Insulation thickness: are 2" board + 3" blanket enough for 850F (and more questions)?

[losangelesdweller]

Hello,

I searched and searched. Apologies if I had missed something obvious. There are many options for insulation, it seems. I want to build a 36" Pompeii. This is for the home, occasional weekend parties. I imagine we'd want to get it as hot as 850F? Instructions suggest 2" insulating ceramic fiber board, and 3" layer 1" ceramic blanket. I'll mesh/stucco over that

Is this sufficient to achieve 850F+ (or should I be using more insulation in either floor or walls)?
Why do people combine other types of insulation with this approach?
Should I be trying to figure out if there are better options (bang/buck)?
Should I be worried about health concerns of the ceramic insulation post build (maybe if my seams on the inside aren't tight enough, etc)? In other words, should I do something (foil?) between the dome and the blanket?


PS. (It actually occurred to me that these blankets should not be compressed, so I am a bit confused about what happens when you stucco over them? You just use heavy, not very sparse mesh?)

Thank you very much, and sorry for many questions.

[david s]

You can render straight onto the blanket, but it tends to be a bit spongey to work against. I find the main problem is that the shape is somewhat lumpy so to compensate the outer render then has to become really thick to get a nice smooth dome surface, unless you like that Fred Flintstone look.I prefer to add a layer of vermicrete over the blanket to get a nice even surface. The stuff goes harder than the blanket and is easier to render a nice thin shell against.The vermicrete is also a good insulator although not as good as blanket, but you can use two layers of blanket and then vermicrete for adequate insulation.

[JRPizza]

You are going to want more insulation under your floor if you want your oven to stay hot for days. Check out RandyJ's recent posts about oven temperatures. Folks with 4-6" of insulation under their floors are seeing some very impressive heat retention and are able to slow cook several meals after firing for bread or pizza.

[losangelesdweller]

Thank you! So I'd do insulation, lath, vermicrete, then stucco?

It looks like insulation I want (I saw another thread when searching for RandyJ) is 4" bottom, 3.5" sides/top. On the bottom, I assume I can just stack two 2" boards (making sure to avoid having seams going in the same direction). Does that sound about right?

[JRPizza]

Stacking is fine. Most people stagger the joints, which is what I did - not sure how much it mattered but seemed like the thing to do. I ended up with 5" of board but that was what worked out best for me. I built a 39" ID oven, so with the 4.5" thick dome I had a 48" footprint. I used Insblok 19, and 2 cases of 2.5 fit almost perfectly with little wastage and only a few small pieces here and there. I was probably going to go with 4" but changed my mind after doing a layout. For the product I bought, you get different number of 12'X36" boards depending on thickness - 2.5 inch had 5 to a case so 10 boards fit well, had very little wastage and gave me the 5" height. I hope to use my heat for several days of cooking - time will tell. I plan on usig the little bit of left over board to insulate my door.

[RandyJ]

Yeah I ran the boards in different directions so as to not have gaps in the insulation. I used the 2"thick board from McGills warehouse. It seams to do a great job and was the cheapest price I could find.

Randy