Yesterday, I was talking with a potential architect and I asked about lower embodied-energy basement wall systems. She mentioned Durisol, which is basically a type of lightweight block made from cement-impregnated wood waste. Builders stack the blocks (which can be sawn and drilled with standard contractor's tools), and then fill the interior with reinforced concrete. Here's a picture of a block emphasizing the texture of the material:
And here is a segment of a wall with this stuff (credit):
This second picture shows the "thermal" version of the block, which is partially pre-filled with rock wool mineral insulation, which reduces the amount of concrete required, and increases the R value of the wall. For below-grade recommended options, this wall system can get up to R-21 (on a static basis - ie ignoring thermal-mass lag effects, which of course is the correct basis for a basement where the ground temperature doesn't fluctuate much).
So this seems like an excellent thing -
- No flammable, offgassing plastic foams
- Much reduced concrete requirement
- Significant amounts of sequestered carbon in the wood waste that the blocks are composed of.
- Fast, simple system for creating a basement wall
So I set out to figure out how much this improved the model straw-bale house that I analyzed a couple of weeks ago if we used this kind of basement wall, instead of a strict poured-concrete foundation with rigid foam cladding. Here's the current model for the straw bale house (with poured concrete basement):
This shows the embodied carbon emissions in the materials, along with the carbon sequestered in the lumber, straw bale, etc. As you can see, the foundation is by far the largest contributor to the embodied emissions and doesn't contribute at all to the sequestration.
We don't have direct embodied energy estimates for the Durisol, but I estimated it as follows. Firstly, I worked with the company's 14" thick R-21 thermal block wall (the highest R-value system recommended for below-grade). Their dimensions data sheet explains that this block is 14"x12"x24" in external dimensions and requires 0.0186 cu. yd. concrete per square foot of wall. That enabled me to directly compute the embodied energy of the concrete component (and assuming the same amount of rebar as in a conventional wall).
Then there is a 3" layer of rockwool insulation (interrupted by the block wall components), and ICE directly has embodied carbon emission data for this material (I assume the highest density version in there).
Finally, there is the block material itself. The material properties sheet says that the material has a porosity (air volume) of about half. The material safety data sheet says that the material is 25% portland cement by volume (and I take this to be of the non-air volume). So the balance must be wood, and I used standard ICE values for the density and embodied emissions values for portland cement and softwood, accordingly.
Here is the overall effect:
As you can see, the foundation embodied emisssions have gone down a bit, and now we have some sequestered carbon in the foundation also. It hasn't revolutionized the situation (the cement content of the Durisol still has significant embodied energy), but still the net carbon emissions of the model home have gone from minus two tons of carbon to minus four tons. Combined with the high thermal performance of this wall system, it seems like a good option if it otherwise makes a strong and durable basement wall.