Best material to insulate mud oven dome?

[martinpolley]

I've read Kiko Denzer's book, and there he says you can use perlite or vermiculite for insulating under the hearth, but for the dome, he recommends using sawdust or wood shavings mixed with clay slip.

Has anyone used perlite to insulate the dome? Or is something like sawdust intrinsically better for some reason?

[patamos]

I think mainly it is a matter of cost and simplicity.
Under a weight bearing surface you need an insulative material that maintains some compressive strength.
In the dome dead air space will suffice.

there could be other reasons i have not picked up on yet...

[martinpolley]

patamos Yeah, the price is right, for sure

But will perlite do a better job? Or let me get away with a thinner insulation layer?

The other thing I'm thinking is that it may be harder to work with. It's OK if you're tamping it into a confined space, but maybe trying to form it into an even layer on top of something else will be a bit more tricky…

[patamos]

In my not so extensive experience, the danger with sawdust and wood shavings is that the higher a proportion of them in the mix (relative to clay and water) the more material will carbonize leaving hollow spaces. This is how the insulating occurs, but the structure does become less stable.

clay and vermiculite are very easy to work with, but tends to shrink a lot as it dries.

All in all clay and perlite work very well together. Fairy easy to lay up with a mild steel or wood trowel. Very structurally stable from the get go. Easy to get lots of perlite in the mix. BUt the dust is no fun to breathe

my two bits

[martinpolley]

patamos I just mixed up my first batch of perlite and clay slip to insulate around the burn tunnel. You're right—it is easy to work with. I think I'm going to go ahead and use it to insulate the dome too and see how it goes.

Thanks

[satamax]

Guys, i don't know what type of oven this is, but the traditional insulation for pizza ovens is sand. Because it acts both as insulation and mass. The oven then doesn't have swings of heat as much as if it was thin vaulted, and with some pure insulation around. Prety sure bread ovens had mostly dirt above.

[patamos]

Ya good point Max.

If you are baking as you fire then the lighter clay=perlite makes sense. Having 1" or so as an inside plaster gives some flywheel effect. But for 'fire first, bake later' or larger and or ongoing use… the higher mass semi-insulative effect of sand makes more sense.
I assumed Martin's is for intermittent home use. ?

[martinpolley]

Hey satamax and patamos! Yes, this is going to be used a couple of times a week tops, to bake a few pizzas and some bread. Just for family use, not some big commercial operation.

I was planning on going with a "standard" three inches of mud for the dome, with insulation over that. And a similar amount under the firebrick floor. But in this oven, it's going to be heated continuously as long as I keep the fire going, so if I have less thermal mass in the dome, it will reach working temperature faster. But it will also cool down more quickly, which means if I want to keep baking for a while, I'm going to have to spend more time fiddling with the fire than I would with a greater thermal mass.

The other thing I'm thinking is if I were to build a much thinner dome, would it be as strong? Wouldn't a dome consisting of an inch of mud covered by several inches of insulation (whether that be perlite/clay or sawdust/clay) lack the structural strength of a nice, thick mud layer? This is my first time making a thing like this, so I don't know the answer

And satamax, regarding sand, I always thought it worked more as a thermal mass than as insulation. I seem to remember reading something that Ernie Wisner wrote where he said they often use sand as part of the thermal mass in RMH heated benches…

[patamos]

I built a big community oven 2 summers back with a dome of 1" of inner cob shell (clay sand with a bit of horse manure). then 4" of clay - horse manure. The 1" of cob plaster. Then finish plaster.
It performs very well for both pizzas and roasting dishes. The 1" of mass has enough flywheel to allow fire tending to slip. The oven has had hundreds of firings by all kinds of inexperienced users and it has worked great for everyone...

Since then i have built a smaller oven dome with 3" of mud before 5" of clay-perlite insulation. Same 6" L-feed fire chamber. My client asked me to beef up the mass (after i had built the fire box intending a similar dome lay up to above) because he wanted enough flywheel to roast for 4 hours post-fire. Seemed like a stretch for that small an L-feed but i went for it.
The result: It takes way longer to heat up. Thus, short duration baking relies more on convection. And requires more fuel resources to get the job done

If a handful of pizzas and the odd 1 hour roast is your main thing, then i'd say go with the thinner mass in the dome.

[martinpolley]

Hey patamos, thanks for sharing your wisdom. I think I'll go for less mass and more insulation

[martinpolley]

patamos Should I do the same with the oven floor? I'm going to be using standard firebricks, so should I have any mass at all under there except for the bricks?

[patamos]

I'd think the bricks will be enough
perhaps with a (clay-rich) clay-perlite layer under that

always worth hearing what other folks have to say too.

[martinpolley]

patamos That makes sense—thanks.

[sothiseeker]

We built a nice stove with clay/perlite/glass bottles underneath standard brick for the oven floor - worked great. FYI, insulated over the 3" clay layer with 4" of clay/perlite covered with decorative cob. Nice little stove.

[martinpolley]

sothiseeker Cool! Is this a "regular" earthen oven or rocket powered? So far, I've only got about an inch and a half to two inches of perlite/slip insulation over my dome. I don't think it's quite enough...