Superinsulated houses need insulation under the slab as well as in the walls and roof, and the most common choice for sub-slab insulation is rigid foam.
The technique was popularized in a 1994 book, The Straw Bale House. Harmon describes the assembly as a “big sandwich,” including two slices of concrete and a filling of straw bales.
“With almost no option other than petro-based materials, bales represent an ideal low-cost solution for quality comfortable floors,” he says. “The straw bale insulated slab uses approximately 20% more concrete than a 6-in. thick conventional slab for an equivalent floor area. The cost is between 50 cents to 1 dollar per square foot of R-50 insulation. No other insulated slab can be so affordable.”
Beware of rot
As appealing as straw bales are from a cost and resource conservation point of view, straw is an organic material that will rot under the right circumstances.
That’s the note of caution from J Chesnut. He visited the owner/builder of a Minnesota straw bale home a few years ago, who said he would not recommend straw bales under the slab because he had found signs of rot in his own home.
The house was described in detail in a post by Jesa Damora found elsewhere on GBA. “The first floor would be concrete slab on rigid insulation if they were to do it again, but they went to great effort to make a sub-slab of straw bale and poured concrete (called a waffle slab),” Damora reported. “They now feel it represents too much labor and concrete to be worth the effort.”