Clinker brick oven and smokehouse

[brickie in oz]

Stone or brick is not an insulator.
All you will do is add more thermal mass which will take more wood.

[cynon767]

Originally Posted by Chattachef

what if I put a piece of soap stone ontop of the fire brick, say an inch and a half. do you think this would increase the insulation. i hyave got the oven very hot ( about 700 degrees) , but it was hungry for wood

Probably hungry for wood because so much heat was going out through the floor. More mass (soapstone or brick) on top of the floor won't help much; it will take longer (and use more wood) to heat up, and only slightly longer to cool back down. The best thing you can do at this point (short of tearing the oven out and rebuilding the floor on top of an insulated sub-hearth) is to insulate beneath the slab. You want to hold the heat in instead of allowing it to radiate out.

The oven works by absorbing heat into the bricks and then radiating it back out again. The more "thermal mass," the longer it takes to heat up, but the longer it continues to radiate back out again. Unfortunately, when it's not insulated, it radiates in all direction and loses its heat quickly. And since the heat is traveling by conduction and radiation instead of just convection, it travels in all directions equally... including down through the floor. That's why we put a layer of insulation beneath the floor of our ovens: it keeps the heat in and makes them much more efficient.

With the layers of brick, mud, and concrete, you have a seriously massive floor... great for baking large amounts of bread or for cooking all day; if you can keep the heat in, that is. It will take a large amount of wood to get it up to temperature, though, even if it's better insulated. Adding more mass will only increase that. Adding insulation, on the other hand, will make the oven hold its heat in, allowing it to get up to pizza-making temperatures more quickly and holding that heat in much longer.

At this point, your best bet may be to add insulation beneath the slab. Whether you use some sort of blanket insulation, a high-heat insulating board, or just some sort of vermiculite fill (though I'm not sure how best to install that on the underside of a slab...), you'll improve the heat retention, heat-up time, and make more efficient use of your wood.

The only other concern I have is that the cement of the slab isn't thermally as stable as the firebricks resting on it. Portland cement breaks down at high temp; that's why we use the lime and fireclay in the homemade mortar mixes. I might worry that, over time, without a layer of insulation protecting it from the heat of the oven, the slab could weaken. What's done is done, though. Go ahead and use it, and see what happens. As long as it holds up, it should continue to work about the same. If it collapses in a few years, you'll know what to do differently next time.

[Chattachef]

Cooked some pizza this weekend and the oven worked good.
Might look into a thermal blanket for the bottom. Under the firebrick and mud I have a 3/8 inch of coragated steel.... This should support the floor.. Any type of blanket I should look for? thanks

[cynon767]

At this point, anything is better than nothing. Ceramic fiberboard would a pricey alternative, but would be both the most effective insulator and the easiest to install on the underside of the hearth. Rockwool or a similar ceramic fiber insulation would be efficient, non-reactive with the cement, but harder to install. Fiberglass blanket would be the least costly, but isn't as high-heat resistant or non-reactive... it would work for a while, but could break down and need replacing soon. Or you could work out a way to use perlite or vermiculite: build a shelf, and find a way to fill it in. I suppose you could make bricks of vermiculite/perlite mixed with cement and put them up on a shelf beneath the hearth.

...Just spitballing ideas here. Whatever you do, good luck. Let us know what comes of it.

[apagios]

for a random thought, if you insulate under your slab, you are going to be holding more heat in your structural slab.

Someone with more experience then I might be able to answer if that's a bad move. Do you really want to keep heat in the structural slab and heat up rebar even more?

If it were me I might simply accept that it will take much more wood to keep hot, and not insulate under the slab to avoid any possible issues with routine heating of a structural slab. I'd be afraid of cracking and having my slab eventually start failing.

If you download the pdf off the Forno Bravo store, there is a LOT of great info and pictures in there (like 70 ish pages of diagramed and photographed instructions)! I've been using that book and this forum as my 'oven bible' since I started in July.

[cynon767]

Originally Posted by apagios

If it were me I might simply accept that it will take much more wood to keep hot, and not insulate under the slab to avoid any possible issues with routine heating of a structural slab. I'd be afraid of cracking and having my slab eventually start failing.

I think that, given the amount of steel he used, it should probably hold up for a while, even if it cracks and starts to break down a little. That's the purpose of rebar in general... even if you get cracks, the pieces are still held together in a relatively stable block. With the corrugated steel beneath the mud & brick hearth, he should be fairly safe. The amount he saves in wood (and heating time) will definitely be worth the effort, IMO.

[Chattachef]

Things have slowed down a little bit. Got the door mounted for the smoke house. Tested it out, the smoke draws great and the house hold heat. Also the roof is taking shape. Next week I am on vacation and I am going to play in the backyard.

Here are some updated pictures.

[Chattachef]

tin roof pictures and smoke house doors