vermiculite insulation

[bisland9]

i am in the process of building an alan scott design oven. i just poured the concrete over the dome this past weekend. my intentions are to eventually clad it with grapefruit size granite stones as a finished coat. my question is regarding the vermiculite insulation. is it best to pour the vermiculite dry first and then cover it with a vermiculite/portland mixture or just use a damp vermiculite/portland mixture with venting right off the bat? thank you in advance.

need real bread,
bernie

[cynon767]

General consensus around here is that it's best to mix the vermiculite and portland thouroughly while dry, add water, then apply the damp oatmeal-consistency mixture. Check out some of the other threads, and look at the photo section on insulation for some good illustrations.

[Rastys]

Gee,
how much thermal mass do you want in your oven?
The bricks in your barrel vault are probably 4 1/2" thick, with a layer of concrete over that to increase the mass, you now need at least 4 to 5" vermiculite cement to insulate the vault AND then a cement layer to hold your rock in place. If you don't do this, you will need all day and a truck load of wood to get it hot and it will drain the heat out and cool down quicker than it took to get hot without being well insulated.
I would not have put the cement layer on, but a thermal superwool or similar insulating blanket or 4" vermiculite insulation straight onto your vault bricks and then your final finish to accommodate your rocks.

Rastys

[Neil2]

The Alan Scott design is more of a bread oven, I think. They generally have more thermal mass that a pizza oven.

[bisland9]

thanks for your helpful replies. i was concerned about loosing insulating value when wetting the vermiculite. as for the thermal mass, i did not even think of the outer stonework adding thermal mass. i considered it cosmetic being on the outside of the insulation. up here in manitoba, canada it may be a blessing. soon, i should have some bread.

thanks again,
bernie

[Rastys]

Bernie,
if you don't insulate well your vault, then your decorative stone outer surface will add to your thermal mass, absorb your oven heat and release it to the atmosphere.
By all means do your rock finish layer but I would put at least 4" vermiculite cement under it to provide your insulation.
Mix your vermiculite 6 pts to Portland cement 1 pt to a porride/oatmeal consistency.
I mixed mine in a wheelbarrow with a flat bladed fork and then carefully added the water till it held together in a ball when squeezed, a bit like a snowball. Little if any water should be squeezed out of it when squeezed. It is quite unusual to mix as the added water washes the portland off the vermiculite but be careful not to put too much water into the mix and that it well mixed before applying it to your vault.

Rastys