©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
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."
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.
Alternative Walls 
Slab Foundations 
Board Insulation 
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 GBAGreenBuildingAdvisor.com. "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 capillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. 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 energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost. 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."
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 rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. 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."
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 (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) or expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.). 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(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. ' panel. The product is basically a sandwich of the mineral fiber insulation and cementitious wood fiber, which I believe is inert to rotting."