Can insulating concrete be used for footings?
One of the tough questions about ‘super-insulated’ building is what to do about footings? Some say don’t mess with them, too risky just take the heat loss hit. Some have used foam under footings. Those opposed to foam point out it’s propensity to ‘creep’ under long term stress and that it’ll deform substantially under rated stress conditions. An alternative, that I’ve seen little discussion of, is lightweight concrete mixed with perlite as the aggregate.http://www.perlite.com/concretemixdesigns.PDF
Why not make the footings themselves out of perlite concrete? It is inelastic, way stronger than footing pressures on the soil and, in my case at least, plenty of safety margin to support the concrete walls and house above under total peak load situations. With a 12″ deep footing of 8-1 perlite I’ll have r20+ footings! What am I missing?
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Replies
Jerry,
I don't have the figures for the compressive strength of concrete with perlite at the tips of my fingers, and I haven't done the math. But if you are confident that you have the correct compressive strength numbers, and an engineer and your local building inspector are on board, there's no reason not to proceed with your plan.
I've had a lot of experience with styrofoam concrete -- this was at at earthquake relief project in Armenia -- and I'm not very fond of the material. But it would probably work fine for footings, as long as you can assure yourself of the compressive strength numbers.
Jerry,
I would ask Perlite if the product can be used under footings. It would be interesting to get their postion on the matter.
Ron,
He doesn't want to put perlite under the footings. He wants to mix perlite into the concrete that is placed in the footing forms.
Martin,
I do believe Ron was suggesting an alternate that avoids the PITA of insulated concrete with the same benefits, an insulated footing. The perlte institute is clearly endorsing the use of bags of loose perlite as under slab insulation, they may also advocate it under footings, it's worth asking. From what I've found the compressive strength of loose perlite is about 60 PSI which is more than most soils will allow. It's certainly a better insulator by itself @ r3+/"
I thought I had read that Jerry once mentioned placing an independent layer of Perlite concrete beneath the actual concrete footings as an alternative to the possibilty of making the footings out of Perlite concrete. But even if the plan is to make the footing out of Perlite concrete, then I would still check with the manufacturer. They ought to be used to the question if they advocate Perlite supporting concrete slabs.
Jerry,
I made some phone calls to the Perlite Institute and a few distributors with an inquiry about the possible use of Perlite concrete either as the footing, or as an independent layer beneath the concrete footing. All contacts told me that they have never heard of that being done and their immediate reaction was that Perlite concrete would not have the necessary compressive strength.
I could not get a definitive answer to the question because it depends on the weight of the building and the Perlite mix, but as you raise the compressive strength of the Perlite concrete, the R-value drops. So it may be possible to achieve the compressive strength, but at an R-value that is so little that it is not worth the extra complexity of the foundation construction.
The main point of my inquiries was to learn whether or not making footings out of Perlite concrete for an insulating purpose is an established process, and whether it is commonly used. The answer to those questions appears to be no.
One person I talked to will be attending a conference with the Perlite Institute soon, and he will pose my question to memebers there, and get back to me with their response.
What compressive strength would be needed for a Perlite concrete footing?