Cob oven hearth insulation

[benny8]

New to the forum. This spring my wife, daughter and myself have planned to build a cob oven over a smoker. Plan on a block base that will serve as smoke chamber, with cob oven on top. We plan on pouring 3" of quikcrete for the structural hearth-then our question comes for the insulated hearth. Cob ovens call for cob base with bottles set in the cob to serve as insulation. If we put bottles in the top layer of cement, will this serve the same purpose? We then plan on setting fire bricks on this layer of bottles/cement. We have decided on the cob oven for a number of reasons-such as availability of materials, ability for the family to get involved, and to see how we like the outdoor oven. I plan on using it as long as possible with someday maybe taking the cob oven off the platform and building an actual brick oven. Your responses are greatly appreciated.

[okn]

Re: Cob oven hearth insulation

Most people on this forum aren?t super keen on using the bottles for insulation. The recommended advice is using a mixer of perlite or vermiculite with cement. You can look through many of the build threads and see what I?m talking about. The next recommended way to add insulation is with the insulating board. Forno Bravo sells it, as do most refractory supply places. It?s more expensive, but if installed correctly will insulate better for the give thickness than the perlite/vermiculite mixers. It's also fast and easy to work with. I think your choice of a cobb oven is great, especially if you can get the family involved. That being said, you can get your family involved in any type of build, considering there are many different tasks to complete. One thing I really didn?t understand much when I built my oven, is how long all the finishing takes. I built my dome (low barrel vault design) in one day. It took me almost as long to screw the cement board on the sides. When I built the dome, my thinking was that if it collapses I can just rebuild it in no time. Now that I did all the other work with finishing it, I?m like that thing better hold up for a good long while. Just some things to think about?

[david s]

Re: Cob oven hearth insulation

Most people making a Cobb oven are doing it this way to either use resources that are close at hand, free and sustainable. Or they are using cob simply because it is the cheapest and quickest way to build an oven. There are superior forms of insulation, but the bottles work and you can simply make the layer thicker to achieve a similar result.
Setting the bottles directly in the concrete may not be so good as it is a fairly good conductor. They would be better filled in with a cob mixture of say 50/50 mud, straw which is much more insulating. Or better still a 4:1 vermicrete or perlite mix, but that requires buying some. Another technique is to smash the bottles, but not too fine, and lay the fire bricks directly on top.I'd say you'd want a layer of this (not such good quality ) insulation of about 6".

[Gulf]

Re: Cob oven hearth insulation

Benny,
Check out this build .

I think that she has done a great job incorporating the "materials available" and some good advice from the forum .