I have been working on a private residence on an island in Lake Winnipesaukee utilizing innovative structural insulated panels (SIPS). The system was developed by a company in Rhode Island for outfitting ships at sea, was subsequently adapted to provide demountable structures for the US military in Afghanistan, and is now being adapted further for home building.
The Winnipesaukee project is the prototype for the residential market, and the impetus for using the SIPS system was threedfold: the logistics of getting material to the island (all heavy material had to come by barge), a limited building season, and the desire for an extremely well insulated house.
For wall panels, the system starts with a block of rigid insulation, typically 4’ to 8’ wide and up to 20’ long. The thickness varies, depending on the desired R-value. Channels are precisely cut into opposite faces of the block with computer guided lasers to receive studs and nailers at 16” to 24” on center. The nailer and the stud are thermally separated by rigid insulation and tied together with a thru-bolt. (In this project the wall panels are 5-1/2” thick, with an R –value of 22. A typical 2×6 stud wall with the same insulation has an effective R-value about 20% less due to heat loss through the studs.) Additional channels are cut into the panel perpendicular to the nailers for wire raceways. Drywall and plywood can then be attached to interior and exterior faces of the panel prior to shipping.
For roof panels (R-40), the rafters are 1” deeper than the insulation to create a continuous air passage for a “cold roof.” Floor panels can also be provided where insulation is required—in the case of the Winnepesaukee house, the master bedroom is located over an unheated screen porch (R-32). After the panels are erected, foam insulation is injected into channels on the edges of abutting panels to eliminate air infiltration and create a tight thermal envelope.
This system has several advantages over traditional SIPS panels. First, by separating the interior nailer and the exterior stud, thermal bridging is eliminated. Second, because the system uses typical stick framing, the panels are easily modified in the field to accommodate dimensional adjustments, additional wall openings, or future additions.