a tale of two houses



Willow HouseHomes made with structural insulated panels (SIPs) are quietly energy efficient. If you're like most Architects, you probably can't even think about designing a new house without thinking about stud framing. Well, think again. My firm won't build another stud framed house if I can help it.
Light wood framing is still a great structural system. Critics misjudged it in the 1830s when they called it "balloon" framing, thinking it so light the wind might carry houses away. The cold wind blew right through them instead.

Insulation was soon inserted between the studs, but even today, every stud is a thermal bridge, and there will always be some uninsulated voids. Vapor barriers were added to keep condensation from destroying the insulation, then air barriers and seal strips were added to improve air tightness. A history of fixes. Over the last 160 years, the strength and resilience of the stud frame has been proven again and again, but energy conservation has always been an afterthought.

Now there is a better way to build a tight, well insulated envelope. "In just one step," asserts Cynthia Gardstein AIA, director of the Structural Insulated Panel Association, "panels provide structure, insulation, sheathing, and air tightness" Structural insulated panels (or 'SIPs'), are a stressed skin sandwich of rigid insulation bonded between two wood-fiber facings. SIPs are typically 4' x 8' in size, with nominal thicknesses of 3 1/2", 5 1/2", 7 1/2", 9 1/4" and 11 1/4", coordinating with conventional lumber dimensions. With SIP prefabrication, a small custom house can be erected, sheathed, insulated and sealed in a single day.

First House

I wasn't looking for an alternative to stud framing when I started to design a house near Woodstock, New York. The house had to deflect north winds and welcome the winter sun, but stay cool in the summer without air-conditioning.

We slowly developed a barrel roof shape which curved down near the ground on the north side, but exposed and shaded three stories of glass on the south side. But the owners wanted more insulation than rafters or studs could hold-R50 in the roof and R30 in the walls. How were we to frame it in wood?

An associate suggested structural insulated panels, and at first I was suspicious of their claimed r-values. The values were credible though, since the joints are far apart and the panels are factory made. Fewer joints also means that SIP houses have air-tightness superior to framed houses. The system doesn't usually need vapor barrier, roof vents, or housewrap. And the local building inspector was all for it. "We just had a panel house go up near here," he said, "I heard the owner played drums for Bob Dylan."

We also wanted to avoid roof trusses because we wanted that curving roof shape on the inside too. This led to another surprise: SIP spanning capabilities. A stressed skin panel develops strength like an I-beam does: the skins act like flanges, taking compressive and tensile loads, while the insulation works like an I-beam web, distributing shear forces and keeping the skins from buckling. Our SIP supplier cheerfully told us we were wasting our money on the 'tree-truss' which supports the midspan of the great room ceiling. Although the 12" roof SIPs could have easily spanned the 24' wide space, even with mountain snows, we put it in anyway.

The house proved to be warm, draft-free, and quietly solid. "Once people have selected panels," says Mike Tobin of AFM Inc., a partnership of SIP manufacturers, "they start to connect the dots, picking better windows and doors, heat recovery ventilation systems, and combined heating systems."

Second House

So when we got a call to design another energy efficient house in the Catskill mountains, we jumped at the chance to use SIP construction again. The house was designed to step down a beautiful mountainside, enclosing a rocky cliff within the center of the house. The panels enforced a simple geometric discipline: a monopitch followed the slope of the mountain, and inside, each ceiling had a gentle pitch. The framing plan was simple, and erection promised to be quick.

Although initially receptive to constuction with SIPs, the owner was concerned about the premium he was paying for this efficiency During pricing, the owner discovered he might have to relocate soon after the house was finished, and the house might have to suit future purchasers for whom the structural and insulating system of the house might be irrelevant. Since the contractor was a proud old framer with little experience in SIPs, he offered a 2% credit to frame the house with 2x6's instead, with half the insulation. Over my objections, we were commissioned to recalculate the structure, this time using stick framing. The house is now being finished, alas, without SIPs. We call it the Marilyn Monroe house, beautiful on the outside, but hurting inside.

To the SIP industry, it's a familiar pattern. "We do well with the quality-conscious 'step-up' homeowner," says Tobin, " but if the owner cares more about $400 faucets than what's inside the walls, he will stick with frame construction." Even though a SIP house can be very quickly erected, small contractors are not willing to bet their businesses on the promised time savings of SIP construction. "But this isn't rocket science," says SIPA's Gardstein, "generally erection savings are clear with the third house."

Using Sips

Though now only half of 1% of housing starts, SIP production is on a 30% per year growth curve, and will rise to claim a substantial portion of the market within a generation. SIPs are part of a 50 year old trend towards engineered wood products - plywood, roof trusses, wood I-beams - all of which are more consistent, straighter, made from younger 'farmed' trees, and more predictable under load and in fires than the conventional lumber they replace.

Most manufacturers have instructional videos, span tables, test certificates, and detail binders. Upon contract award manufacturers will provide signed and sealed panel shop drawings for approval, prior to fabrication, shipping and erection.

So a stick framed house is no longer the best house you can build. "The difference in construction quality is a revelation," says David Wright of Better Building Systems, a SIP prefabricator and erector based in Grass Valley, California, "its like the difference between site built and shop built cabinets." If you haven't started thinking about using SIPs for your next house, you're missing something.

In regions with low cost energy and labor, and in low cost or speculative housing, stud framing will remain common. You may not see SIPs in these sectors for a decade or more. But for high-end homes in high fuel cost or high labor cost areas, and wherever owners are interested in real quality, structural insulated panel construction is now the method of choice. But it's really only a matter of time: I believe that in most houses SIPs will soon replace stud framing.

Architect Colin M. Cathcart