I know this question is stupid - insulation under

[Ed_]

Orrin, dmun,

I'm planning to build a shallow frost protected slab similar to what's shown in the link dmun included. Two inches of extruded polystyrene insulation has the same insulating value as about eight feet of soil, so you effectively raise the frost line (almost) all the way to the insulation. (The insulation changes the balance between the cold air above and the roughly constant 50F of the earth several feet below.) You do need to extend the insulation a couple of feet beyond the slab.

So while the slab itself will of course get quite cold, the earth beneath it will remain above freezing. Along with adequate drainage, this is apparently quite effective at preventing heave.

Ask me again this time next year whether I still think this is a good idea.

[dmun]

I just got it! DUH.

The earth under the frost line is always fifty-something degrees. The wide spread insulation sheet keeps the frost from penetrating the soil, and the warmer stuff underneath keeps the footings from freezing.

[Orrin Dean]

That makes more sense, but what keeps the soil around the edges from heaving. If the frost line goes down even 48", I would think that would still rock the foundation, because you still have the earth around it to consider. Maybe, all the settling with our house this year is making me flakey!?!
Oh well, Ed, I'll give you the benefit of the doubt. You've been researching this longer than I.

Yes, This would be different from the topic. I'm still thinking floor=foundation. How thick should the oven wall be for a standard residential WFO, for pizza and a lot of random bread baking? 3"-4", with insulation around it?

[dvonk]

Yes, James, I've seen that, course.
It seems for me that all that folks building Scott's ovens do not know that its not feet into home purpose requirements. And as I've seen, Alan knows that and do not make any notices: some people get in touch with him and he was guiding them during construction.
Woodfiredpizza.org - Pizza Oven Construction - this is quite small oven, nothing like one that may be used as a pro bread oven.
I think it's not quite frankly.

[Ed_]

Originally Posted by Orrin Dean

That makes more sense, but what keeps the soil around the edges from heaving. If the frost line goes down even 48", I would think that would still rock the foundation, because you still have the earth around it to consider.

You've hit upon the reason that the insulation is supposed to extend a minimum of two feet from the edge of the poured concrete. The six inches of well-drained gravel under the insulation should also help mitigate any tendency to heave at the edges, since water that isn't there won't freeze. If the frost line under the edge of the slab is raised to six inches below the insulation, there should be no trouble.

But of course it's all just theory to me at this point!