Hearth insulation on custom oven

[Custom Oven]

I have a custom built oven with the hearth floor fire bricks built on a 1.5-2" stone slab. Beneath the stone slab is open air area for holding extra wood. I want to add some insulation to maintain hearth heat during use. The FB insulation board looks like a good option to apply to the stone base underneath given the apparent ease of installation and insulation capability. How do I adhere this board when there will be open space beneath it? Thanks.

[SableSprings]

Welcome Custom Oven! Unfortunately you need to place the insulation layer between the stone slab and the cooking floor fire bricks. Currently you are heating both the fire bricks and the stone slab beneath (as I read your post). Here's a link to a great cross section of a WFO showing the proper positioning of insulation layers.

https://community.fornobravo.com/for...901#post406901

Can you post some pictures of your oven and installation to better confirm "the problem"? The best option would be to lift the entire oven up so you could place the insulation into the proper position between the oven and the stone slab, but I suspect that's not an option. You might be able to place some insulation board underneath just the cooking floor if you can remove those fire bricks, place the board on top of the stone, and then replace the fire bricks. It would entail working with a cooking floor lifted 2" higher in your oven...

Putting insulation under the cooking floor only would not be as heat retentive, but it would help quite a bit...again if you could work with a "lifted" cooking floor. Also, do you have insulation over the oven dome itself?

[Custom Oven]

Thanks for the response. I agree that the insulation should be between the stone and fire brick but that would require me to disassemble most of the oven. I'm looking for a simple solution to preserve heat retention of the hearth fire bricks. Think I'll get some insulation board and support it against the stone base underneath. Any suggestions on how to do this - metal rods holding insulation board up against the stone? Thanks.

[UtahBeehiver]

I agree with Mike that the best place for the insulation is between the fire brick and the stone slab. What temps are you seeing on the bottom of the stone slab inside the wood storage at the peak temp of your oven? Concrete with aggregate has a K value (thermal transfer) of about 1.7, stone upwards of 2.0, fire brick 1.4 so the stone slab will act as a heat sink even moving the btus to the walls of the stone structure. If the stone temperature is relatively low, say 100 F I would try some type of insulation less expensive before springing for ceramic fiber boards, maybe even structural foam, or fiberglass to see if it helps or not. You can drill concrete anchors in the bottom to secure.

[SableSprings]

Thanks for the response. I agree that the insulation should be between the stone and fire brick but that would require me to disassemble most of the oven. I'm looking for a simple solution to preserve heat retention of the hearth fire bricks. Think I'll get some insulation board and support it against the stone base underneath. Any suggestions on how to do this - metal rods holding insulation board up against the stone? Thanks.

As Russell noted, use something cheaper than ceramic fiber board. Also your idea of bracing some insulating board against the base may be the quickest way to see if it's worth going for a more permanent attachment (or superior insulation product). Obviously, if your stone temp gets up to 100F, then the insulation will help keep the fire bricks in the 150-200F range longer. Trying to keep high temps longer means that stone needs to be closer to those higher target temps...and that might produce a structure that "can burn you on the perimeter".

Also, what temps are you trying to "preserve" in the fire bricks? The fire brick is going to bleed heat into the stone until it reaches equilibrium. The greater the differential, the faster the transfer between the layers. Insulating under the stone, just under the oven will not stop the thermal transfer to the sides of the stone slab. You will achieve better heat retention for the entire structure but that includes the firebrick and stone. So, again, if you want to retain 400F-500F in your fire bricks for several days, that means you are going to have to heat that stone up near those temps as well. Be aware that you will be using more firewood to heat the oven and stone slab to get to your retained heat goal.

I don't mean to be such a pessimist, but not knowing all the factors of this build (type of stone slab, location, upper insulation, etc.) makes it difficult to be positive about finding an easy solution. Pictures would be very helpful. Please keep posting...this is going to be interesting (and hopefully will result in enough gain for you to be satisfied).

BTW: I truly appreciate folks like you that test/question the "common knowledge and staus quo"...that's when the best discoveries and innovations are made...thank you for being you!

[Custom Oven]

I only burn the oven once every 1-2 weeks so the bricks are cold. The dome and sides achieve the temp I'm looking for (650F+). The hearth is about 550-600F when initially moving the ash to the side in order to start cooking pizzas. However, by the 8th pizza, the hearth temp fades to 400-450F. The bottom of the stone portion supporting the hearth fire bricks is exposed to open air and no doubt leaking a fair amount of heat. I'm trying to preserve/build heat on the hearth during the whole cooking time. Will post some pics. Thanks for everyone's input.