“This is not how my dad taught me to frame a house!”
Then again, dad heated with oil at 25 cents a gallon.
The use of Structural Insulated Panels (SIP) is on the rise but it is still fairly uncommon. So why was this type of construction chosen for TerraHaus?
Short answer? To avoid thermal bridging—conductive materials that allow heat to bypass insulation. In standard construction, wood in the form of studs, top plates, headers, etc. rather than insulation may make up over 10% of a wall, and wood is not a very good insulator.
Structural Insulated Panels (SIPs) are one of the key pieces of technology that makes it possible for TerraHaus to achieve its remarkably low space heating fuel requirement, the equivalent of under 80 gallons of fuel oil or ½ cord of wood for 2000 square feet per heating season, less than 10% of the fuel used in a standard code-compliant home constructed in our area.
SIPs are sandwiches of oriented-strand board (OSB) sheathing and some type of foam core insulation. TerraHaus uses a relatively new foam insulation, graphite-coated expanded polystyrene (EPS) by Neopor. EPS was chosen for its low global warming and ozone potential (see previous blog post). The graphite coating is an innovation that creates a radiant barrier which boosts the R-value of our 8.25-inch panels from R-33 to R-36. After cellulose is added to our cavities, we’ll have an R-value of 50.
A SIP is strong enough to be used as a load bearing wall but it is often used over timberframe or stick built construction. TerraHaus is framed with 2 x 4s and, around the south windows, some heavy posts. This framing creates a chase for electric utilities, provides vertical strength to support the second floor and roof, and will be filled with cellulose insulation to add to the total wall insulation.
Again, a thermal bridge is a gap in the insulation that forms the thermal envelope of the house. The problem with standard framing is that wood has an R value of only about 1.4 per inch. In standard 2 x 4 construction this means that every 16” on center you have a thermal bridge of 1 ½” (the actual width of 2” nominal lumber) where heat can bypass the insulation and conduct through the wood rapidly. The real R value of a wall ends up being only 80% or so of the R value of your cavity insulation.
SIPs construction on TerraHaus forms a continuous barrier without thermal bridging from studs. In addition, the seams are minimized because, unlike 4 x 8 sheating, the OSB-covered SIPS are up to 8 feet wide and they run the entire height of the wall. The window and door openings are precision cut at the factory based on the architect’s specifications. After installation, each seam is filled with spray foam and sealed with tape.
Can you minimize thermal bridging in standard framing? Yes. Many green builders have switched to Advanced Framing. Advanced framing is too complex to go into in detail in this post, but I’ll mention the highlights: 2 x 6 construction 24-inches on center (rather than 2 x 4s at 16 inches), two-stud corners (rather than three), rigid insulation included in the center of headers, minimization of unnecessary cripples and jacks, and single top plates. These changes necessitate some other modifications such as metal bracing and drywall clips on corners, rafters or trusses set directly over studs, and 5/8” drywall rather than ½“. As architect Matt O’Malia points out though, Advanced Framing still results in serious thermal bridging. Rigid insulation on the outside of the sheathing can help, especially when installed over new types of sheathing that can be taped for air sealing.
Doug Fox, Center for Sustainability and Global Change, Unity College