Can I build my oven on ceramic fibre board

[Nikkit]

Hello there,
I am about to build my first brick WFO and would like the best insulation possible, this I believe is Ceramic Fiber Board. My question is this, I have purchased 3 X 20mm boards and want to build the oven dome directly on the boards and fit the oven floor within the dome. Is this going to be ok or will the boards disintegrate under the pressure of such a heavy structure?
kind regards,
Nikki

[SableSprings]

The Ceramic Fiber Board through Forno Bravo and most other sites is designed to easily support the oven's weight. The important factors in WFO ceramic boards are compression >70 PSI & 0.07-0.08 w/m k at 400C thermal K factor. Remember that some ceramic board readily absorbs water and we recommend putting several weep holes through your hearth slab, a layer of porcelain tiles to keep the board off the concrete surface, your ceramic board, a thin layer of sand/clay mix (if needed to level), and your cooking floor bricks. You can either cut your bricks to fit inside the dome perimeter (leaving a gap of 3-5 mm for expansion allowance during firing) or lay your dome perimeter directly on the cooking floor. Either way works, as long as both are on the insulation board. Some prefer the brick floor inside the dome simply because you can replace cooking floor bricks easier if one gets damaged.

Hope that helps...looking forward to your build thread. Remember that the more complete your plan & understanding of the goals for your oven build & use...the easier the process will be. Don't be afraid to spend lots of time looking at other builds and taking notes of various techniques & design features that will work for you.

[Nikkit]

Thank you Sable Springs, I appreciate that.

[masterofcoal]

Sorry to hijack the thread but I have the same question and I want to understand the SS’s response better.

why are there weep holes if I use a ceramic tile? I dont want to drill into my beautiful slab
is 50mm cf board enough or do you suggest vermicrete below it?
is it a good idea to have a plastic vapour barrier between vermicrete and the board? or in case no vermicrete, between concrete slab and board?
Thank you!

[SableSprings]

The mosaic tile sheets hold the insulation board above the cement hearth platform and the gaps between tiles create channels for moisture to escape. If you create weep holes by drilling or during the slab pour, you are providing an exit route for any accumulation of water. We don't recommend putting a layer of plastic between the hearth & base insulation because it will trap and pool any moisture creeping in from the oven perimeter, up through the concrete slab (concrete is not waterproof), or down from the oven/door & landing.

Fifty mm of insulation board is a common & adequate amount. Many builders want (and do add) a perlcrete layer between the hearth and insulation board to increase heat retention for their oven. Perlcrete & vermicrete both hold a lot of water when mixed and it can take a long time to dry them completely... Making a tile/weep hole drain system even more important. I hope this helps answer your questions.

[masterofcoal]

Thank you it does. What i am not sure of is the vermicrete, is this under or over the board? If it is over, then how will weep holes help? I dont want any water in the board

[SableSprings]

If you are going to use perlcrete and ceramic board to insulate the base of your oven, the perlcrete would be below the ceramic board. The ceramic board is the better insulation & comes in sheets, making it easier to level (for the cooking floor bricks laid on top of it).

Here's the sequence from the bottom up;
1) Reinforced concrete hearth slab with 4-5 weep holes (1/2" or so in diameter). Underneath, glue some insect screen to keep bugs from using the holes as "homes".
2) Mosaic tile sheets (placed upside down...backing mesh on top). These square sheets of tiles don't have to match and extras or discontinued patterns are often available for very little money. Align gaps between tiles to the weep holes as much as possible.
3) Perlcrete or vermicrete (you will need to make a form to contain it while it cures). A handfull or two of clay in the mix aids its workabiliity. There's a lot of water held by this material and it's really good to let it air/sun dry as long as possible (at least a week or two if possible). Even when cured, the material is crumbly on the edges so most people leave the form on as long as possible or even make a thin border of mortar or concrete around the perimeter.
4) Ceramic board
5) Possibly a thin layer of sand/clay to make a very level surface on which you will lay your cooking floor bricks.
6) Cooking floor firebricks (and/or dome base brick chain).

You can build your dome directly on the cooking floor or cut your cooking floor perimeter so the dome base will be built just outside the actual cooking floor. In either case, your oven must be entirely isolated from the hearth slab...so make sure to make your insulation base layers large enough for the entire oven's footprint.

I hope that clarifies how the recommended base is designed & built. Looking forward to your build.