Insulation: Which is first? blanket or concrete

[mgreene]

I have finally got the oven built. We are now waiting for it to dry out before insulating.
We are going to have the dome inclosed with exterior walls and ceiling that will consist of stucco and stone. The dome will not be exposed to the outside.
So???
1. Do we cover the oven dome with the FB Blanket, as stated on Page 55 (Pompeii Oven Instructions), then p. 56 cover with insulating concrete?
or
2. Do we cover it with concrete first like what some of the pictures look like(P. 47, 49,) Also, according to the Allen Scott book he used foil first (that was to help with expansion) then put concrete on the foil, then insulation.

It mentions how all the motar and concrete need to dry prior to insulation. So it doesn't make sense to put FB blanket then concrete, cuz i would need to let it dry another week or 2??
Help, Michelle

[RTflorida]

No need for any extra mortar/concrete. Some have added more for extra thermal mass or as in my case, I wanted a smooth an uniform surface before proceeding with the blanket and vermiculite concrete.

Since you are going with an enclosure, you have a couple of options. Simply fill the entire structure with loose vermiculite or perlite (at least 4" all around, more if you can afford it). Second, you could just use insulating blanket (minimum is 2", but I would go with 3" or 4"). Third, use a combination (2" blanket then pour in several inches of loose perlite or vermiculite).
I would go through the entire curing process before sealing up any oven, you need to let the steam escape (will keep minor cracking from becoming blowouts)

Skip the foil, that whole "slip plane" issue has been debunked. Aluminum foil disolves when left in contact with masonry. It could make a little sense ((if it didn't disolve) if you were building an igloo, but you are not. The blanket actually provides all of the flexing needed to avoid cracking on the exterior of an igloo.

RT

[Ken524]

Hi Michelle,

You definitely want to put the blanket on first.

There isn't any harm in putting the blanket and vermiculite/cement on right now. Many folks wait on the vermiculite/cement until after a few curing fires to make sure everything's ok under the hood.

Skip the aluminum foil. From a different discussion:

Originally Posted by dmun

No foil! It's useless. It rots out in contact with the caustic portland cement, but before it does it creates a vapor barrier that prevents you from drying out your oven. Stomp out the foil meme.

Good luck curing! Let us know how it goes.

[Ken524]

Looks like I spent too much time composing my post. RT beat me to it!

[GianniFocaccia]

I too have been curious about the relative efficiency of respective oven insulation layers and the optimal R-Value per layer outwards from the dome exterior. Could you put an initial layer of insulating mortar (Kaolite?) on the dome followed by rock wool and vermiculte? Just wondering...

John

[dmun]

Could you put an initial layer of insulating mortar (Kaolite?) on the dome followed by rock wool and vermiculte?

Insulating castables are very expensive. If you have money to burn, just use extra refractory blanket insulation.

[Neil2]

You don't need anything fancy. 4 inches of vermiculite (or perlite) mixed 12:1 with normal Portland cement placed directly on the dome bricks will suffice. (Mix gently so as not to break up the vermiculite particles and thus reduce insulating value). You don't need chicken wire or stucco lath or anything like that.

If you have an enclosure, make sure it is ventilated. Some people just fill up the inside of the enclosure with loose vermiculite.
I went with the igloo shape. If I were building an enclosure, I would still put the 4 inches of 12:1 vermicrete over the dome just to make sure it stays in place.

You could put this on before or after the initial curing - I don't think it matters too much. People have done it both ways with equal success.

[mgreene]

Thank you, Michelle