Thermal Break

[JACK]

It seems to me that most ovens are built without a thermal break between the dome and the outer entrance area. Mine will be a 42" Pompeii.
My dome will be housed in a brick (dog house) type shell. It will have 4" of perlicrete under firebrick floor.

Should I worry about it or just lay it all up solid?

[UtahBeehiver]

I would say at least 50% of the builders have thermal breaks between the inner arch and vent area. Builders who want solid multiday cooking often install thermal breaks. 4" of 5 to 1 pcrete (recommenteded under floor) is your minimum amount of floor insulation. Heat loss from the oven opening is also a big culprit so a good insulated door is a must for preserving BTUs.

Attached is the K values for p/vcrete but it is your choice.

[rwiegand]

I didn't use a thermal break and my oven is at 225 deg 72 hours after firing with a badly cobbled together door. A real insulated door will probably do even better, but that seems good enough for my needs.

[JACK]

Utahbeehiver, I think I feel a little negativity about using 4" of perlicrete under my oven floor. I am new at this and would like all the info I can get from those of you (especially you ) that have already built ovens.

I have read that 4" of perlicrete has the same insulating quality as 2" of cal silicate board.

Any info or advise would be greatly appreciated.

Every thing I have looked for here in NC is so expensive , especially insulation board and Heat stop 50.

I sure don't want to mess it up over a few hundred dollars.

Thanks
Jack

[UtahBeehiver]

P/v crete is a good option, it just takes more thickness than CaSi or Alsi board. So it is a budget and material availability issue for you to decide.This applies to the dome insulation as well which can be 8-10 to 1 p/vcrte or ceramic fiber blanket. Some have tried rockwool or reg fiber glass insulation, I would not recommend either. Spend a couple bucks and download the "cheap" eplans from Forno Bravo, it will give you a good baseline for the oven design. Home brew is a viable option for your mortar, I used it on my build. 3 1 1 1 sand, lime, portland, fireclay. Fire clay will be the hardest material to procure but and bricklayer supplier should have it and it is relatively cheap.

[JACK]

Thanks UBH, I have the plans, So many choices. As I said earlier my dome will be housed in a doghouse so to speak . I may just dump it full of perlite right on top of the fire brick dome. The thickness of insulation doesn't bother me at all, I stand 6' 5" so with 4" of insulation my hearth will be about 50" high.

How did your home brew Home brew mortar hold up over time?

Jack

[UtahBeehiver]

The beauty of the pompeii ovens is that design is for the most part self supporting and the mortar is a filler agent. I have not notice any degradation of the homebrew on my build which I did in 2012 and I have not seen any reports of homebrew failures of ovens that used the DIY mortar. Pouring of perlite into the enclosure works good too. Should leave some type of access so if settlement occurs you and add more. To cut down of the perlite requirements some builders have block the corners of the structure with scrap ply or sheet rock. 4" of 5 to 1 pcrete floor is your min. If you formed with with a 2x6 it will give you 5.5". Couple more items, be sure to install weep holes in the concrete hearth so water can egress out. Also, p/vcrete will has lot of water in it due to the mixing process so you really need to let it dry out well, say 2 -3 weeks before laying the floor.

[mk e]

It seems to me that most ovens are built without a thermal break between the dome and the outer entrance area.

I used one....its a little more labor but my time is free to me so I did it after the concrete was hard I filled the break with a portland/vermiculite mix, as as I put under the slab. Probably little difference either way it just made me feel better.