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.”
In Chesnut’s opinion, it would be impossible to keep straw bales dry when used beneath a slab, making the potential for rot high.
He adds that water has a way of finding its way through concrete by hydrostatic pressure and capillary action. “I’ve worked on cob/straw bale construction with a natural builder who goes through great lengths to avoid the use of any ‘unnatural’ materials,” Chesnut says. “But even he compromises where the wall meets the foundation with the use of rigid foam board and wet applied capillary breaks to ensure water is not wicked up from the ground into the straw bale. “
Dylan Eide adds another point: while straw bales provide excellent R-values, this technique of insulating the slab requires a great deal more concrete than a conventional slab.
“Considering the high embodied energy cost of concrete, the associated CO2 pollution, and presence of heavy metals increasing its use could very well be counter-productive,” Eide writes. “After energy production concrete production is one the largest sources of CO2.”
Get the bales out of the ground
Lucas Durand is among those who would be a little nervous putting draw bales below grade. But he wonders whether they might be used above grade as a “sub-slab” or “sub-floor insulation.”
“Maybe some type of platform structure lifted a short distance above final grade by piers and straw bale infill underneath the platform,” he writes. “Loose fill cellulose on top of the straw bales between the joists of the platform structure. The base under the straw bales could be well drained gravel with an impermeable membrane between the ground and the straw.
“A parged exterior but maybe with a rainscreen cladding as well for suspenders,” Durand adds. “I imagine the straw bales and cellulose would have to be able to dry to the inside of the platform structure above. Wide overhangs, and all-round kick-ass drainage would be critical.”
Durand’s accompanying schematic shows the assembly clearly. But, he adds, “there are many unresolved issues that I haven’t thought about like air sealing, attachment of rain screen cladding over parging, drying potential of under-floor insulation, etc. etc.”
Mineral wool and other options
AJ Builder wonders whether Roxul, a mineral-wool insulation made from spun basalt and recycled slag, is another alternative to foam.
Maybe it is, but Dan Kolbert writes that Roxul is “easily several times the cost” of either extruded (XPS) or expanded polystyrene (EPS). After checking his records, he says he paid about $50 per 4-ft. by 6-ft. sheet of 2 3/8-in. Roxul, making it about twice the cost of 3-in. EPS.
“The 2 3/8-in. Roxul DrainBoard is R-10, while the 3-in. XPS is R-15,” adds GBA senior editor Martin Holladay. “So the Roxul is more than 3 times as expensive as the XPS in terms of $/R.”
J Chesnut also speculates that it might be possible for a manufacturer to produce a sub-slab panel out of the same materials used to make Faswall or Durisol blocks, which are types of insulated concrete forms that use wood fiber and cement rather than foam.
“I think they make poor wall assemblies but seems like they could make a rugged sub-slab insulating ‘SIP’ panel. The product is basically a sandwich of the mineral fiber insulation and cementitious wood fiber, which I believe is inert to rotting.”