I know this question is stupid - insulation under

[Ed_]

Re: I know this question is stupid - insulation under

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!

[dvonk]

Re: I know this question is stupid - insulation under

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.

[Orrin Dean]

Re: I know this question is stupid - insulation under

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?

[dmun]

Re: I know this question is stupid - insulation under

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.

[Ed_]

Re: I know this question is stupid - insulation under

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]

Re: I know this question is stupid - insulation under

I don't think I understand the practice of insulating the concrete slab on the foundation. The insulation around the oven walls is necessary for heat retention and structural preservation. But does the insulation under the foundation keep the oven level? Or keep the slab from cracking? Any heaving would crack the foundation with the insulation.
My brother-in-law is working with his town government installing fire hydrants. He was saying from this winter the frost line would be closer to 60"-72". With this factored into the equation, what does the insulation do? Does it keep the ground inside the foundation from freezing?

I think we have two different questions going on here. Orrin seems to be asking about a anti-frost foundation using insulation board as shown in this link:

ESB: Frost-Protected Shallow Foundations

It sounds interesting, but I don't think anyone here has tried this technique yet, and can speak from experience. So I think the short answer is, I don't understand it either. I can see where a heated building would use the heat from the building to keep the foundations from freezing, but I don't understand how it works with unheated structures.

[james]

Re: I know this question is stupid - insulation under

I wonder (surfing brick oven sites) why Alan Scott's following (and probably Alan by himself) pour the insulation hearth under the structural but not vice a versa?
What a thermal mass and what a firing it should be (even despite the fact they put a floor bricks in the side!)???

The only reason to build up that much mass in the oven floor is for a commercial bread bakery. I've said many times that the Bread Builder's oven is basically a shrunk commercial bread oven -- where the definition of "commercial" means that it is designed to bake 3-4 full batches of bread from a single firing. That is hundreds of loaves of bread from multiple hour firings.

My cooking floor on my Scott oven was about 9" -- 4 1/2" of brick and 4 1/2" of concrete. A residential pizza oven floor is 2"-3" and a commercial pizza oven floor is 4".

The 9" floor is a real no-no for a pizza oven. Hands-on experience taught me that it is pretty much impossible to keep the cooking floor at high temperature -- because the heat of the fire is wicked away from the inside of the oven to the outer edges of the 9" thermal mass. The Scott design is basically a whoops -- don't do this at home.

Here is a graphic of draw a while back that tries to capture this visually.

James

[Orrin Dean]

Re: I know this question is stupid - insulation under

I don't think I understand the practice of insulating the concrete slab on the foundation. The insulation around the oven walls is necessary for heat retention and structural preservation. But does the insulation under the foundation keep the oven level? Or keep the slab from cracking? Any heaving would crack the foundation with the insulation.
My brother-in-law is working with his town government installing fire hydrants. He was saying from this winter the frost line would be closer to 60"-72". With this factored into the equation, what does the insulation do? Does it keep the ground inside the foundation from freezing?

[dvonk]

Re: I know this question is stupid - insulation under

I think sometimes his plans actually have the insulating hearth floating or hanging on the block walls

They do have, indeed!

Woodfiredpizza.org - Pizza Oven Construction
Wood Fired Brick Oven Tampa October 2004

[Dutchoven]

Re: I know this question is stupid - insulation under

I think sometimes his plans actually have the insulating hearth floating or hanging on the block walls...there is a oven build site brickoventampa or something...his oven did not have a structural layer of concrete under the oven itself.
My oven was done in that way and I can say it rather stinks sometimes!
Dutch

I wonder (surfing brick oven sites) why Alan Scott's following (and probably Alan by himself) pour the insulation hearth under the structural but not vice a versa?
What a thermal mass and what a firing it should be (even despite the fact they put a floor bricks in the side!)???

[dvonk]

Re: I know this question is stupid - insulation under

Well, it's clear that increasing of thermal mass will be huge, but how good it for home baking?

[dvonk]

Re: I know this question is stupid - insulation under

But most of them are using it for cooking real small batches of pizza and family-size amount of bread - not like pro using it at all. And I've seen many times words like "when oven cooled slightly, we bake blah-blah-blah" - so, after 1.5-2 of firing, floor still transfuse heat to the hearth.

[Frances]

Re: I know this question is stupid - insulation under

The FB plans had the hearth that way round at first - or so I'm told, bit before my time really. The heat loss through the floor proved to be too great though.

I'd think that would be the same for a high mass oven though too, that it'd work better with insulation all round the outside of the heating mass...

[gjbingham]

Re: I know this question is stupid - insulation under

I assume to increase the heat retention of the bread oven. Good for baking multiple loads of bread. Bad for pizza - it takes way more fuel to get up to pizza temps. I think in the book, The Bread Builders, the plans don't even show insulation under the hearth.

[dvonk]

I wonder (surfing brick oven sites) why Alan Scott's following (and probably Alan by himself) pour the insulation hearth under the structural but not vice a versa?
What a thermal mass and what a firing it should be (even despite the fact they put a floor bricks in the side!)???