"Bonding" multiple layers of ceramic board insulation

[mrpubnight]

I'm about to get started on my dome, having already poured my slab, and was wondering about the ceramic board insulation.

I have an opportunity to get 1" board for a much cheaper price than I can get the equivalent 2" or 4" but I'm wondering how (if at all) I would go about 'bonding' the layers together as I would want to put at least 3" worth of insulation down below my oven floor.

Also, I have also seen using a light layer of fire-clay mortar (fire-clay / water) between the ceramic board and the firebrick oven floor. Does anyone have experience with this?

Thanks in advance.

Frank

[Gulf]

Hi Frank,

Welcome to the forum. There is no need to bond the 1" layers of insulation together. Just stagger the joints beween layers.

If the insulation layers are uniform and flat, and the firebricks are a uniform thickness as well, there is no real need for the clay imo.

[UtahBeehiver]

Gulf is right about being able to use 1" thick CaSi. Be sure to check the specs on the board, there have recently been some builders that have gotten CaSi with low compressiblity ratings, ideally it should be over 75 psi with 5-10% compression and K factors in the range of 0.55 Btu.in (hr.ft2.F) at 500 F

[mrpubnight]

Thanks guys. The board is Insblok 19 which I think is pretty standard although the compressive strength is lower (~35 psi).

Would a layer of firebrick be better - would I need insulating firebrick? Given the costs, I can get 4.5" of standard firebrick at a lower price than 3" of Insblok. I recognize not all insulation are the same.

Trying to compare the insulating qualities of both and determine some options.

[UtahBeehiver]

An additional layer of fire brick, not insulating firebrick (IFB) will give you quite a bit of thermal mass (also means it will take longer to heart up as well) but will not be an effective insulation option. I am not sure what you mean Insblok 19 as being standard. There are many other CaSi or AlSi board manufacturers out there with better compression ratings. These ovens are really heavy and need appropriate structure underneath them.

Have you considered putting down a perlite or vermiculate/concrete layer down as insulation under the fire brick. It is less expensive than CaSi board with fairly good K factors. For under the floor a 5 to 1 ratio is recommended and at least 4-6" thick