I know this question is stupid - insulation under

[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!)???

[gjbingham]

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.

[Frances]

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...

[dvonk]

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.

[dvonk]

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

[Dutchoven]

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

Originally Posted by 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!)???

[dvonk]

Originally Posted by Dutchoven

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

[Orrin Dean]

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?

[james]

Originally Posted by 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!)???

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

[dmun]

Originally Posted by Orrin Dean

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.