Help with Insulation for vault oven

[Gulf]

Rusty,

Leveling your stand with sand for your fiber board is probably just fine. As long as it is not so thick that it can not be contained and could sift out at the edges. If it is that thick, you might would want to go for the extra insulation that a vcrete layer will offer (that also means waiting on the vcrete layer to dry). That is up to you, and also what your finished height will allow. I'm not sure why you would need to further level the firebrick on top of the fiber board. Most, level the oven floor brick with fireclay. If you lay your oven wall on top of the fiber board, you can use mortar where needed to level them. If you are laying them on top of the oven floor brick, a thin (no stick layer) is usually used. I've seen aluminum foil or newspaper used to keep the mortar from sticking to the floor brick. I would guess that parchment or butcher paper would work just as well.

Note: I like the practice of a moisture barrier between the hearth slab and the insulation layer. I also like the practice of elevating the insulation layer above the hearth slab.

I hope this helps.

[Rusty Nuts]

So progress has been slow but after taking out the base bricks and relaying them on CF board I am happy with the insulation happening now and was very excited that when I took the formwork away the main roof stayed up! So the final build will be a brick box that is topped with a 2 inch slab of stained wood (macrocapa) and the chimney will run through that and into the wall. My question is about the chimney joints. I have cut the pipes (ceramic field tiles with a 10mm/bit less than a half inch wall) and will seal these together with fire mortar at the joins. I feel that over time this will possibly crack and let smoke out. I was thinking of wrapping the exposed chimney section in chicken wire and applying a render to it. My question is what sort of render would be suitable? can I just use a cement/sand mix or should I be using a homebrew 5:1:1:1 (sand, lime, fireclay, cement)? or even just a cement/sand/clay mix?

or should I use the fire mortar thickly at the joins and then a more basic render elsewhere?

This does not need to be weatherproof, just seal the chimney to stop smoke coming out.

One note on the clay for a render (not a home brew) is it OK to just dig clay out of the ground (like from a building site or where a new road cut has gone through) or do I need clay powder that has been dried?

Sorry if this sounds a bit confusing, it's getting late and my brain is switching off.

Cheers for the help.

Russell