40” new build

[MikeD55]

So I’m planning my first new build. 40” oven with an 16-18” dome using a combination of arch bricks. I’d like to do something with a thicker floor for retained heat. Was gonna do 3” of foamglas and 1” CaSil and a 4” floor. Here’s my question:

1-was thinking of using IFB under the dome and surrounding the foamglas/CaSil. This would require 3 rows. Should i mortar the first row down to the concrete counter OR should i let it sit dry and allow the dome to “move” as it expands and contracts OR mortar that first row down?

2-instead of using IFB surround the space around the floor/below the dome extension out the Foamglas or CaSil essentially letting the floor “sit” in a well of insulation and let the dome sit directly on top of the insulation. This would have the added benefit of not having to cut so much IFB.

thanks
Mike

[rsandler]

There's no need to mortar down the bottom layer of insulation, whatever you end up using; the weight of the oven will keep it in place. This forum strongly recommends putting in weep holes and a layer of (unmortared) impermeable tiles under the floor to provide an air gap and channels for moisture to reach those weep holes. Trying to mortar in a layer of insulating firebrick would interfere with that, so another reason not to mortar.

I'm unclear what you're suggesting in your second question. The dome usually either sits on the insulation, or else on the firebrick floor, depending on your preference. You'll want insulation completely surrounding the dome and floor one way or another. I'm guessing you already have since you've been hanging around the forum for a while, but do download the Pompeii plans and read them if you haven't yet. They are a little dated and there's some newer innovations, but those are a good starting point for basic construction questions.

Couple thoughts regarding what you have in mind for the floor:
-With sufficient insulation (4" under floor is a good start), a standard 2-2.5" floor is more than enough to bake a batch of bread the day after fire; multiple batches are possible if you plan around a decreasing heat gradient (start with a bread baked very hot like baguettes, then follow with a more enriched dough). If you plan on making more than 1-2 batches of 6-8 loaves at a go, a 4" floor could be handy, but otherwise it will likely be overkill.
-Make sure you're comfortable with the resulting height of the floor for operating the oven. 4" floor + 4" FG/CaSi + a layer of IFB is adding more height than typical. Not necessarily a bad thing, just would be wise to mock it up and see if you like it, and adjust if not.

[MikeD55]

Thank you. Regarding my second question my thought was instead of 1-2 chains of IFB, I was wondering if it’s possible to cut a layer of CaSil or FoamGlas in the shape of a ring. Surrounding the floor, sandwiched between the “main” insulative layer and the dome.

[NewEnglandNewb]

For the second one, I’m curious why are you trying to insulate between the floor and the dome? I can see having a small gap if your dome is around your floor rather than sitting on it since they may expand/contract at different rates during heating and cooling cycles. Most people seem to just add a layer of cardboard during construction to achieve this and just let it burn off and fill with ash. For retained heat cooking I would think you would want the floor and dome temperatures to equalize, so heat transfer between the two would be desirable. What am I missing?

[MikeD55]

Not between the floor and the dome. But under the dome as well as the floor. The floor will be 4-4.5” thick for retained heat but rather then cut two extra rows of firebrick on the bottom of the dome surrounding the floor my thought was to add an extra ring of insulation at least 2” thick beneath it. Yes I will be incorporating a 1/4” gap between the floor and the dome.