The house we are building currently was framed using Structural Insulated Panels (SIPS).  We have worked with SIPS on multiple projects now, and I am a huge fan.

While traditional, stick-built, homes are framed with studs (2×4, 2×6, etc.) every 16 or 24 inches, SIPS have an outer and inner skin of OSB (sheathing material) and are filled with foam.  There are different types of foam,  but all SIPS share the main feature of eliminating the vast majority of framing members in the walls, each of which represents a thermal bridge or uninsulated space in the wall.  Furthermore, every variety of foam used in SIPS (Polyurethane, XPS, EPS) have higher R-values than traditional fiberglass or even cellulose insulation.  On top of that, SIPS homes are sealed much more tightly than stick-built homes, causing the insulation and housing envelope to perform even better.

Another advantage to using SIPS is that the prefabricated package eliminates a huge amount of waste that is a byproduct of typical timber framing.  Furthermore, labor costs are reduced because SIPS framing happens much more quickly than traditional framing.

This said, there are a couple of downsides to using SIPS.  The obvious one is the added material cost.  Although with reduced waste and labor, as well as smaller HVAC system (due to lower heating and cooling loads), many in the industry say that SIPS are actually comparable to stick-built homes in terms of total cost to the homeowner.  A second issue is the wiring.  Because the walls are all solid members, you cannot wire the house in the traditional ways (at least not the exterior walls).  In this case, we requested the SIPS manufacturer to provide chases in the foam.  We drilled holes through the skin of  the SIPS to access and pull wires for wiring.  Not horrible, but somewhat more time-consuming than the traditional methods.  A third issue — and this one is shared by many homes that are super-insulated or “tight” — is that you end up with a house that does not “breathe” or easily exchange outside air.  This is ameliorated by adding some type of mechanical ventilation, typically in the form of an energy recover ventilator (erv) or a heat recovery ventilator (hrv).

I’ll talk more about the HVAC system in this home in a later post.