Building up base approaching oven floor height and need input

[Daron]

Yes I have downloaded the plans the site offers and reading thru but as I read answers to newbie questions I realize I have a lot to learn before I go further. I am approaching cooking floor height but need to design my insulated floor install. I am hoping to add an insulated floor up to lower lip shown then place fire brick floor on the lip. Then build outer floor up to top lip level and start my dome there but that gives me a 26 inch inner cooking area. If I set floor out to outer rim I get 28 inch interior cooking area but will be robbing room to build my dome to proper thickness. I read I need insulation UNDER where I start the first ring of fire brick of the dome. My current design did not have that. And is 9 inches the absolute thickness for the dome? If so I will need to build out the last few courses of rock to give me enough surface area. Don't want to screw this up after all this work. THANKS!

[david s]

If you are building your oven using firebrick the minimum recommended wall thickness is 4" so the bricks are usually (depending on your brick size) cut in half. If you go the cast route then you are not relying on the strength of mortar joints so most cast ovens are 2" thick. I think 9" thick is overkill and may take all day to get to high (pizza) temperatures.

[fox]

The overall thickness will largely depend on how much insulation you use, 4” is a good thickness plus plaster finish so say 4” brick 4” insulation and 1” finish plaster = 9”.

[UtahBeehiver]

By placing the fire bricks on any uninsulated area, IE lip of the concrete circle, the heat from the fire brick will transmit down thru the concrete which will act as a heat sink. The floor and the dome "must" be isolated by insulation for effective oven performance.

[Daron]

Thanks Russell, I've now learned that it should create a complete evenly insulated cooking environment so I'll take away the top few inches of my concrete cylinder so I can pour a proper floor to build my dome off of. I read the fiber matt insulation is prone to breakdown with moisture intrusion so leaning towards vermiculite insulating layer above reinforced concrete floor then insulation then refractory mortar to build off of.. If I can insure base would stay dry I'd do a matt floor but still prefer vermiculite coating then refractory cement layer then waterproof stucco on dome. Feedback anyone? Answers are less than I expected.

Daron

[SableSprings]

Daron, first--nice looking start for your stand! However, I'm a little concerned about your last response/plan. Ceramic insulation board doesn't break down...it just can easily absorb water, rendering it useless as an insulation until the moisture is driven off. We've had several folks on the forum who've had the lower board get soaked but all have recovered with several drying fires. One of the reasons we recommend putting weep holes in the hearth concrete and placing porcelain tiles between the hearth and base insulation board is to give any water that intrudes, channels to escape. Although vermiculite or perlite insulating concrete (5:1 - v or p to Cement) is often used because it is less expensive...however, it also will absorb water (which requires a long time to remove/drive off) and takes twice as much thickness to equal ceramic board (2" board = 4" of perlcrete or vermicrete). Currently there is a ceramic board available that is water resistant...a really good thing!

So, on adding weep holes or channels for water to escape from the surface of your hearth is very worthwhile (sheets of mosaic tiles work really well to provide the channeling to the weep holes). Your cooking floor bricks are laid directly on the insulation board or insulating concrete layer...no refractory mortar needed here. You may need a thin layer of sand/clay mix to level the cooking floor bricks...but just a thin layer.

Most folks choose the easiest method (IMHO ), of then starting their dome build on top of that cooking floor's perimeter. Having some of the cooking floor bricks protrude beyond the dome base is OK because they'll be covered by the outside dome ceramic batting or 10:1 insulating concrete and whatever final stucco or finishing material to want to attempt to use as a waterproofing layer. I say attempt, because there are many options to protect your oven's insulation from water/moisture intrusion but they all take consistent maintenance. The best protection for your oven from the elements is to put it in an enclosure...just saying...

I hope that helps a bit...here's a nicely documented oven base build that uses these hearth/insulation principles:
https://community.fornobravo.com/for...andpoint-idaho

Enjoy the journey!