Oven Insulation

[Naushad Kasmani]

Hi. Thanks FB for providing the plans to building ovens. I have made my oven using firebricks 30% alumina and cement fondu instead of the refmix as it was locally available. The outer casing is a dome shaped made from stucco - standard portland cement + sand plaster. I could not source the vermiculite locally and had to opt for fibreglass wool insulation. The insulation is 4". The base insulation i have used insulating firebricks. However, the problem is that, while the inside temprature can reach to 400C, the outside temprature ranges between 60 - 70C on the outside dome. Is this a normal or shall i have to add more insulation?

The other problem i face is the chimney. I have connected a metal hooper but the smoke all comes out. Local engineers say that i have to use a blower exhaust fan as my oven is indoor and the outlet is about 40 - 50 ft away. Is there another way to this?

[dmun]

Re: Oven Insulation

If it's fourty to fifty horizontal, yes, you probably need mechanical ventilation. Vertical or near vertical, it should vent its self.

As to your insulation, domestic fiberglass batts aren't nearly good enough an insulator for your dome. Another problem is, that at least in the US, they have an organic binder which burns at oven temperatures, degrades what insulation value they have, and stinks up the place.

If you can't get vermiculite or perlite, you might look for volcanic pumice, the larger chunks used in landscaping might be suitable for insulation, or you could use a layer of the insulating firebrick outside your dome, which should reduce the temperatures on the outside to where you could use ordinary fiberglass.

[Neil2]

Re: Oven Insulation

"inside temprature can reach to 400C, the outside temprature ranges between 60 - 70C on the outside dome."

You oven is not as well insulated as it should be. The outside surface of the dome should be no more that 35 to 40 C (90-100 F) after firing to 400 C (750 F) for an hour and a half.

[Naushad Kasmani]

Re: Oven Insulation

Thanks dmun for the quick response. Yes my chimney outlet is 50ft horizontal. I tried using normal exhaust fans but they got too hot and blew off inside the chimney box. I will just go for blower exhaust in this case though my worries are that i hop it wont suck away the heat from the oven chamber.

Honestly speaking, in the town where i live, Mombasa kenya, i think it would be difficult to locate either of your preferred insulation types. Could you kindly highlight what the outside dome tempratures should be in a properly insulated oven? i can reach the 400C tempratures for cooking pizza and i should say the pizza;s turn out really great. i also achieve the white inside dome.

The cost of the insulating firebricks is way too high to accomodate covering the whole oven block.

[Naushad Kasmani]

Re: Oven Insulation

Thanks Neil, i have added up another 2" all round the dome with the fibreglass wool that remained from the initial coverup. Do u think that the cover style matters as far as insulation is concerned. I was wondering that the outside of the dome could have gotten hot as the stucco was directly placed above the insulation thereby compressing it slightly?

[Naushad Kasmani]

Re: Oven Insulation

Do u think the fireblanket (the one that is used to put off fire) can provide some form of insulation? though the thickness is <1"?

[Neil2]

Re: Oven Insulation

The stucco covering might have compressed the fiberglass wool thus reducing the insulation value. Part of the reason vermiculite or perlite is used is that it is non compressible. (Local terminology may be different but sometimes vermiculite is referred to as "blown rock".)

If practical, you might add more insulation over the stucco (without removing it). This might help. I'm not familiar with the "fireblanket" you describe, but adding this won't hurt.

Add this all over the existing dome as an experiment and see if it makes a difference. If it does you can leave it as it is or consider re building the insulation layer.

As to the ventilation issue, note that the largest volumes of smoke are produced when you first fire it. During the cooking phase there is considerably less smoke. Maybe some of the others on this site may have some suggestions with respect to mechanical ventilation.