Using only vermiculite and vermicrete for insulation?

[ThorsSmith]

I am in the design and materials gathering phase of my 42” Pompeii oven build. I found a local supplier of vermiculite that is very reasonable and am trying to stay very cost conscious. Is there a level of diminishing returns on how thick an insulating layer is? Would an 8” layer of 5:1 vermicrete under my oven floor be overkill? I’m planning on a gabled enclosure around my oven so i can use loose vermiculite or 9:1 around the dome. How thick is too thick? Most of the threads I’ve read here have people using Ceramic board under their floor and blanket around the dome. Has anyone here made successful ovens without the use of boards or blankets?
Ive really been enjoying the forums and have learned an enormous amount so far.

[UtahBeehiver]

It depends on what you want to ultimately do with the oven. Are you cooking pizzas for a party then maybe something the following day or so or are you after multi-day or high production cooking?
There have been many successful builds with V or Pcrete, it just takes more thickness to equate to higher tech insulations, IE K values for 5 -1 vcrete (under floor) is about 0.75 where as Themogold 12 (CaSi) is about 0.55 at 500 F. A number of builders that have enclosed ovens have dry filled vermiculite or perlite in the structure cavities. Just remember to leave an access point as these materials will settle.

[ThorsSmith]

I used to do a lot of bread baking and would like to again now that i have space to build my own outdoor oven and not heat up the house. So i was thinking late night pizza party Friday, warm it back up Saturday and bake bread, then maybe a roast Saturday afternoon to evening, and if it still had heat to do something Sunday morning that would be great too. I don’t really see myself doing a lot of the really long low temp stuff that some people talk about like drying fruit or yogurt. Dry filling makes sense and i can build a little door into the front of the gable to fill after it settles.

[david s]

Prior to about five years ago the new generation of ceramic fibre blanket (bio soluble and exonerated as a carcinogen) was prohibitively expensive so for the ovens I built then I used only vermiculite and perlite. I have found that a mixture of the two creates a more workable mix than either of them alone. This only becomes a problem when making a lean mix. For a 5:1 mix it doesn’t really matter much but going down to 10:1 to make it more insulative then workability becomes more problematic.
Apart from its superior insulating qualities, the insulating blanket being flexible, works as an expansion joint in addition to being dry. Wet vermicrete/perlcrete against the inner dome is a recipe for expansion and cracking. This can largely be overcome by applying the vermicrete in layers and allowing it to dry before adding a subsequent layer. I find a thickness of around 1 and a half inches with a week of drying after each layer works well.
Not sure how much bread you want to make, but increasing thermal mass and or insulation may be a consideration.
I think 8” of 5:1 is overkill (4” normal) baybe you could compromise and go to 6”. As the water in the underfloor insulation is the most difficult to eliminate, doing this in layers and allowing drying in between is also advisable.
If you are planning an enclosure then dry vermiculite or perlite is preferable. You can cap it off at the top With an 8:1 vermicrete before roofing over it. Also to save on vermiculite or perlite you can fill the corners with plastic bottles with their lids on.

Attached is an experiment I did re drying vermicrete that might find informative. Vermicrete insulating slab copy.doc.zip

[fox]

Yes vermiculite works, there are lots of documented builds on this forum but ceramic fibre works much better with less thickness.
8” inch of Vmix under your oven will take an age to dry out and some time to install where as 2” inch of ceramic board can be built on straight away.
Where I live ceramic blanket and board, works out as a cheaper option than vermiculite so there is little point in me using vermiculite.
My own oven was built with a 5 “ inch vmix base but I have since built five more ovens that use ceramic board and apart from the ease of use, they warm up quicker and stay warm longer.

[ThorsSmith]

That is all great information, I’m going to make some phone calls and send some emails tomorrow for some local pricing/availability of ceramic board/blankets. Perhaps without having to cover so much shipping they will be more affordable for me. The time to dry on the under the floor portion was not something i had thought of. Those drying times are massive, and i live in a very humid environment, being between 3 major rivers. I have been debating doing an extra layer of brick under the floor and using an extra inch of home brew over the dome when it is finished for a bit extra thermal mass. I have a fantasy in my head about baking a bunch of bread for friends/family or church groups.

Fox are you using 4” of ceramic board under your most recent ovens?

[fox]

Yes, 2 x 2” in fact, same thickness of blanket over the dome.
if you use two layers of brick. it will take forever to saturate them with heat and use up huge amounts of fuel.
i am not an expert on this aspect but I would think the bottom layer will keep absorbing the heat until they become saturated and that will take many hours if not all day!

[david s]

Instead of two layers of brick, you can turn the floor bricks on edge which should then give you 4” thick floor.

[Slackbladder]

Help. I have just cast a vermicrete base and dome
but after 5 days it’s just not setting
mused 5 to 1 mix and cast 80 mm depth using 2 to 5mm vermiculite and Portland cement
not drying at all
sorry if I’ve butted in on someone else’s post but
i couldn’t find how to start a new post
any help would be greatly appreciated