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shepster 06-21-2009 01:14 AM

Vermiculite only insulation?
After a winter of doing nothing I have now got my oven ready for the final build! Right now its the basic igloo shape and awaiting insulation. Can I just use a vermiculite mortar rendered all over or do i have to use ceramic blanket as well? Has anyone else done this and if so what mix? I have got loads of vermiculite so want to void more cost and being in the UK it is only going to get occasional use!!! Also, how thick should it be.

Wookie 06-21-2009 01:27 AM

Re: Vermiculite only insulation?
I used both a ceramic blanket; enclosed with chicken wire, as well as a mix of vermiculite and cement render. The blanket was about 3 inches and the vermiculite was between 2-3 inches in 99% of places. I have found that this amount of insulation retains a significant amount of heat overnight. (provided you close the door). Using vermiculite alone (mixed with mortar of course) should be fine, but do a few layers and let it dry fully before doing the next one. I found the mix to be about the consistency of thick porridge - helps to stick to the chicken wire!!

Rastys 06-21-2009 01:49 AM

Re: Vermiculite only insulation?
Vermiculite cement insulation will be fine alone without a ceramic blanket providing it is thick enough.
You can mix it in the ratio of 1 part portland cement to 5 parts vermiculite.
You can add 1/2 part fireclay if you need it stickier so that it will stick on vertical surfaces. 2" of vermiculite has the equivalent thermal insulation of 1" of thermal blanket so I'd put 5 to 6" all over the dome and sides
If you plan on leaving it as an igloo, then render it with a 1/2 to 1" waterproofing render. You can put the layers on one after the other, don't have to wait to dry between coats. I put 3 x 1" layers on one after the other over a wool blanket and then topped it odd with a waterproofing render. I plan to tile it once I find tiles appropriate or shelter the oven with a roof extension of the pergola area.


jzman123 06-21-2009 08:51 AM

Re: Vermiculite only insulation?
If you are building a house around the oven, do you need to make any adherent insulation, or can you just fill up the entire void space with loose vermiculite?



Neil2 06-21-2009 10:17 AM

Re: Vermiculite only insulation?
"Can I just use a vermiculite mortar rendered all over or do i have to use ceramic blanket as well? Has anyone else done this and if so what mix?"


I put several layers of heavy aluminum foil over the brick and a 5 inch vermiculete layer directly over that.

I used a 1 part portland, six parts coarse perlite and 6 parts vermiculite mix. (a 1:12 ratio). I found the perlite made it more workable. The weaker the mix, in terms of portland content, the better the insulating value.

karl 06-21-2009 10:28 AM

Re: Vermiculite only insulation?
A layer of insulation (by a blanket, mat or similar) would allow the fire bricks to expand and adjust more freely. And you will probably see less cracks in your outer layer.


david s 06-21-2009 02:25 PM

Re: Vermiculite only insulation?
Using a lower proportion of cement in the mix will make it insulate better. More cement will make it stronger but insulate less. For the same reason don't use sand in the insulating mix. 10:3:1 vermiculite, water, cement works ok, you'll think "this'll never work" but it sets up hard enough. Another tip is to do a layer about 2" thick then leave it for a few days to dry before repeating the next layer. There is lots of water that you need to get rid of.

Lars 06-21-2009 02:58 PM

Re: Vermiculite only insulation?
I am doing a sort of 'gradation' of insulation.

Since I was not THAT concerned about retaining a lot of heat --the oven is out in the country now, and don't really have a way to do extended baking sessions, I just put the minimum 2.5" vermiculite in the floor.

Then, I also had kind of a small footprint on my base, so I only had room for about 3.5" all round the bottom of the dome. At about 10" up the sides, I added 1" of ceramic blanket all around AND vermiculite on top of that.

Next I plan to put about 2" of blanket and 4" of vermiculite over most of the top of the dome. I also used 'metal lath' which is great for stucco, and I drew it first to see how much of a 'pleat' I would have to make ( for mine it was about 3/4" every 6" ) this gives the metal lath the beginnings of a spherical form. Actually, the first 6" up was only 1/4" every 6" around, then on the second, 1/2" roughly, etc. )

DavidS is right, there is much water in the vermiculite mixes. That portland vermiculite stuff is really interesting. Gives me ideas for other types of uses.

I know when something is really hot, the concept of heat rising is probably not a big factor, but I figured it can't hurt to 'up' the insulation on the dome, even if the base is somewhat scant.

Second fire today!!!

jzman123 06-22-2009 09:57 AM

Re: Vermiculite only insulation?
Can you use loose vermiculite instead of vermiculite/portland?

This question relates back to dome construction. Does the insulating cladding provide any structural support?

If I am building a house around the dome, then why not just fill up that entire void with vermiculite and don't worry about mixing in any cement.

The thermal properties should be better without the concrete, yes?

Any thoughts?



Lars 06-22-2009 10:23 AM

Re: Vermiculite only insulation?

Yes, they say loose vermiculite insulates better. It's not bad mixing up the portland/verm. stuff, though. I think, long term, the mix would stay in place better, and be easier, down the road, if any repairs were necessary.

The cladding does not really offer any structural support. If doing a dome, though, like I am, it will provide a fairly stable surface to put stucco onto.

That being said, I am on my 2nd fire, and I burned a small wood fire for several minutes. The bricks on the top of the dome didn't even get warm!!! WITHOUT any insulation on them! Obviously it takes a fairly big fire to bring this oven up to temperature, and, therefore, insulation or no, it will take a while to cool down.

Once the moisture gets out of the dome, I am sure the 212 F. temperature will be easier to surpass. I imagine that takes several curing fires.


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