Vermiculite 4" 5" 6" Does it matter?

[fxpose]

I searched but I couldn't find an answer.
Does it make any difference whether I use 4" or 6" of vermiculite/perlite insulation under the floor? Is there any downside going with 6" as far as placing bricks on top? I understand the R value will increase but is it better than going with the recommended 4"?

Thanks!

[eprante]

I think it was Dmun who said, you can have too much thermal mass, but not too much insulation. I have 5" under my floor.
Eric

[fxpose]

I guess what I'm trying to ask is will there be any 'settling' as you go thicker? I know it has good compressive strength but over time will they eventually settle somewhat under the weight of the oven?

[Neil2]

"I guess what I'm trying to ask is will there be any 'settling' as you go thicker?"

No. A vermiculite/perlite/portland mix will not settle to any appreciable degree.

[fxpose]

Thanks Neil, that's exactly what I wanted to hear.

On a related note, is it all right to place the vermiculite/portland mix over a freshly poured slab, giving, say, a couple of hours setting time in-between?

[RTflorida]

'Back in the day' when I built my oven, it was recommended to pour the insulation layer the same day as the support slab. The idea being - the slabs will bond together better. As we all know, nothing is going anywhere once it is all built, so the bonding theory is debunked. Many have waited a day/week between pours, since those original instructions were posted.
It is entirely your call based on time and energy level, take a day or two off, or go straight to the insulation layer.

RT

[fxpose]

Thanks RT, that's exactly what I thought. I can do both on the same day as there will be no re-forming to do. The insulating layer will be formed exactly like the slab.

[Neil2]

I would wait a day or two. I would then check that there is no dishing or ponding on the new slab and fix it with a bit of grinding. You want the vermicrete layer to drain out if any water gets in.

[Tscarborough]

To the original question. It is a (non) simple engineering question to calculate, but the reality is that 4 to 5 inches will probably not become heat soaked (thus requiring more depth) in the time frames we are operating the ovens.

As for the compressive strength, my brick chimney is sitting on up to 24 inches of 8-1 perlcrete and I have no concerns of settling at all.

[fxpose]

I decided I'm going to instead use an insulating board over the slab as I found a local dealer who sells 2" and 3" boards in 24"x36". The 3" boards are less expensive (per sheet) than the 2" boards. The 3" boards are more dense as well (per cu/ft). Are they not as effective as 2" boards?