Pearlite/Vermiculite Dome Insulation questions

[EricU]

Hello,

I am in Northern California, planning a 42" oven for the backyard. I will be using 8" wide CMU for the base support and a concrete slab/insulated concrete slab for the dome hearth base. Outside the oven dome I will use 4" wide CMU to build the walls outside the dome which will be faced with a stone/brick combo.

My questions are regarding the Pearlite or Vermiculite insulating parge over the dome and the mix used as the insulating base

1. Is one better than the other (Vermiculite vs. Pearlite)
2. For the hearth base, what mix do I use?
3. For the dome insulation parge, what mix do I use?
4. For the dome insulation parge, how thick?

Also, could someone provide a link to some build pics showing a dome insulation parge? I tried doing a search here and got so many threads that I kinda just kept going off topic checking things out!!

Thanks for any help, Eric.

[david s]

1. Perlite is a marginally better insulator, but vermiculite is easier to handle.
2. Usually 5:1 for hearth base
3. I like to use 10:1 for dome insul. because more cement means poorer insulation and 10:1 is enough to hold it together.
4. Depends how much insulation you want, but don't go less than a couple of inches or it won't stick together that well.
I'm sure there will be folk who disagree with my response. There is more than one way to cook a goose, skin a cat, etc. etc.

[david s]

I've found that for every 10L (2 gallons for those imperialist neanderthals) of vermiculite, use 3L (about the standard beer consumption on a Sat. night) of water.

[Glosta]

Please allow me to jump in with a related question. Why use any portland cement in the dome insulation perlite/vermiculite?? Many of the builders enclose their WFO dome in a small house with a roof. If the portland cement reduces the insullation efficiency of the perlite/vermiculite; why isn't it just poured in as dry insullation?? This is how it is used when insulating concrete block walls and attics. Is it for fear that an exterior water leak may wet the perlite/vermiculite? Since it is inorganic, it won't rot. Is there a concern the wet vermiculite will settle?? It would dry out with subsequent oven use. A Portland/vermiculite mix will also absorb water, but would continue to hold its shape. -----Any input would be much appreciated. Thank you. Glosta

[BackyardPermaculture]

Glosta, I believe the vermiculite is generally used without cement inside enclosures; that's certainly how I'm planning to use it.

Cheers,
Mick

[dmun]

Why use any portland cement in the dome insulation perlite/vermiculite?

A lot of builders don't want to go the trouble to build an enclosure, want the look of a semi-sphere, or want a less visually massive object in their backyards. It's quite difficult to build a freestanding dome enclosure to encase loose insulation, hence the vermiculite concrete.

why isn't it just poured in as dry insullation?

Pouring in loose vermiculite/perlite is a good approach. It can be surprisingly fluid, so make sure any gap in the enclosure big enough to let light through is thoroughly sealed

[david s]

Glosta,
You are correct, only use the cement and water if you are doing an igloo. I was assuming the original post was referring to an igloo. Pouring the stuff in dry for an enclosure is simpler and also eliminates the problem of having to remove the moisture in the layer once it has set.

[Xabia Jim]

I think the answer is accessibility as both perlite and vermiculite are similar for this use. I used perlite and a lot of it. I was happy with the results.

do a search for perlcrete. I mixed the cement with the water first, then folded in the perlite until it was like lumpy oatmeal. (somewhere there is a thread for my second build.... the Benjamia Bakehouse oven where I used this. The material set up fine and floated in water)

You don't need a lot of cement to make an insulation layer...it is not structural. If you are going to use dry insulation, vermiculite might be better. You have to make sure that you have all the cracks sealed or it can run out like sand!

The one rule here is thicker is better. I put a 6 inch layer in the hearth and had quite a thick outer dome of used insulation firebrick with perlcrete over it.


Good luck!

[EricU]

Originally Posted by dmun

A lot of builders don't want to go the trouble to build an enclosure, want the look of a semi-sphere, or want a less visually massive object in their backyards. It's quite difficult to build a freestanding dome enclosure to encase loose insulation, hence the vermiculite concrete.

No semi-sphere for me on this one,

I will be building a complete vertical wall enclosure that will be faced with rock and brick outside the oven dome. This enclosure will be built from 4" wide CMU block above the hearth with the standard 8" wide CMU as the hearth support. And yes this thing is getting big (I am currently designing it in AutoCAD).

With this setup, do I need 6" (min) of dry pearlite or vermiculite?


Thanks again, Eric

[BackyardPermaculture]

Eric, the concrete blocks will not improve insulation; they will soak up lots of heat if you don't insulate to the same levels as commonly discussed around here.

Just remember that blanket/wool insulation is most efficient, and that dry pearlite/vermiculite is more efficient than when mixed with cement. But a ton of concrete isn't going to keep heat in your dome.