Insulation under the oven floor

[DavidApp]

Hello All

I am about to start my build and have a question about the amount of insulation under the oven floor.
I have bought enough Fiber Board to give me 2" of insulation under the floor, dome and vent.
Would an extra 1" of Vermiculate cement make much difference?
Has anyone cast the 1" thick tiles of Vermiculate cement and after they have cured set them in the concrete slab. I was thinking 12" x 12" x 1" tiles.

[vtsteve]

Re: Insulation under the oven floor

I've seen members cast large blocks (4-6" thick), but it's too crumbly as a thin tile. If you could put enough cement in the mix to hold it together, it wouldn't be a very good insulator anymore. Low-cement vermicrete is about half as effective an insulator as the refractory boards.

[Les]

Re: Insulation under the oven floor

Hello All

I am about to start my build and have a question about the amount of insulation under the oven floor.
I have bought enough Fiber Board to give me 2" of insulation under the floor, dome and vent.

I'm a little confused, you can't put the board over the dome. That requires the blanket. Two inches on the hearth is on the low side (IMO) Anything you can add to that will help, vermiculite is cheap... Don't set it in your concrete, pour your hearth - let it dry and then do your layer of vermiculite. The forms for that layer takes very little bracing because it doesn't weigh squat...

[NCMan]

Re: Insulation under the oven floor

I'm a little confused, you can't put the board over the dome. That requires the blanket. Two inches on the hearth is on the low side (IMO) Anything you can add to that will help, vermiculite is cheap... Don't set it in your concrete, pour your hearth - let it dry and then do your layer of vermiculite. The forms for that layer takes very little bracing because it doesn't weigh squat...

In addition to adding a layer of vermiculite or perlite, you may want to consider a layer of plastic in between for a vapor/moisture barrier. Also, since you are in the planning stages, you may want to take into consideration the finished elevation of your oven floor, since adding layers of different insulations can affect your oven floor height.

[DavidApp]

Re: Insulation under the oven floor

Thank you for the replies.
VTsteve.
I have cast a small test block and I must admit it was crumbly, not sure what the percentage was but it did break in half as I got it out of the container I cast it in.
I was thinking I may avoid the drying problems I have seen others having with the perlite/vermiculite concrete.

Les
I do have a box of blanket for the dome. I had gathered that 2" of Fiber Board was the normal amount of insulation that was being used these days. I was just trying to get in a bit more by setting it into the concrete slab and thus not increase the height of cooking floor above the slab. I will have 2" for the fire brick and 2" for the fiber board so I was trying to get the insulating concrete flush with the slab.

NCMan
You mentioned a vapor barrier. This is not the first this has been mentioned. Should I put it between the concrete slab and the insulating layer?

I may pour a 5" concrete platform/slab with a 1" deep depression formed to the oven and vent profile. When the platform has set I will then fill the depression with insulating cement. Then place my 2"Fiber board on top of that.

Thank everyone for their input.

[Tonyp]

Re: Insulation under the oven floor

Hi David,

If I understand your description it sounds like the concrete hearth slab under the actual oven will only be 4" thick then? If so, then be sure to reinforce for the slightly thinner slab. Have you built the support wall for the hearth slab? Why not lower the height so you can incorporate a thicker insulation layer and keep the whole hearth slab a full 5"?

[NCMan]

Re: Insulation under the oven floor

Thank you for the replies.
NCMan
You mentioned a vapor barrier. This is not the first this has been mentioned. Should I put it between the concrete slab and the insulating layer?

Yes. This keeps the water/moisture in your slab from wicking up into your insulating layer. Place it under as well as around the edges, the best you can. The insulating layer will be across the whole oven area, correct? Also, if I follow you right, are you only pouring one inch of insulation under the board? If so, why is that? Can you not pour more? I guess some is better than none. I would recommend going about 1:8 ratio (Portland cement/Vermiculite) for better insulation value. That's the advice I got when building and it's done well. Also, the insulation layer you plan to pour should have no reinforcement in it. It's not needed and it would be counterproductive to what it's designed to accomplish.

[vtsteve]

Re: Insulation under the oven floor

If it won't throw you too far off schedule, your best bet is to add 2 more inches of board. You'll be well insulated, with the least possible height increase. The plans really should say 4 inches of board, 2" minimum.

[DavidApp]

Re: Insulation under the oven floor

Hello All
Nothing is set in concrete so far. I am trying to get everything straight before I start my build. My last oven was a two color brick oven with no insulation. They have changed the design since I did my build.
I am aiming for an oven floor height of 44" and have bought some half height blocks to try to achieve this dimension.
I can go with a 5" platform and dispense with the insulating concrete layer. Also I can add an extra 1" of fiber board to bring it up to 3"

Thank you for your input.
David

[tonybaker]

Re: Insulation under the oven floor

as heat rises and ash on the floor is a surprisingly good insulator, what you need is something that will really soak up the heat that is reflected from the dome when you push the fire to one side. My oven has 3+ inches of perlite over 6 inches of concrete. In the middle of the floor are two very high density firebricks that are designed to take up the heat.
It works well and heats up quite fast. Not sure if you can get perlite blocks where you are?

[DavidApp]

Re: Insulation under the oven floor

Hello All

Just to be sure I have this correct.
Sheet of plastic between the concrete slab and the FB.

I am going with 3" of FB under the hearth and vent.

The Forno Bravo directions call for a mix of Fire Clay, sand and water to set the hearth bricks. Will the FB tend to soak up the water in the mix?
Would a dry mix be better.

David

[Tonyp]

Re: Insulation under the oven floor

Sounds good to me David. I tried to make a slurry and found it a pain as the ceramic board was flaking while I was trying to spread it and then it sucked the water out and the mix became a paste.. I think some builders coat the surface of the FB with margarine or something to prevent that but I didn't want to add oil to the FB. I ended up just using dry silica sand for a pool filter. Really just a matter of what works for you to get the hearth bricks fairly level and even across the tops.

[DavidApp]

Re: Insulation under the oven floor

Hello Tony

I am going to do a test run with a dry 50/50 mix fireclay and sand spread out with a notched trowel to see how well I can level the hearth bricks.
I will post my results.

David