Insulation hearth thickness limit

[Mongo]

Is there a practical limit on the thickness of a 'vermicrete' insulation layer?
Can I pour a 6 inch layer of vermicrete on which to lay my oven floor? How about 8 inches?

I want the extra thick layer not so much for the insulation factor (although I consider that a bonus) but to raise the height of my oven floor.

Bonus question: I've seen it mentioned but could someone point me to a primer on vermicrete or comment on the benefits of mixing vermiculite with perlite?

Thanks!

[UtahBeehiver]

This will help you of K values for various ratios of vcrete, typically for a floor it will be 5 to 1 and dome insulation 8-10 to 1. DavidS is our local contributor on p or vcrete. Typically hand mix, not mechanically, and David suggest a mix of the two (perlite/vermiculite). The only drawback is this insulation type does require quite a bit of time to dry out before you can lay the floor brick. Once the floor brick is down, it takes a "really" long time to drive the moisture out during the curing process.

[Mongo]

Thanks very much for the data. Compression strength is what I'm concerned with.
Does anyone have experience with using a 7 inch thick 5:1 vermiculite/perlite mixture under a 42 inch oven floor?
Looks like a 4:1 mixture increases compression strength but I'd hate to lose the insulation quality of the 5:1.
I'd also hate to see it crumble.

Again, thanks in advance.
Great folks here with great information and ideas.
I'm currently playing with bricks while waiting on my structural hearth to cure.

[david s]

Thanks very much for the data. Compression strength is what I'm concerned with.
Does anyone have experience with using a 7 inch thick 5:1 vermiculite/perlite mixture under a 42 inch oven floor?
Looks like a 4:1 mixture increases compression strength but I'd hate to lose the insulation quality of the 5:1.
I'd also hate to see it crumble.

Again, thanks in advance.
Great folks here with great information and ideas.
I'm currently playing with bricks while waiting on my structural hearth to cure.

A 7" thick vermicrete slab contains a lot of water and as one litre of water makes 1540 litres of steam that is a lot to eliminate. It is also vital to use the correct amount of water in the mix. For every 10 parts (by volume) of vermiculite or perlite you need to add 3 parts water.Too little and the mix can be rather crumbly, too much and the cement washes off the grains to the bottom of the cast slab leaving the top too lean while the bottom is too rich. The hydration process will take up a bit less than half of the added water leaving tons of free water to get rid of. You can get around the excess water problem by doing the stuff in layers and allowing sun and wind (weather permitting) to do much of the drying for you. Lay one layer about 1.5 inches thick and allowing a week to dry before adding a second layer. A cheap garden moisture meter can help monitor the moisture level if you're really anal about it. I've found a 50/50 mix of perlite and vermiculite makes a better mix than either of them alone and a handful of powdered clay for every litre of cement also helps in making the mix more workable. A mixer tends to degrade the grains of vermiculite and perlite and sticks to the mixer blades so I always mix by hand in a barrow where you can see more closely what is going on. Mix the ingredients dry then add the water slowly, braking any lumps that form with the back of your spade. The stuff is so light this job is not hard. The correct amount of water is also indicated when a little pools in the bottom of the barrow. This is a whisker too much, add a little more dry material so it no longer pools. The attached experiment should show you how the water takes longer than you'd think to dry out. Hope it helps.The heat will eventually push the water out but could take months of firings. It is a good idea to provide some weep holes in the supporting slab so the moisture has an escape path. This is important both in the initial drying and also should any water find its way in there subsequently.
Vermicrete insulating slab copy.doc.zip