Hearth Insulation Question

[Aquaponicsguru]

I have seen glass bottles used in cob ovens. Does anyone know if this would be feasible in a Pompeii style build using the glass bottles with vermi/pearlcrete? Any advice would be greatly appreciated.

[RandyJ]

Re: Hearth Insulation Question

I think you would be better off just using the vermicreete. That is what the plans call for so I think that is the route that I would take. If you want to go for more insulating value then you would need to step up to some ceramic board.

Randy

[Aquaponicsguru]

Re: Hearth Insulation Question

Thanks. That is what I did. You were the only person that responded to my question so I thank you for that as well. I just set the cooking surface in fireclay/sand yesterday and will be starting on the dome today... wish me luck.

I do have another question... my oven is 36" and I plan on using a 6" stainless steel insulated chimney. Is a 6" chimney sufficient and, if so, how big should the vent opening be a the top of the arch. It is hard to see in the instruction manual that I downloaded for a Pompeii oven.

[NCMan]

Re: Hearth Insulation Question

I think you would be better off just using the vermicreete. That is what the plans call for so I think that is the route that I would take. If you want to go for more insulating value then you would need to step up to some ceramic board.

Randy

Or....just pour a thicker slab of vermicrete, if you have some flexibility w/the height. Also, the ratio of the mix affects the insulating value some. I used a ratio of 1:8 and it works well.

[RandyJ]

Re: Hearth Insulation Question

NC man I was saying to follow the plans verses doing the glass bottles in the insulation layer. Going with more is always a good idea. Be that more vermicreete or going for a more hi tech solution like the ceramic board. I myself went with 4" of ceramic board under the oven, and 3" of ceramic blanket and 36 cf of vermiculite over the top. I am always a fan of go big or go home.

Randy

[NCMan]

Re: Hearth Insulation Question

I agree. Your oven is well insulated, for sure. Great advice.

[david s]

Re: Hearth Insulation Question

Or....just pour a thicker slab of vermicrete, if you have some flexibility w/the height. Also, the ratio of the mix affects the insulating value some. I used a ratio of 1:8 and it works well.

I think 8:1 is pretty lean for under floor insulation that has to support a heavy load. I think the plans indicate a 5:1 ratio. Check the compressive strength in the attached table. (I think the values for compressive strength for 5:1 are incorrect and should read 175-225) but it is considerably more than the 8:1 strength, about double, with a far smaller loss in insulating value.

[NCMan]

Re: Hearth Insulation Question

All I know is I did quite a bit of research and asked around a lot. I feel 1:8 works fine, as it did for me and it appears to work fine for others. I have heard of people going as lean as 1:12 and it worked fine, it seems. I will stick to the 1:8 ratio in the future and would not personally consider going leaner than that and wouldn't recommend it to others.