As an architect, I’ve been using structural insulated panels (SIPs) since 1992, when they were relatively new. Since then, I’ve designed several hundred residential and commercial projects using SIPs. In my opinion, they are superior to conventional framing systems in almost every way — SIPs are simply better insulated, stronger and faster to build with than standard stick-frame construction. SIPs also help conserve forest resources, because they produce almost no waste.
As worldwide timber quality and availability continue to decline and the costs of labor and energy increase, SIP construction should become more popular and cost-effective. I predict that in the next 10 years, the U.S. construction industry will adopt SIPs as the system of choice.
Advantages of SIPs
Structural insulated panels are typically composed of rigid foam insulation sandwiched between two skins of oriented strand board — kind of like a s’more. The exterior skins are attached to the foam core with a high-strength adhesive.
The foam core material is often expanded polystyrene — the same material used in ice chests and shipping “peanuts.” Some manufacturers use polyurethane or isocyanate foam cores. The skins can be plywood, metal or other rigid sheet materials, but oriented strand board is used by the majority of SIP manufacturers. The foam core serves as a spacer and insulator between the exterior skins. (Keep reading for more specifics about these materials, and their environmental impacts.)
Altogether, the structure acts as a monolithic whole — as opposed to stick-frame construction using 2-by-4s, where hundreds of individual nailed connections hold the studs and skins together. As a result, SIPs are twice as strong as a wood-framed house, which is a real advantage in locations that experience tornadoes or hurricane-force winds.
SIPs are also extremely energy efficient. Compared to a typical stick-frame house, a house built of SIPs will require about half the energy to heat and cool throughout the year in most climates. It’s not just the thickness of the polystyrene inside that makes SIPs more energy efficient — it’s that the whole panel is designed to function as one structural unit. The thermal “tightness” and resistance to heat flow work with the insulation value to achieve exceptional comfort and energy efficiency.
Cost and Savings
I have heard claims that SIPs are “cheaper than stick-frame or the same cost as conventional systems.” That may be true in some parts of the county where labor cost is high or skilled labor is unavailable. In my experience, using SIPs usually costs slightly more than stick framing, adding about 5 to 15 percent to the total cost of the home. (The framing, insulation and associated labor is usually only about 10 to 15 percent of the total cost of a new home.)
However, SIP construction is a better value than conventional wood-frame construction. When properly built, SIP buildings will be straighter, truer and smoother, which makes it easier to install the doors, windows, siding and sheet rock. The energy savings, structural stability, speed of construction and labor savings will pay for the extra cost of SIPs. In most cases, these savings will mean you’ll recover the extra costs in two to four years, depending on your location.
If you decide to build with SIPs, start by looking for qualified builders on the Structural Insulated Panel Association website. Or, do an online search with your state name, plus “SIPs.” If you plan to build your own home, consider looking for an experienced SIP installer to help you and your crew.
SIP building is actually easier than stick framing if you go about it carefully. I have introduced several professional contractors to SIP building who are now so enthusiastic about the quality and technology that they no longer want to build stick-frame houses.
Here are the basics of how a SIP home comes together. SIPs come in 48- or 96-inch wide panels that can be up to 24 feet long. The builder will use shop drawings, aka “panel drawings,” as a map to determine all construction details. These panel drawings delineate each roof and wall plane (yes, SIPs can also be used for roofs), giving each panel a discrete identification number. The architect and contractor each review and sign off on these drawings before the panels are ordered.
SIPs are manufactured in a large regional plant. They also need to be “fabricated,” which is the special cutting and additional preparation specific to a certain building. A manufacturer can provide panel drawings and fabrication, but fabrication is a secondary service. There are also contractors who provide panel drawings and fabrication in addition to panel construction.
A SIP building that has been fabricated in a shop before arrival at the construction site takes about a quarter of the time to frame as a SIP building fabricated on-site. Fabrication inside a shop allows continuous production unaffected by rain, snow, heat and windy site conditions. Plus, foam scraps from fabrication can readily be recycled.
The placement of SIPs is called panel erection or installation. Installing SIPs is not difficult. After the concrete floor slab or wood floor platform is completed, the process goes quickly through the finished structural shell stage. First, the builders attach a sill plate to a slab with anchor bolts.
Window, door and skylight openings can be cut into panels before installing them. The wall panels are usually joined with a proprietary system, unique to the manufacturer. Handling and installing the panels requires two people. The panel being placed slips into the previously set panel and over the sill plate. A caulking gun is used to place a bead of caulk/adhesive mastic on all joining surfaces. After the panel is set in place, it is common to place a few screws using a cordless screw gun, one at each joint to tack the panels in place. Exterior or interior wall braces help stabilize and align the panel wall until the top plate is installed and a corner connection completed.
SIPs are meant to be installed with an eighth-inch joint tolerance, and there should be a slight space between each panel for expansion and contraction. When the wall panels are up, the builder caulks and inserts the continuous top plate. This helps true-up and stabilize the wall. Later, when the walls are up, a nail gun is used to “nail-off” each joint on each side with 8-penny nails at 6 inches on center.
SIPs and Green Building
Many environmental experts consider SIPs to be a green product. SIP construction reduces wood waste, and the thermal properties of SIPs provide a long-term investment in energy savings and conservation. However, there is a healthy ongoing discussion about some of the chemicals used in SIPs.
One substance of concern is pentane gas, which is a low-level pollutant that is used in forming the expanded polystyrene. Manufacturers are required by law to take extra caution to contain the pentanes and other gases used in manufacturing, preventing them from releasing into the environment.
Another concern is that SIPs that use expanded polystyrene are treated with a flame retardant called HBCD (Hexabromocyclododecane) to prevent flammability. HBCD is toxic to aquatic organisms, and the potential health effects on humans are not yet clear. However, very little of this chemical will enter the environment, because it is enclosed in the insulation. (By definition, expanded polystyrene is closed-cell.) Outgassing is not a problem, as it is with some other foams.
Once the SIPs have been manufactured they are considered environmentally safe by the Environmental Protection Agency, and once they are enclosed in the walls and roof there is no danger to the environment. Personally, I think SIPs pose little risk to the consumer and are definitely a green product. However, I’m pleased by any dialogue between manufacturers of building products and the general public, because we all benefit when consumers pay attention to the environmental effects of the products they use.
I think that all materials used in construction should be sustainable and non-resource depleting. SIPs go a long way toward saving energy resources in a cost-effective way, and in my opinion they are an outstanding choice for green buildings. Someday soon I expect that rigid foam plastic will be made from soy or other organic agricultural products. There is also room for oriented strand board to become greener. For example, it can be manufactured from sustainably harvested wood chips and perhaps other natural products, such as rice hulls or hemp.
The Future of SIPs
The construction industry is sometimes slow to change and adopt new technologies. However, as interest in sustainability and energy efficiency continues to grow, I expect that not only will the use of SIPs continue to increase, but that the product will be manufactured with ever more environmentally friendly materials. I look forward to SIP systems becoming a major player in the search for more energy independence and the quest for zero energy buildings.
Building With Structural Insulated Panels by Michael Morley
David Wright, architect with Solar Environmental Architecture Group, is the author of The Passive Solar Primer: Sustainable Architecture. Wright was first featured in MOTHER EARTH NEWS in a Plowboy Interview in 1977.