Builders and consumers who want energy efficiency but don’t have the means or the desire to build with straw bale or adobe must rely on improvements to stick frame materials and methods. Structural insulated panels (SIPs), enclosure systems typically made of two engineered wood panels that enclose a rigid insulating foam core, may not seem like an obvious choice for environmentally conscious builders, but they’re widely praised for offering energy and resource efficiency, construction speed, and design flexibility.
Used in place of the traditional combination of studs, sheathing, and insulation found in standard construction, SIPs are suitable for walls, roofs, and floors. The first homes using SIPs were built in the early 1950s and are still occupied today. The industry has grown most rapidly in the last decade, however, and Bill Wachtler, executive director of the Structural Insulated Panel Association (SIPA), estimates that there are now more than 10,000 SIPs homes in the United States, and more than 30 manufacturers. International demand, particularly in Japan and Europe, is also driving the expansion.
Are SIPs for You?
SIPs lend themselves to simple, boxy design, but they also allow for design flexibility. Builders use SIPs for many different architectural styles; they work well for open floor plans because their strength and efficiency eliminates the need for excess interior walls.
Although SIPs products differ slightly, they all require similar construction techniques. Openings for windows and doors can be cut into the panel on site or pre-cut at the factory. Panel connection methods vary from engineered wood splines to steel joints, but all types are designed for rapid assembly, and panels are made with prerouted chases for electrical wires.
Proponents claim that careful planning to minimize waste makes a SIPs project as inexpensive as stick framing. If a builder works from plans designed specifically for panel efficiency and uses cutoffs from one part of the house elsewhere in the construction, costs will be much lower than dropping SIPs into plans for a traditional stick frame house. (Some companies will convert plans to accommodate panels, which they then fabricate.) For builders, the big advantage of SIPs is rapid, one-step construction of the shell, which provides savings that can trickle down to the homeowner.
SIPs are one of more than 100 environmental features in the Energy and Environmental Resource Center built by Atlanta’s Southface Energy Institute in 1996. “I’d like to see SIPs used more in production building, especially when windows and doors can be pre-cut at manufacture,” says Mike Barcik, director of technical services for Southface. “This is the most practical, efficient use of this technology.”
The downside to SIPs is that the number of contractors with experience using them is still limited. Although manufacturers claim their products are “builder-friendly,” Barcik says, “you might have to educate and train a crew.” But, says Wachtler of SIPA, “Every year it’s getting easier to find architects and builders familiar with SIPs. Interested builders and consumers can use membership in our association as a screening mechanism to find reputable manufacturers who are rigorous with testing and exceed building code specifications.”
SIPs products still lack a national product standard, and not all panels are tested for code compliance as a final unit. Financing can be jeopardized if all components of the final product aren’t compliant.
Manufacturer claims about R-value (resistance to thermal penetration) vary, and panels are available in different thicknesses and dimensions. In testing performed by the Oak Ridge National Laboratory, a four-inch SIPs wall showed better thermal performance than both two-by-four-inch stick and batt construction and two-by-six-inch construction. The same study showed that SIPs perform at approximately 97 percent of their stated R-value overall, losing only 3 percent to nail holes, seams, and splines. And when the Florida Solar Energy Project did a side-by-side comparison with stick frame construction in the early 1990s, the SIPs home outperformed the stick home by 30 percent, even though great care was taken to ensure tightness and energy efficiency in the stick home.
Barcik lauds the product chiefly for this reason. “We think it’s an excellent technology that combines continuous R-value with airtight construction,” he says. The key word is “continuous”—the lack of wood studs means no thermal “bridging” breaks up the material’s insulating effect.
Evaluating the Materials
Stick frame construction relies on old-growth dimensional lumber, while the oriented strand board (OSB) that forms the outer layers of SIPs is from new-growth wood that regenerates quickly. A SIPs house does use less wood overall, but how much less is debatable. In a demonstration conducted by the University of Oregon’s Center for Housing Innovation, the panel house used 55 percent less framing lumber than the reference house, but factoring in the OSB use meant the demo house actually used only 5 to10 percent less wood.
The use of petrochemical-based polystyrene is another environmental conundrum. All foam comes from nonrenewable fossil fuels, and their manufacture involves the use of toxic chemicals. But expanded polystyrene (EPS), the most commonly used foam in SIPs, is very stable once manufactured, has excellent insulative value, and does not contain ozone-depleting chlorofluorocarbons or hydrochlorofluorocarbons. “My unscientific view is that EPS insulation is a good use of petroleum,” says green builder Jonathan Meyer of Meyer Sons Builders in Edgewater, Maryland. “It’s long lasting and saves much more petroleum over its lifespan than the small amount it takes to produce it.” To further reduce the impact, Meyer Sons Builders ships clean foam scraps back to the manufacturer, where they’re reground and used in nonstructural roof insulation.
Air Quality Issues
SIPs homes are extremely tight, a feature both good and bad for indoor air quality. Although external pollutants won’t easily seep into the house, there’s not enough natural air exchange for good health and comfort. Mechanical ventilation and a dehumidifier are recommended.
OSB and waferboard (either may be used in SIPs) contain phenol formaldehyde, not the nastier urea formaldehyde, so the amount of formaldehyde emitted is less than 0.1 parts per million. The water-based adhesives and the EPS layer do not contain formaldehyde, and the panels are effectively sealed off from the interior by the sheathing layer and drywall. Chemically sensitive persons or those concerned about emissions from the phenol-formaldehyde glue in the OSB layers may use an aluminum foil diffusion retarder under the drywall.
Strength & Performance
Because their structure is analagous to an I-beam, panel walls are significantly stronger than stick construction walls. The foam core provides rigidity, and the engineered wood skins provide compressive and tensile strength. SIPs also offer excellent dimensional stability and have shown high resistance to earthquakes and hurricanes.
The panels’ durability is also a function of the exterior material that covers them. Any type of cladding or siding can be nailed directly to the panel walls; vapor barriers are not necessary. Although delamination was an issue early on, most products now carry a lifetime warranty against it.
Pest control issues give potential users pause, because polystyrene is a very attractive home for carpenter ants and other pests. R-Control panels have received positive reviews for the pioneering development of
EPS pretreated with nontoxic borate, and borate-treated OSB is also on the horizon.
Foam’s insulating qualities are also exploited in a similar enclosure system called insulated concrete forms (ICFs). These stay-in-place EPS forms are filled with concrete and used for walls and foundations. The Southface Energy Institute used ICFs for the foundation of its building. Barcik says that using ICFs above ground level gets expensive, “but it’s great for foundations and dugout basements.”
Some product developers are seeking alternatives to the panels’ foam component. The future of one product using compressed straw as an alternative core is uncertain because the company developing it went out of business. Leo Ojala, a SIPs sales and marketing veteran, has created Ingenuity Wood, which manufacturers a “No-Stress Panel System” from mineral wool insulation and plywood. “I was initially attracted to SIPs because of their excellent energy performance,” Ojala says, “but I don’t see foam-core SIPs as a durable, long-term product.” He worries about potential degradation of the OSB, especially if used on the roof. Ojala also takes issue with the environmental consequences of nonrenewable, petrochemical foam products. Ojala has chosen mineral wool rather than cellulose as the insulating core for his product because it won’t absorb water, and it’s naturally fireproof and pest-resistant. He prefers plywood because it “holds nails better than OSB” and doesn’t require the high-energy manufacturing process and heavy use of adhesive associated with OSB. The assemblies are screwed or nailed, techniques that are stronger and more reliable than adhesive-based SIPs, Ojala argues.
Ojala admits his product isn’t perfect. Mineral wool is an energy-intensive product, plywood is made from old growth wood, and his product doesn’t reduce the overall amount of wood used by stick frame construction. But he believes the product is a step in the right direction, and to offset concern Ojala plans to incorporate sustainably harvested wood as the company grows.
AFM R-Control Building Systems
Intermountain Building Panels
The Murus Company
Southface Energy Institute
Structural Insulated Panel Association
Gig Harbor, WA
Winter Panel Corporation