Damage Limitation for Floor Insulation

[tompowell2468@hotmail.com]

Hi All,

This is my first post, have read the forums but make the beginner error of underestimating the importance of floor insulation. Just fired up my oven for the first time properly and the bottom of my concrete got extremely hot to the point where a small crack appeared. I used a mixture of perlite, straw (was trying to save money!) and cement to insulation the floor but only 2 inches. In hindsight, should have gone for vermiculite board. My question now is, how much thickness of board do I need.?From my photo, you can see I have began pulling out the perlite insulation. I have 90mm to play with. My firebricks are 38mm thick leaving potential space for 50mm of board. Do I need 50mm of board or will 25mm be enough. Not sure how effective the boards are.

Many Thanks

Tom

[wotavidone]

So, you've got a concrete slab you built the oven on? I take it your dome bricks go all the way down to the concrete slab?
Those two bricks across the front are about 90 mm and hold the floor bricks and insulation in behind them?
Use the full 50mm of board. That is not really enough if you are using vermiculite board, most say 4 inches, but without a full rebuild it is all you can do.

Having said that, 2 inches of perlcrete (even with straw in it or maybe because it had straw in it) should have slowed the heat transfer to the concrete slab slow enough not to crack.
I suspect the oven was still wet. Mine took about 10 "big scary" fires before I could reliably get the floor to 60-90 second Margherita temperatures.
Any more photos of the oven?

[tompowell2468@hotmail.com]

Thanks for the reply,

Yes, it is built on the concrete slab. My dome was made over a sand mold using refractory castable. From the picture, you can see the edging bricks with the castable starting on top. You were right in thinking that the floor was still damp as all the water from the sand mold had seeped through to the floor space. Even still, I think it is the right decision to redo the floor. I say I had 2 inches in perlite but in reality it was probably more like and inch and a half with the rest being loose sand to lay the clay bricks.
I am going to buy the vermiculite board. My next question: I have some vermiculite stove fire bricks, 1 inch thick and enough to cover my floor space. Do these act like vermiculite board and if so would it be ok to have an inch layer of vermiculite stove bricks and then a 1 inch layer of vermiculite board?

Thanks

Tom

[UtahBeehiver]

I believe what Wotavidone is getting at is with the dome walls sitting right on the concrete hearth, heat from the dome will transfer directly to the hearth acting a heat sink. This will affect the ability of the oven to maintain heat as well as transfer heat down to the hearth although not as much as the "wet" vermculite/straw mix. before you buy the v-board, you need to do some research to make sure it will work for you. The board should have a thermal K value of at least 0.07 to 0.09 W/m C at 300 to 400 C and a compressive strength of 0.5 mPa. Not enough info on Vbricks to say whether they will work.

[tompowell2468@hotmail.com]

I understand what you mean now. Yes, unfortunately, I hadn't considered that. Hopefully as I am only planning to cook pizza's and not have the need for long periods of heat retention, that shouldn't affect it too much (as long as I sort the floor insulation out.

Thanks