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Q&A Spotlight

How to Insulate a Slab Foundation—With Straw-Bales?

Can straw bales be used as horizontal insulation under a concrete slab?

To avoid rot, straw bales must be kept dry. Lucas Durand calls this a “concept sketch,” and appends a few caveats: “This idea is less than half-baked. There are many unresolved issues that I haven't thought about — like air sealing, attachment of rainscreen cladding over parging, the drying potential of the under-floor insulation, etc.”
Image Credit: Lucas Durand

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.

But it’s not the only choice. In a post at the Q&A forum, Roy Harmon floats another possibility: the Archibio Sandwich Slab, which uses straw bales instead of foam as a thermal insulator.

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.”

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13 Comments

  1. J Chesnut | | #1

    clarification
    My mention of Faswall and Durisol concerned an idea to develop a panel product of the same materials (cementitious wood fiber and mineral wool insulation) that does not yet exist. Faswall and Durisol are blocks like CMUs and are of little use for subslab insulation. If Roxul rigid mineral wool panels can work under a slab then this idea would probably only constitute an option even more expensive yet.

    "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."

    Now seeing this quote I wonder if I incorrectly recalled the discussion with the owners of finding evidence of rot under the slab. I know the owner found rot in the walls which was subsequently rectified. I also am positive that they advised against the straw as a subslab insulation per Damora's quote but maybe I'm incorrectly conflating the two recollections. Still I would be skeptical of a subslab installation considering how prone straw is to moisture damage if it can't dry properly if wetted.

  2. Emilio Garazgos | | #2

    Bad idea. Period.
    Why are we even talking about this in 2011 when it was clearly pointed out in the 1997 "Pilot Study of Moisture Control in Stuccoed Straw Bale Walls" report to Canada Mortgage & Housing Corp (CMHC) that such constructions are the equivalent of placing a moisture-susceptible material into a moisture-impermeable bell jar. In that almost 15 yr -old study, the investigators cut into strawbale wall and floor assemblies that were about 10 years old and then inspected/photographed the condition of straw.

  3. Lucas Durand - 7A | | #3

    Response to Emilio
    Emilio,
    Thanks for linking that CMHC study - interesting stuff.
    It's not clear from your previous post whether "Bad idea. Period." refers to insulating below grade with strawbales or to strawbale building in general.

    If you mean insulating below grade with strawbales is a bad idea, I'm inclined to agree.

    If you mean strawbale building in general is a bad idea, I diagree. The conclusions in that CMHC study read more like good advice for any structure (strawbale or not) and not like an indictment of strawbale building. From the "conclusion":

    What's not done lightly with wood should not be done with straw... not yet, anyway.

    Fair enough.

    Wood held in the same moisture state and duration would rot similarly, we have observed, but would avoid or delay that condition in most of these cases if only because a wood frame wall drains better and the "net drying regime" is strengthened.

    More attention moisture management.

    ...cement rich stuccoes may well be too impermeable to water vapour — not "breathable" enough — to use freely as exterior skins in cold country... and the lime rich stuccoes too permeable to liquid water for driving rain country...

    More attention to climate-specific variables.

    ...we think the problem of excessive moisture can be avoided altogether at the design and construction stage, although we would not be confident building stuccoed straw bale in all regions.

    The all-important design-stage... The perfect time to sort out all the pesky details.

    I think there is potential for more modern details to correct some of the regional deficiencies caused by universal application of traditional techniques - for example adapting rainscreen claddings to strawbale construction in rainy maritime climates...

  4. Bruce King | | #4

    Don't put straw bales under your floor!
    Emilio is right, and thank you for digging up that CMHC study. I've been tracking, designing, building, studying and researching straw bale buildings for 15 years, and love them and the wonderful kooky people you meet around them. But they do need extra protection from moisture, and every attempt I know of to use bales to insulate floors has failed in that the bales do rot. Sometimes just a bit, sometimes a lot, but either way they stop insulating.

  5. Emilio Garazgos | | #5

    Strawbale . Good idea. Eh ?
    The title of this piece asks if straw bales (SB) as under-slab insulation is a reasonable proposition.

    It is that proposition to which I refer: re "Bad idea. Period" (above)

    The report cited (above) is what, 15 years old now ?

    There are also a good number of other research reports on SB construction done under the auspices of CMHC over the subsequent 15 years-- an investment of a not-insignifcant amount of Canadian taxpayer funds - that look at pretty much every aspect of building with straw (including thermal performance, vapour permeance of various plaster skin choices, inexpensive DIY in-wall moisture sensorss etc ) with the intent of providing would-be SB builders with an arsenal of useful, factual information to improve their chances of making durable, healthful,energy-efficient, low environmental impact buildings.

    I think that it would be reasonable to assume that those govt funds would not have been invested if initial investigations showed that SBC was a "Bad Idea".

    There are currently thousands of SB buildings around the world. (The numbers and locations of many can be seen at sbregistry.greenbuilder.com/ )

  6. Lucas Durand - 7A | | #6

    Response to Emilio.

    The title of this piece asks if straw bales (SB) as under-slab insulation is a reasonable proposition.

    It is that proposition to which I refer: re "Bad idea. Period" (above)

    Well then, I guess we agree.

    I think it is still worth noting though that that CMHC study's focus isn't below grade use of strawbales, nor is the study conclusive in its findings. More from the "conclusion":

    A much wider field project should be conducted across Canada and at least northern tier USA, further refining and using the method and procedure as developed in the pilot.

  7. Lucas Durand - 7A | | #7

    Response to Jonathan
    Jonathan,
    Thanks for the information. I have been considering using drainboard recently.
    Unfortunately, I'm not sure that I'll be able to use a rock-wool board under my slab...
    Roxul makes a rigid board "RockBoard 80" which is (I think) their highest rated product in terms of compressibility (only slightly higher than "DrainBoard"). At 10% deformation it only comes in at 743psf - 5.16psi.

  8. Jonathan Lantz-Trissel | | #8

    Roxul prices lower than XPS per square foot
    In Virginia I just ordered 2 3/8" Drainboard (R-10) 4x6 sheets at $19.03 a piece. 1" for $7.97 (R-4.3). 2" XPS (R-10, 4x8 sheet) was quoted at $32.75. So for equivalent R-values, the mineral wool is about $.80 per sq foot vs $1.02 per sq foot for the XPS. Scott, Would it make sense to update the above article to acknowledge that mineral wool might be far more competitive with foamboards than stated?

  9. Robert Nemoyer | | #9

    Rockboard80
    Lucas,
    Would this indicate that 25 psi extruded insulation would not deform as much? I am wondering if it would work under my footers as well as under my slab. I would like to put 2 inch thick xps underneath a 24 wide by 12 deep footer. But I will have a heavy wall on that footer. A concrete sip of 2 1/2 concrete 7 inches of xps and 2 1/2 concrete on the inside. This would be on a ranch with a basement so it would be almost 20 feet high including the basement wall. I want it under the footer because if the footer is on the ground the inside was of 2/12 inches would conduct the cold up the wall. I could try to put the insulation over the footer and under the inside concrete skin but I suspect that would be even worse because the weight wouldn't be concetrated even more? Another option is getting more expensive 40 or 60 psi insulation under the sabor under the internal skin.

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Robert Nemoyer
    Robert,
    In case you didn't see it, some of the questions you raise are discussed here: Foam Under Footings. Be sure to read the comments that follow the article.

  11. User avater
    Todd Collins | | #11

    Why not Perlite???
    Have you considered bags of perlite? While it isn't the best for footers due to deflection, it will work below a slab/basement floor.

  12. Michael Chelnov | | #12

    I have used Perlite in bags under a slab. Bought from a perlite manufacturer, these were reasonably priced. see the link below from the perlite institute. also I have also attached the pdf from the Perlite Institute.

    https://www.perlite.org/wp-content/uploads/2018/03/perlite-underslab-insulation.pdf

  13. User avater GBA Editor
    Martin Holladay | | #13

    Michael,
    For a discussion of the use of perlite under a concrete slab -- along with links to further information resources on the topic -- see "Building a Foam-Free House."

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