Fiber Board

[torrie]

My stand is completed and I am about to start the dome. I have a 7in. layer of vermiculite/cement on top of the concrete slab. Is fiber board necessary over the vermiculite?

Another question about the first course of bricks, do they need to be mortared to the vermiculite insulation? The stand build went well but this is my first attempt at a oven so there is a lot to learn.

[moderator]

Re: Fiber Board

The fiber board will out perform vermiculite at 2 to 1 in insulation if you use about 5 to 7 parts vermiculite to 1 part concrete. So you have about 3 1/2" of equivalent insulation, that is usually sufficient. The insulation is a new world solution to vermiculite. Just make sure the vermiculite base is strong enough to support the dome as it tends to be spongy. Adding the insulation will improve performance so it won't hurt.

The weight of the dome should compress the sponginess of the vermiculite, so you should not need mortar below the brick. However, it never hurts to use it on the base exterior as a sealant and limit dome movement during expansion / contraction. Mortar has different thermal properties than brick, but it will transfer heat unlike vermiculite which is an insulator. So if you do use mortar, just make sure it is not acting as a heat conduit drawing heat away from you dome to the exterior facade without an insulator below / beside it.