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JoeT62 06-21-2007 04:24 AM

More questions on vermiculite!
Has anyone ever looked at the different types of vermiculite and the different possible ratios in any sort of scientific way? I know some have said that a coarse grain would be better (bigger pieces of insulative material) and some have said fine grain would be better (less heat conducting cement between pieces).

James - I know you had posted that experiment on the exterior insulation (inches of insulfrax vs. inches of poured vermiculite). Has anyone done this with portland cement/vermiculite looking at grain size and ratios?

I'm probably getting into areas of more academic interest than practical significance (let's face it, the pizza's gonna taste great anyway!), but it might be cool if we could come up with the "ideal" mix for the insulating concrete.

maver 06-21-2007 06:50 AM

Re: More questions on vermiculite!
Well, if you are looking at ideals for the loose stuff - throw perlite into the equation. For me, this was a bit less expensive at one of my local independent hardware stores than vermiculite, and the insulative value (K value) appears slightly better than for vermiculite. I think the large particle small particle question may be decided based on how you are using it - loose fill vs concrete aggregate. However, the limiting factor for most is lack of choice.

And the ideal... ceramic panel and ceramic blanket insulation as is available at the FB store. Having built my oven with the "cheaper" perlite, I think my cost savings went up the flu when I add in the additional time involved in dealing with the loose stuff.


wlively 06-22-2007 06:23 PM

Re: More questions on vermiculite!
Agree perlite is cheaper and better insulator.

Also I think bigger is better. You mix the portland and perlite/vermiculite dry, then add just enough water to hold together. I did not have loose cement to glob everything together, the mix was very "airy" (Is that even a word. :)).

nissanneill 06-25-2007 04:18 AM

Re: More questions on vermiculite!
I can't speak of prices in the US as I am down under in Australia. I used a 1" themal blanket, covered with chicken wire and then plastered with 3 x i" layers of vermiculite cement. The posting:
shows the construction and a close up of the vermiculite cement is seen at:
posting #2. The vermiculite is covered with a 1/2 to 3/4 inch plaster render layer in readiness for mosaic tiles.
Even after 4 hours of very hot fire and pizza cooking, the outside of the dome is completely COLD. I am very happy with the insulating properties of both the blanket and the vermiculite. It is sold here in 100 litre bags and weight 6Kg and cost Aus$26, similar to your 4 cubic feet bags.
I hope this helps you decide. Regards and good luck with your build.


JoeT62 06-26-2007 07:41 AM

Re: More questions on vermiculite!
I found a less expensive sourse for the vermiculite at a local pool supply warehouse, so that worked out nicely. I also got them to fix the accessory panel on my hot tub, so killed two birds with one stone!

Your design raises another question for me: the mosaic dome is quite beautiful, and my wife (after looking at a couple of the examples here) is leaning toward that - do you have any thoughts on how well that would stand up to the freeze/thaw cycle (New England)?

james 06-26-2007 08:38 AM

Re: More questions on vermiculite!
That's a very good question. I always tell people to not worry about the oven itself, as the insulation that keeps the heat in for cooking also works to keep the oven warm during a cold winter.

But what about external tiles? I think the question could also extend to any external tile work, for example on the landing.

What do our tile experts think?

CanuckJim 06-26-2007 10:35 AM

Re: More questions on vermiculite!

Can't call myself a tile expert, but I have stuck on a lot of veneer stone, both cultured and natural, in a freeze-thaw region. My recommendation would be not to go with ordinary tile grout, because it's too porous. The tile could be stuck on with something like ThinSet, which would be fine because it would be covered. For the grouting, expecially on vertical surfaces, I'd go with Type N and very fine brick sand. After 28 days of curing, I'd coat the grout lines with stone/cement sealer. I'd also coat the tiles themselves if they are at all porous. That should keep out the elements just fine. It's water intrusion that you want to avoid, which will lead to lifting and/or spalling.


JoeT62 06-26-2007 12:57 PM

Re: More questions on vermiculite!
Thanks Jim! I am still not sure about making the time/effort investment in the mosaic, but on the other hand if it looks like junk after a couple of years, I can always build an enclosure later. Food for thought.

Archena 06-26-2007 01:24 PM

Re: More questions on vermiculite!
Do a mosaic on a cheap birdbath the same way you plan to do it on the oven (or try a couple so long as you can tell which technique you used on which). Shove it in the yard (or in a corner if you really goof it up - in which case you should really consider stucco ;) ) and see how well it holds up. If a birdbath can take it the oven certainly can.

JoeT62 06-26-2007 02:50 PM

Re: More questions on vermiculite!
Oh I know it will hold up through the summer, but I'm more interested in it's year to year durability.

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