Thermalite aerocrete blocks for hearth insulation?

[Carl]

I know the conventional wisdom is to use perlite/vermiculite concrete or insulating board under the hearth, but I came across these Thermalite aerocrete blocks in the shop the other day: http://www.hanson.co.uk/67/blocks/th...cts/turbo.html
I should probably say straight away that I've got no affiliation with this company!
They're made from waste pulverised fuel ash, and have a thermal conductivity of 0.11 W (m.K), compared with 0.094 W (m.K) for vermiculite concrete (taken from this website http://www.william-sinclair.co.uk/in...retes?OpenPage. It seems pretty close to vermiculite concrete in insulating ability, and it'll be very easy to lay these. They're cheap too - about 65p a block (~440x210x100).

I took my blowtorch to one earlier and was able to heat a spot up to bright yellow after 10 minutes...

... and could comfortably keep my thumb about 4cm away from the heated spot once I turned the blow torch off...

The discoloration was only about 1-2mm deep - obviously the blocks wouldn't get this sort of direct heat unless my build goes badly wrong There's some more pictures on my blog if anyone's interested.

I'm thinking about using these for my hearth insulation as they'll be so convenient and cheap. I know that there's been some discussion about using AAC blocks, but are these different? They're made of 80% ash, and I know ash is a traditional insulator for ovens. I'd be interested in hearing what people think.

[asudavew]

They look good to me...

But let's see what everyone says.

I'm still thinking about using lava rocks..

I took a bunch and made a brick out of lava rocks and cement. Then I let them cure a few days.

The only test I could think of was putting half of the brick in my gas BBQ. So thats what I did.

The bbq was over 600 degrees F and the half brick on the inside was hot enough to turn water to steam, but the half brick sticking out was still cool to the touch.

I think it would work fine! Just don't want to make an insulating error.

[dmun]

AAC is made up of sand cement, and aluminum powder, which acts in a chemical reaction to produce the bubbles which create the air spaces which insulate.

Aerated autoclaved concrete - Wikipedia, the free encyclopedia

I haven't heard of the product you mention, but I know that fly ash from power plants can be used as a partial substitute for portland in regular concrete.

If it works, it'll be great. We haven't heard from anyone yet who has used AAC for the oven.

[Carl]

Well here goes, I'm trying it.



The blocks were extremely easy to cut to shape with a hand saw, and mortared down just like regular blocks. They do suck a lot of water up though, and I'll be trying to keep them dry from now on to save me an enormous drying session when I first use the oven.



I've put a thermocouple under the blocks, and will also put one in the heath firebrick, the oven void, the wall etc etc. I'll let you all know how they work out with some real numbers. Fingers crossed they work OK.

[Frances]

Great!! I saw these in shop right after I'd poured my vermiculite floor. They'd be a lot easier to get hold of (around here anyway), cheaper, as well as easier and faster to put down. I was thinking of putting them under the floor of my grill (which will sit right beside my oven one of these days...). I'll be really interested to hear how it works out as oven insulation, good for you for trying it!

Frances

[Bacterium]

I've used AAC as an insulator when I made a charcoal BBQ box once......we have a version of AAC here called Hebel. I found with the heat/cool cycles that it became brittle or developed cracks.

Carl the thermalite stuff sound interesting though.

[rcspott]

Carl

is the material your using water proof, do you think it will hold up well outdoors?

Thanks

Rich

[Carl]

As far as I know it's waterproof - it behaves just like a very lightweight concrete block (which I suppose it is). It soaks water up like a sponge though, so I'm keeping it dry as best I can. The top of my oven will be rendered anyhow, so this bit shouldn't get wet, plus I'll be putting a roof over the whole thing, so I should be OK. (hopefully )

I don't think I'd want the blocks exposed to the elements in a finished structure, because I'd be worried about the water wicking into the oven. They do make versions of this block that you can use for exterior walls, but I think these are designed for interior walls. You should find more on the link to the manufacturers website.

[Carl]

I promised to post my results with the AAC block - so here goes. The graph below is after a gentle 3 hour fire, and the 'below hearth insulation' line is the thermocouple that's below the AAC block that's below the hearth bricks. I'm pretty pleased with that - only getting to ~25 degrees above ambient.

I'll post more from larger fires when I have them. I also need to make a door to fit the oven - that's why it's cooling so quickly!