Recycled tire rubber for foundation wall, footing, and slab insulation
Has anyone investigated the use of recycled shredded tire mulch, or the running track playground surfacing made out of the same material, or the patio tiles made out of it, for thermally isolating a foundation from the soil? Rubber seems to be between R-1 and R-3 per inch, and various products claim a compressive strength of 500-1250 PSI (based on a quick Google, though I doubt this is rigorous apples to apples testing).
I’m not seeing a lot of alternatives to heavy-duty EPS/XPS out there that are both available in the US and don’t have any issues with insects or freeze/thaw. There’s the option to mix boric acid into the foam plastic mix, but I’m unclear on cost/availability there. They stopped offering Foamglas, and that apparently had freeze/thaw issues. I’m curious about using AAC blocks, but there’s availability issues and we would expect them to be very brittle. Their counterpart styrofoam-laced cellular concrete might be worth investigating (perhaps as an entire foundation wall pour), I’m not sure. https://www.structuremag.org/?p=7776 says “The standard cellular concrete mix weighs 30 pcf, with an average compressive strength of approximately 100psi (14,400 psf).”
The mulch in particular seems like you could just dump on top of aggregate and enjoy a thermally broken (moreso than rock), water permeable solution, it’s reasonably priced, and for multistory footings in particular, it’s dramatically stronger than both cheap low-density low-PSI grades of EPS, and expensive high-density high-PSI grades.
While somewhat toxic in breakdown, it is being used in flower beds and patios and children’s playground. It’s recycled. Chemical breakdown seems to be mostly a product of UV exposure, which is not a huge consideration except on the exterior of the foundation wall. The only big downside seems to be that it would increase a structure’s flammability (again, exterior of the foundation wall only).
https://www.aaastateofplay.com/recycled-rubber-mulch-2-000-lb-super-sack/
https://www.amazon.com/RevTime-Interlocked-Outdoor-Playgrounds-Backyard/dp/B07C7MMZQ5/
Has anyone seen this considered before, or have any theoretical reasons why it would be a good/bad idea?
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Replies
Burninate,
it's an idea I've never heard, and I'll be following this discussion with interest. I've read research on adding rubber to the substrate under foundations to dissipate vibration in high seismic zones, but never for insulation.
Example of the tile at my local orange box, to put a hard ceiling on price possibilities for those:
https://www.homedepot.com/p/Envirotile-Reversible-16-in-x-16-in-x-0-75-in-Slate-Brick-Face-Flat-Profile-Rubber-Paver-MT5001608CM/302907057
$4.50 for 16"x16"x0.75" is equivalent to $40 for 1 cubic foot of those tiles, but if even a single layer achieves, let's say, R-1 for $2.53/sf, that would double the insulation value of the concrete wall without making an un-dryable condensation/mold issue like a number of interior insulation options do. I can't really speculate about what a purpose-made sheet of this material, hung from the top of the foundation wall, would cost, but we know the raw materials would be below this figure.
Further searching indicates the Rockwool boards are actually rated for subgrade wall installs these days... which dramatically mitigates the exterior foundation wall issue, I think. Termites can't chew on those, and they only run around $1/sf for Comfortboard 110 at 1"=R-4. The footing and slab applications (where the very high compressibility of rockwool can be an issue) remain challenging.
Rockwool is rated for use under slabs, just not under load-bearing footings.
https://www.rockwool.com/applications/floors/underslab-floor/
Burninate,
There is a type of building called an "earthship" that has foundation walls made of old tires filled with packed soil. This method is not code-approved, and these walls require a lot of hard physical labor to build. But people do it.
More information here: "Earthship Hype and Earthship Reality."
Martin,
I think the OP is just suggesting using rubber, either in sheet or pellet form, in place of foam as sub-footing and exterior wall insulation.
I came across a product called Glavel, made from recycled glass blown into gravel with air pockets trapped inside, seems similar to some lava irocks, but made from recycled glass. Apparently its 1.7 R/inch and is considered clean fill. I didn't get a quote on the cost for my project, but seems really neat.
https://www.glavel.com/
https://foursevenfive.com/glavel-foam-glass-gravel/
Malcolm,
Comfortboard 110 is rated at 8.5psi (1220psf) to achieve 10% compression, well below the 25-100psi you can get out of higher density EPS/XPS. 4 inches of concrete will prestress it to 0.34psi; 8 feet of concrete (as in a footing), to ~8psi and 10% compression at build time. Whether they will bother recommending it or not, whether you can stand on it without destroying it or not, I don't know that it's a great idea to use a normal slab on that; The resistance to compression is far below what you would expect out of a compacted gravel or sand surface. The failure in question would be cracking under particularly strong point loads, and loads that are arranged in a colinear fashion, like shelving. An extra-thick multiply-reinforced slab or a tensioned slab would be a whole other matter, probably mitigating this entirely: For any substantial loading we are leaning heavily on tensile strength of the slab in order to spread out the forces over a sufficient tributary area of slab insulation.
https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/brochures/commercial/COMFORTBOARD-110-for-Continuous-Insulation-Brochure.pdf?f=20180618125814
Martin,
I am similarly skeptical of the Earthship idea, but I wouldn't put that emphasis on it - the relevant technique is that Earthships use "discarded tires to hold MANY TONS OF SOIL" rather than "DISCARDED TIRES to hold many tons of soil". Earthbag or rammed earth would be equally effective, which is to say, moderately effective if confined to a mild desert climate, and not effective at all elsewhere. They're not actually using the rubber for all that much.
I am interested in rubber not because I have thirty tons of free rubber burning a hole in my pocket, I'm interested in finding materials that can do what the current set of recommended materials & techniques can't do. Foam is currently popular but it has downsides in several of the applications it's used for.
What I *do* have thirty tons of, in my parents' house, is random junk arranged on basement shelving, in the vicinity of https://www.uline.com/Product/Detail/H-3914/Industrial-Shelving/Heavy-Duty-Steel-Shelving-72-x-24-x-72 . I wouldn't understand laying out a slab incapable of surviving that sort of loading.
Vashonz,
Glavel does indeed look interesting.
https://www.greenbuildingadvisor.com/article/sub-slab-mineral-wool
Slabs here are typically poured on EPS rated to 10 psi, which isn't far off the 8 psi for Rockwool. If we are happy to build two story houses on footings laid on a substrate of soil with a bearing of 2500 psf, I'm struggling to see the problem with non-loadbearing slabs on half that capacity. Live loads or point loads exerting that kind of pressure just don't occur in residential construction.