SIPs FAQs
Q: What is a SIP?
SIP stands for Structural Insulated Panel. Most SIPs consist of two layers of wood sheathing laminated to a foam core. Some SIPs come with other facing materials: metal, drywall, fiberboard, even plastic. When installed in a home, the SIP provides both structure and insulation. While most commonly used in walls, SIPs can also serve as load-bearing floor, ceiling and even foundation components. Most SIPS are designed carry the entire range of structural loads in a building; however, SIPs with one drywall facing are just used as cladding panels around timberframe structures. (Note: A related category of SIPs doesn't use facing materials; they use either wood or steel framing members placed between or within tall blocks of foam.)
Q: What gives a SIP its strength?
The resulting sandwich panel product acts like an engineered I-beam, resisting both compressive forces from above and buckling forces from the side. Engineering tests show that, depending on the type of structural test being administered, SIPs range between two and seven times stronger than traditional framing. In some SIPs, integral studs used as splines for joining panels together also help carry the structural load.
Q: Are the high efficiency claims valid?
Four key factors make the average SIP home very energy efficient. First, foam provides higher insulating value per inch than traditional fiberglass insulation. Second, there is much less wood framing within the typical panel, which again increases total R-value. Third, the foam is continuous and is not susceptible to the commonplace flaws found in average batt installations. Fourth, foam doesn't allow air infiltration around it; with reasonable attention to detail, panel joints can be sealed to provide house tightness that averages between 20% and 40% (for house with wall and ceiling panels).
Q: Does the foam lose R-value over time?
Urethane foam stabilizes at a relatively high R-value (about R-7 per inch) once it is placed in a home's wall and ceiling system; at that point, the R-value of a fully cured urethane panel is about twice that of a fiberglass wall. Expanded polystyrene insulation (EPS) used in most panels offers lower R-value (about R-4 per inch) but maintains its R-value once the panel arrives at the job site.
Q: Are SIP homes too tight for good indoor health?
No. Research indicates that the best way to provide a home with good indoor air quality requires two essential steps. First, the home has to be built as tightly as practical; it is easier to control air movements in a tight home than in a leaky one. Second, you need some upgraded mechanical ventilation features that can provide a steady flow of fresh air.
Q: Can SIPs be used in a passive solar home?
The National Renewable Energy Laboratory (NREL) designed a number of passive solar homes that were eventually built in the sunny but cold climate at the South Rim of the Grand Canyon. Researchers found that the SIP building shells actually performed better than expected. SIPs and solar can work well together, as long as heavy heat storage materials, such as a concrete slab, are included within the building shell. But to be comfortable, solar homes-built with SIPs or traditionally framed-should incorporate high-performance, low-e coated window products.
Q: Are building codes a problem?
Most panel manufacturers have listed their products with regional code bodies, which facilitates acceptance of their products by local code officials. Yet while SIPs have been around for several decades, some code officials still aren't familiar with them, so check with your local building department.
Q: How do you wire SIP houses?
Wiring a SIP house takes a little extra planning prior to construction. Nearly all panels come with some type of pre-cored chases in the foam. You simply fish the wire through those chases to where the wire is needed. While panels are being placed, some minor cuts are normally made to accommodate switch boxes beside exterior doors.
Q: What about SIPs in fires?
To the surprise of some building researchers, data from extensive laboratory fire testing as well as reports from house fires indicates that SIP structures tend to be more resistant to house fires that standard wood-frame structures.
Q: Are ants and termites a problem in panels?
No more than in conventional construction. In climates where either termites or ants can cause problems, panel manufacturers recommend that all homeowners use the same preventive treatments (topical sprays around foundations, termite shields, etc.) they would use in a stick framed home. Note, however, that foam within panels does not provide food value for insects.
Q: Is a SIP a "green building product?"
SIP homes require considerably less framing lumber than a conventionally framed home. Note too that 95% of a tree can be used when it is cut into wood chips to make OSB for SIP facings, vs. 63% of a tree when it's sawn into solid lumber. SIP homes pre-cut in the factory mean less job-site waste is shipped to the landfill and more of it is recycled at the factory. While it several times more energy to make foam than fiberglass, and the foam itself is a petroleum product, lifetime savings in heating and cooling requirements will offset this difference in favor of the SIP home.
Q: Do SIPs walls cost more than frame walls?
Generally speaking, a little more. The materials definitely cost more, but labor savings should partially offset any increase. But SIPs can be competitive if the design is optimized to incorporate them. That means doing things like designing house dimensions at two-foot and four-foot increments, pitching roof SIPs up at steeper angles to include loft square footage, and similar adjustments. A number of builders scattered around the country use SIPs for their entry-level housing products. Builders who repeatedly use SIPs have figured out ways to minimize any cost difference.
Q: Are panels a nearly foolproof product?
No. Like anything else, panel manufacturing requires considerable quality control in the factory. Make it a point to ask any manufacturer about their QC process. Additionally, a knowledgeable designer or engineer needs to review plans to make certain that the panels selected for a particular plan are capable of meeting the structural loads involved. And later at the job site, assembling panels into a strong, airtight and watertight shell requires someone in the lead with some SIP experience.
Q: Bottom line, are SIP houses better?
Assuming the SIPS are properly designed, manufactured and installed, a SIP house will be stronger, more energy-efficient, and more comfortable.