ovens and earth bag construction

[Annie]

Greetings,
I am building an igloo style oven on an earth bag constructed base. The back of the oven will butt up against a curved earth bag wall. (Earth bags are adobe filled sand bags, 12”thick and are covered with 2” of adobe and 3 coats of lime plaster). I want to construct the dome as close to the wall as possible to give more space in front. I was hoping the final oven would appear to extent out from the wall behind it.

My question is: how close can I set my first course of fire brick to the earth bag wall and how much insulation would be necessary in that area? Could I leave room for the 1” of the mortar and 1” of Insulfrax and then allow the wall to take the place of the 4” of vermiculite?
any input on this would be very helpful!

[dmun]

If you should ditch anything, I would say it's the one inch of mortar. What's that for? Extra thermal mass? You don't need it. The adobe will have no insulation value, I'd make the wall a little thinner in that area so you can get more insulation blanket in the space. One inch of the blanket just isn't enough to prevent heat loss (and bag burning).

[Annie]

Thanks,
I know that adobe is used for ovens here in the west, but have been unclear as to it's insulation ability. (Plenty of thermal mass) So....if I put more layers of blanket would it reduce the over all thinkness of the final oven?

[dmun]

Let's back up a bit. What is the material of your dome? Is there any insulation underneath the cooking floor?

To answer your question, No, adobe has next to no insulation properties. When it finally dries out (12 inches? really?) it will have the same thermal characteristics as concrete.

Remember the FB forum mantra:

Insulate! Insulate! Insulate!

[Annie]

Let me restate the question,
I am building a igloo style oven per the forno bravo plans. I am working in limited space and the back of the oven will butt up to a curved mission style earthbag wall. (adobe in sand bags)Unfortunately I built the base and the wall before I knew what the heck I was doing with the oven. I have discovered that inorder to build a 36" oven per the plans, I will have to extend the base with a "landing" in front of the oven door. I'm afraid the look will be funky and unattractive I want the exterior of the oven to blend into the wall behind it and get the oven as far back into the curve as possible. I understand that insulation is very important but is there a way to reduce the exterior diameter without sacrificing insulation???

[dmun]

You could reduce the thickness of the back wall of the dome by half without sacrificing anything structural: there are only a few bricks that you would have to cut, and they would be on the lower rings, that heat up last. My whole dome is only 2 1/4 inches thick and it works fine, although I now think a thicker dome is more structurally sound. With two inches of insulating blanket between the thin back wall and the rear wall you should be fine.

Any drawback? When you are throwing logs in your oven with wild abandon, remember you're aiming at the thinnest part of the oven.

Sorry if I misunderstood your question originally.

[Frances]

This is just a thought, but maybe you could also reduce the length of the entry archway and curve the chimney back a bit after the vent - that could give you more room on front of the oven, too.


As for the landing in front of the oven... if you build the oven so it looks as if its coming out of the wall (which sounds like a cool idea btw) I would imagine that it would look good if you had a free standing shelf poking out from the wall just underneath the oven door. It doesn't have to be out of cement, could be marble, granite, slate - or whatever goes best with the adobe surroundings.

[SpringJim]

I also think that reducing the wall thickness in the back will work fine, particularly if you fill the space with board or insulating perlcrete. If that back area is solid, (between the adobe and the oven) it will help to also support the dome in that area and insulate where you need it.

The thin FB board would be best but you can suppliment with the perlite mixture.

Pictures might help!

[Annie]

Awesome!
So ....I reduce the size of the brick at the back for a couple of courses, then add 2 inches of blanket and fill in any additional space between the wall and the oven with vermiculite. Sounds good!

Now by using an extra inch of the blanket all around, that reduces the vermiculte by 2 inches... correct? 1 inch blanket = 2 vermiculite?

Someone say pictures? Be advised.... this project is my baby!

[BaconGrease]

wow that is going to be one sweet sitting/standing/cooking/eating area.