In this post, I continue looking at the simple whole house embodied carbon emission model I studied here for a conventional home, and here for a straw bale home. Today I'm looking at what happens if we use SIPs - structural insulated panels - as the wall system. SIPs are large structural panels consisting of two skins of oriented strand board, plywood, or similar, bonded to either side of a thick layer of rigid foam (polystyrene, polyurethane or similar). SIPs are probably the leading technology for building super-insulated houses, and there are dozens of manufacturors in the United States producing them.
In particular, I assume that to get thermal performance comparable to a straw bale building we need to use a 10" SIP, containing about 8" of rigid insulation, giving about an R-40 wall once the insulation has aged a decade or so.
(Also, I note that in response to this comment by Jim Camasto, I added 2" of rigid foam insulation under slab and outside basement walls to the model for all three houses. That added about half a tonne of embodied carbon emissions in each case).
Here are the embodied and sequestered carbon emissions for the SIP house:
Comparing this to the conventional home:
Basically, the SIPs make modest increases in both the embodied carbon emissions and the sequestered carbon (essentially because the extra skin of OSB contains more material than the studs in the conventional home, as well as the additional insulation).
Of course, if the houses is heated/cooled with carbon based energy, the emissions saved through the life of the home, versus the conventional home, will dwarf these embodied/sequestered differences.
Comparing again the straw bale home:
the main environmental advantage of straw bale from this perspective is the sequestration.
In particular, if the operational energy of the house is to come entirely from renewables, so there will be no operational carbon emissions, then straw bale offers the possibility of having the house carbon negative on a embodied basis as well as on an operational basis.
There are of course a number of other considerations:
- The aesthetics of straw bale are quite different, with thick walls with slightly wavy surfaces, versus the more conventional rectilinear appearance of SIP walls.
- SIPS are much more widely available and familiar to architects, contractors, banks, and building officials - straw bale is still a bit fringe and is often done with significant amounts of owner labor. A SIP building is probably lower risk.
- SIPs probably present larger issues with offgassing of toxics into the interior space.
- Straw bale walls probably present larger maintenance issues, especially in cold wet climates (though certainly it can be done).
Personally, after this exercise, I still want a straw bale house :-)