing exterior walls that are better because the highly variable climate in metropolitan Washington, D.C., demands it.

“We're in an area where you don't know what it's going to be outside,” says the principal of Chevy Chase, Md.-based Meditch Murphey Architects. “We're in the middle [of the Atlantic Coast], so it could be cold, rainy, or hot and humid.”

The building envelope is perhaps the least sexy topic to discuss with a client, but few areas of a house are more important. As the first line of defense against the elements, the wall (and roof) is of utmost importance, but many design professionals overlook elements that could have potentially damaging consequences.

“Not being careful about keeping the walls watertight—that's, by far, the single biggest error” design professionals make, says architect Peter L. Pfeiffer, FAIA, of Barley & Pfeiffer Architects in Austin, Texas. “It's not just another thing you have to do; [keeping the moisture out] is one of the most important things you have to do.” But it's more than a matter of moisture protection, says Pfeiffer, who's also a building scientist and green design consultant. Architects must design for “total wall efficiency” based on their regional climate.

“If you're in a very cold climate, having high R-value is clearly going to benefit you,” Pfeiffer says. “Having something like a solar radiation barrier probably isn't going to be all that important, but a good air-sealed wall is very important.” And because almost all homeowners use air conditioning in the summer, the house has to be tight and efficient to contain the cooling, he adds.

Galen Ohmart, AIA, LEED AP, has developed specs that work well for his rainy Oregon climate. “I like ICF, aerated concrete, and Faswall [by Philomath, Ore.-based ShelterWorks], which is made with wood fiber and concrete,” says the principal of SOLARC Architecture and Engineering in Eugene and Portland. If Ohmart is doing wood framing, he thermally separates the inside wall from the outside. “I like using staggered-stud framing with 2x4s on a 2x6 or 2x8 plate,” he says. “Then I prefer sprayed foam insulation, which really fills the cavity between the studs.” The staggered method, he says, breaks thermal bridging because studs never touch the inside wall. If cost is an issue, he foams the first 2 inches to seal the building, then uses fiberglass for the rest of the cavity.

CommercialWrap is 50 percent heavier than the manufacturer’s standard residential product, so it has good tear strength and allows up to 270 days of UV exposure. Architect Peter L. Pfeiffer uses it on his houses in and around Austin, Texas.

Angela M. Dean, AIA, LEED AP, principal of AMD Architecture in Salt Lake City, says she optimizes passive solar for her mild climate. She also specifies ICF for basements and Faswall or structural insulated panels for the main structure. “It's going well, but it's not the least expensive option,” she says, though it allows the wall to breathe. Dean is also a fan of 2x6 framing with blown-in foam or cellulose insulation to cut thermal bridging. “By code, you can still get away with 2x4 framing, but it's not a good idea,” she says.

Washington's heat and humidity, and its moderately cold temperatures, mean Meditch Murphey's walls must be highly insulated and high-performing, which is why Murphey keeps refining his construction methods. In the 1980s he switched to 2x6 studs, and about a year ago he started using closed-cell foam insulation. “We like that [foam] is a moisture barrier and a smart barrier,” he says. “It works both ways.”

Like many architects, Murphey favors closed-cell foam because it offers high insulation value and tightly seals wall cavities. “Leaks bring in humidity, and humidity brings in lawyers,” he says. He's doing more rainscreens, which he says “allow the building to dry out, and, in theory, stop the wind and reduce the pressure differential.” He also has adopted a process that eliminates traditional sheathing. “We use metal bracing to stiffen the studs and 2-inch rigid foam board outside, and then we apply the housewrap,” he explains.

The technique, sometimes referred to as “outsulation,” is becoming popular with building scientists. “The concept is that you're not just putting insulation between the structural members, but also around the structural members to protect the building from being exposed to the outside,” Pfeiffer explains. “It cuts down on thermal conductivity.”

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

Credit: Georgia-Pacific

“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.”