What's a SIP?

In a structural insulated panel (SIP), foam is sandwiched between two sheets of wood. The most common SIP has oriented strand board (OSB) facings and expanded polystyrene (EPS) foam. The OSB and EPS are bonded together to make one strong unit.

R-values depend on the thickness of the unit and range from R-17 to R-46. My R-17 wall SIPs are about 4 1/2 inches thick.

A SIP house has no frame--the SIPs are both structural support and sheathing. To build a wall, you essentially stand the panels on end and connect them edge to edge with splines. To install a window, you cut a hole in the SIP. More photos.

The facing: OSB

Most SIPs are faced with oriented strand board (OSB). The wood often comes from fast-growing plantation trees. The glues contain some formaldehyde, but the offgassing is minimal. It's not like particleboard; it's more like exterior plywood. You can also seal the OSB to stop offgassing.

The foam core: EPS

The current EPS manufacturing process uses no chlorofluorocarbon (CFC) or hydrochloro-fluorocarbon (HCFC) products. Once the EPS is done it doesn't offgas. The manufacturing process doesn't sound pleasant, and EPS is a petroleum product, though a recyclable one.

A green system?

Energy-conscious builders are using more SIPs and less traditional framing. Most publications compare only these two approaches, and SIPs come out ahead for several reasons:

• They use fewer and smaller trees or "waste" wood.
• They build tighter, more insulated structures that need less heating and cooling.
• They create less waste on site. Often, you send your plans to the SIP manufacturer, who cuts your windows and doors in the panels, recycling the scrap.
• They build stronger, quieter structures, thanks to greater shear resistance and solid walls.
• They go up quickly and can be built with unskilled labor. This is one reason Habitat for Humanity is using them.

But how do they compare environmentally to straw bale, a natural system that also offers a high R-value?

• SIP manufacture must release unhealthy stuff, and the raw materials are trucked to the factory. Straw farming probably requires less chemical input and less driving around.
• SIPs are probably not available locally. Mine were made in Tennessee and trucked to Indiana. Most straw bale builders can get straw locally.
• SIP walls are relatively thin, which is useful if you want a small footprint (one of their appeals to me). A straw building with the same interior square footage would need more foundation and roof, which increases the use of materials and the amount of land disturbed.
• SIP walls contain some formaldehyde, supposedly well contained in the resin but still a concern to chemically sensitive people. The OSB can be sealed to stop offgassing, but that's an extra step and product. Straw bale walls have no formaldehyde.
• The real-world statistics vary, but straw bale probably has a higher R-value than the most commonly used SIPs. At the same time, a SIP house is probably tighter, which reduces heating and cooling loads. My gut feeling is straw comes out ahead.
• To get rid of a SIP house responsibly, you would have to ship the walls to a place that could recycle them. A straw house would happily molder away in place.

Why did I pick SIPs?

I originally planned to build a straw bale home. Two concerns stopped me: moisture and money.

My site is in humid woods, on a ridgetop with wind-driven rain. I didn't want to fret about rotting straw in my walls. The larger footprint required by the thick walls also gave me pause, as did my apparent sensitivity to straw.

I was also concerned about banks' willingness to mortgage my house if I had to sell it or needed emergency funds. I need to be able to easily and quickly get the value out of my home if I end up in severe trouble.

SIPs offer an energy-efficient and financially safe alternative, though they're not as green as natural systems.

Links

Yet more on SIPs.

A SIP manufacturer's view of SIPs and sustainable building.