Insulation questions

[ironchefshort]

I have built one oven and am now planning to build my second. This is a non-kit oven with a firebrick core similar to an Alan Scott design. I'm not planning to do the suspended hearth as he does but instead place it "on" the cement block base separated by 4" of vermicrete and hopefully 2" of foam board. The primary use of the oven will be for bread and I hope to get as many bakes out of it as possible, my last was not very well insulated and I was only getting 4-5 bakes out of it.

My questions pertain to the order in which I will do the different "layers" of the hearth. My plan is to do the block stand, then the vermicrete, the foam layer and then the hearth slab with the firebrick hearth on top of that. Will the foam layer support all the weight that will be placed upon it without issues? I plan to do 22-26 firebricks across (on edge) for the hearth width and 9 firebricks deep. I'd like to have 4.5" of cladding on the sides and about 6" on the dome area. Would I be prudent to also use some sort of ceramic blanket on the sides/dome cladding? Do I need to add loose vermiculite over this blanket layer if I were to do that? My plans are to do a steel stud structure around the back part of the oven with steel siding, so I'd have the open air space to fill around I think.

Any thoughts/comments are welcome. I've done this once, as I've said, but I'm still not an expert by any means. I do computer work by trade so this is a leap for me to move into "designing" an oven.

Thanks,

Ed

[egalecki]

Re: Insulation questions

I have 4 inches of vermicrete (more or less) with 2 inches of board on top, followed by my floor and the dome. My dome does sit on top of my floor. The board I used, from Harbison-Walker, has taken the load just fine. The board from FB does just fine as well.

When you say foam board, do you mean styrofoam? That won't work anywhere in your design. As the bottom layer it will compress and be useless, and as the layer under the floor it will melt.

If you use proper insulating board, you'll be fine. If you put it on top of vermicrete, it will insulate you very well indeed.

[ironchefshort]

Re: Insulation questions

Yeah I misspoke and did not mean foam. I was meaning the board, similar to the one from FB. Been confusing myself I guess. Thanks for the input. I've never used the insulating board (nor seen it) so I have no idea what it is or even what it can take in the way of weight on top.

Ed

[Les]

Re: Insulation questions

Ed,

I can take a lot. I used 4 inch thick board, and as a test, I drove my 3/4 ton truck on top of it. It only compressed on the edge where the tire rolled over it.

Les...

[gjbingham]

Re: Insulation questions

I'm curious about the order of your layers under the oven. If you place the hearth slab on top of the vermicrete and ceramic insulation board, you will loose heat out the sides of the hearth slab, unless it is isolated and surrounded by insulation (IMHO).

It seems to me that the proper order should be block base, hearth slab, and then the vermicrete layer - at least the size of the external dimensions of the oven, at a minimum. A few inches larger than the oven might reduce heat loss to the surrounding/underlying layers. This vermicrete layer could additionally be surrounded by concrete out to the edges of the block base if desired, and to a depth equal that of the vermicrete.

After that, at least in my little mind, comes the ceramic board, upon which the floor bricks are placed. You might consider two layers of floor bricks for additional heat retention for multiple bakes. Perhaps that is what your idea of layering of concrete over the insulation was intended to do. (?)

You have more experience than I in building barrel vault ovens. That's just my thinking. I'm curious what Dutch and Cjim think. Regardless, it seems to me that you're already doing really well getting five bakes from a single firing. I think I remember reading in Reinhart's book "The Bread Baker's Apprentice", about a pro baker somewhere around the San Francisco area who can get 10 bakes per firing. That's got to be some kind of record!

Best of luck with your build.

[ironchefshort]

Re: Insulation questions

GJB,

I plan to buy some ceramic blanket and will run that all the way from the top of the dome to the vermicrete level on the sides/back/front. Hopefully I'll be able to encase the oven in that with about 50 sq ft. I plan to pour loose vermiculite to fill the gaps between the oven and the enclosure that will be built.

If I were to put the hearth slab away from the firebrick hearth would I not lose most of my heat storage? It is the concrete that holds most of the heat from the fire and then "feeds" the firebricks heat when the temp inside the oven drops from baking. So putting the insulation below the hearth slab makes good sense as I don't want the heat escaping from the heat source.

Jim, if I'm not mistaken, can get 10 bakes from his oven. That is pretty amazing...especially for the energy it takes to make up 10 bakes of bread!

Laid out my block base last night in my barn...oh the anticipation of Spring.

[gjbingham]

Re: Insulation questions

I thought you might be using the concrete over the insulation as a heat reservoir. I wonder how the thermal properties of concrete compare to other refractory products, such as high duty firebrick and/or others?

In my small brain, it seems that the higher the thermal conductivity of the hearth placed over the insulation and under the oven floor, the more efficiently you'll capture the heat and have it available to recharge the floor as it cools during the baking process.

Where's dmun???

[glennb]

Re: Insulation questions

Concrete doesn't handle the high temps very well either.
If you were aiming for a higher heat retention, then i'd be tempted to use a double layer of fire bricks (on edge) in the hearth and then keep that fully insulated from the concrete slab. You could also use the High Duty fire bricks which I "think" are heavier than the normal ones and might hold heat longer than concrete.

A double layer of fire bricks on edge would be about ~250mm thick as a heat slab.

[Dutchoven]

Re: Insulation questions

A double layer of fire bricks on edge would be about ~250mm thick as a heat slab.

I think this would be a great way to go although I don't think both layers on edge would be necessary. If you laid the first layer of firebricks directly on the insulated layer on edge you would have about 4.5 inches depth, then add another layer laid flat giving you an additional approx 2.25 inches for almost seven inches total. I would think that size heat sink would give you a nice long bake time...especially if you insualate it very well. I have a barrel vault built very much like you are referring to and I get about 6 loads per firing...I would do it with everything on top of the insulation if I had to do the same oven over again...
Best
Dutch

[ironchefshort]

Re: Insulation questions

I appreciate all the good input I'm receiving. It hadn't really occurred to me to do a double layer of the firebrick and I especially like the idea of having one layer on edge and another laid flat. Something hangs in the back of mind that I read while doing my first brick oven about having too much firebrick mass can actually cause the hearth to be overheated. I cannot seem to find that reference now though. Maybe someone else has run across that.

So if I were to go the route of having 2 courses of firebrick for the hearth, equaling 6.75", I would then place the CA board directly underneath the firebrick hearth. Under that would go the vermicrete layer followed by the hearth slab. Am I correct in my thinking? How thick would I need to go for the hearth slab since heat retention is no longer the main purpose?

My other train of thought moves to refractory concrete for the hearth and cladding. The main reason I turned away from this initially was the cost. I had planned to mix Portland cement with sand/fireclay/lime in a 1:3:1:1 mix respectively, as I had read that somewhere on here in a thread. I thought that might be a good substitute for the $35 a bag stuff I had been getting quotes on. Most of those were mortars and not concretes anyhow. I didn't know if the refractory concrete was better at conducting heat or if it was simply used because of the low heat threshold of Portland cement.

Ed

[Dutchoven]

Re: Insulation questions

Ed
Hopefully this can help you...here on the forum we refer to a structural slab that is beneath the vermicrete/ceramic fiber board or combination thereof...then goes the firebrick floor...I personally don't think that you will overheat the floor just simply thanks to the firebrick finding equilibrium...depending on the design of your block walls and how you are reinforcing the structural slab could be thick or thin...someone correct me if I am wrong but I think the plans spoke of 4 inches thick on the structural layer?...as far as the cladding goes...I am on the fence as to the necessity of thick cladding of concrete...especially if when you design you vault arch you don't use a full height brick for the soldier course and are careful to fully encase the catenary arc in the brick...
Best
Dutch

[glennb]

Re: Insulation questions

So if I were to go the route of having 2 courses of firebrick for the hearth, equaling 6.75", I would then place the CA board directly underneath the firebrick hearth. Under that would go the vermicrete layer followed by the hearth slab. Am I correct in my thinking? How thick would I need to go for the hearth slab since heat retention is no longer the main purpose?

Ed

Ed,
That sounds about right in order of materials. You could simplify your build by using all vermicrete or all CA board under the firebricks though. I would use the CA board, maybe as a double thickness (2x2" boards) on the concrete slab as insulation between concrete slab and firebricks. This will also give you a nice level surface to place the bricks on too.

Concrete slab should be ~4" thick with reinforced steel (mesh or bar) throughout the slab to provide structural support only (no heat should go into this slab).

[gjbingham]

Re: Insulation questions

3.5 - 4 inches for the hearth concrete sounds right. The purpose for using refmix, from my understanding, is that it has properties that are very close to, if not identical, to that of firebrick. This is a good thing because the mortar will expand and contract the same amount as the bricks during thermocycling. Fewer broken mortar joints (cracks) is the intention.

Additionally, the refmix adhears much better to the bricks (I hear) than the homemade variety.

That all said, I used the homemade variety.