Fully Framed in a Day

Structural insulated panels have many advantages in today's building world, key among them that they go up fast, withstand more stress than stick-frame construction, and conserve energy. That's all due to the way they're constructed, like giant ice cream sandwiches with two "wafers" of 7/16-inch oriented strand board (OSB) around 3 to 12 inches of expanded polystyrene foam (also known as beadboard), all of it bonded together with special glue.

It seems hard to believe that walls with no studs could be strong, but in fact these are three times stronger than conventional construction, according to Jim LeRoy of PanelPros, the Northeast installer for SIPs made by Insulspan, which manufactured the ones used in the Carlisle project. "The panels form a skin that's very much like an airplane fuselage," says LeRoy.

Those panels are fabricated using software that translates architectural drawings into machine codes, which in turn lay out panel sizes down to the fraction of an inch. "It's a one-day job to manufacture them," says Insulspan founder Frank Baker. "Contractors often wonder how long they'll have to wait for the panels, but usually we're ready before they are."

Speed and strength are just part of the SIP equation. Because SIP walls have no studs, as long as they're properly sealed there's no place for heat and cold to seep through. "It's a total energy envelope," says Baker. So while an SIP's R-value — the standard measure of thermal resistance — is already higher than that for fiberglass batts (a 6-inch SIP wall is rated at R-24 to R-27, versus an R-19 for a 6-inch stud wall with fiberglass), the overall performance is even greater. According to the U.S. Department of Energy, SIP construction can result in a 12 to 17 percent saving in energy costs, though tests currently under way at the Oak Ridge National Laboratory suggest that that saving may be even higher. A home built with SIP walls generally calls for smaller, less expensive heating and cooling appliances, notes TOH plumbing and heating expert Richard Trethewey.

However, because SIP homes are so tight, Richard recommends installing an air exchanger to cut down on the moisture trapped in the house. This added expense offsets the energy saving in the first few years. Moisture has also proven to be a problem with SIP roofs in colder climates. If the joints are not properly sealed, escaping moisture vapor can get trapped beneath the roofing, eventually rotting the OSB. That's why when working with SIPs Tom takes no chances. On a simple gable roof, he ventilates by nailing furring strips to the OSB and adding another layer of 1/2-inch exterior-grade sheathing before putting on the felt and shingles. "This keeps the underside of the roof shingles cooler and the top side of the panels dry," says Tom.

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