AAc blocks instead of Vermiculite insulation under slab Pompeii Oven Construction Discuss AAc blocks instead of Vermiculite insulation under slab in the Pizza Oven Design and Installation forums; So, do we feel that this product can replace the superisol board? If so, it is dirt cheap. You can ...

[Les]

So, do we feel that this product can replace the superisol board? If so, it is dirt cheap. You can buy 24 panels (4" x 24" x 32") for $100.00.

[dmun]

Not only that, they will make a custom reinforced-floor panel that's both support slab and insulation layer in one.

I'm wondering if they will cast a dome-in-a-block that all you have to do is line with firebrick and mortar.

[Xabia Jim]

Did you notice the Jumbo Blocks? Two would give you a 4 foot square hearth, 12 inches thick...

[dmun]

This AAC stuff has given me a great idea. Since it's cheap, and you can cut it with a pruning saw, you could make an approximation of a dome shape by cutting and stacking layers of the four inch sheets:



This drawing, in wireframe, gives the idea.

I've been thinking about building an oven upside down for some time. Instead of a temporary sand form, you could build it right in the structure of the oven insulation block, bracing your bricks with damp vermiculite as you build.

Here's a cleaned up, upside down version of the same drawing:

[vtbread]

Hi Guys,

now we are thinking in the right direction. Why not just mix up hightemp refractory and cast it into the negative upside down form?

But seriously here are the material properties of Superisol
MatWeb - Online Material Data Sheet


Based on the data here presented as a direct comparison:

AAC SuperIsol

Density 0.03 0.008 lb/in³
compressive strength 609 377 PSI
specif. heat capacity 0.24 0.2 BTU/lb-°F
Thermal conductivity 1.7 0.5 BTU-in/hr-ft²-°F

Since the AAC I have is 12 inches thick, I can saw it in half for a 6” insulation layer below the oven. This would get me about the same insulation value as a 2” layer of SuperIsol.

[Les]

That's somewhat disappointing. I don't want to raise the cooking floor height an additional 4 inches. I may have a little wiggle room with the grade I set my pavers at. The advantage of using it is obviouslyy cost – this stuff is CHEAP.

[maver]

Originally Posted by vtbread

Since the AAC I have is 12 inches thick, I can saw it in half for a 6” insulation layer below the oven. This would get me about the same insulation value as a 2” layer of SuperIsol.

Remember that although the insulating value for 6" AAC equals the insulating value for 2" superisol, that 6" AAC is going to absorb more heat than superisol. The insulating value comparison means that if you put your hand on the underside of either hearth materials after the oven is fired the tempertures outside the insulation should be about the same. But the AAC is still a bigger heatsink than superisol, meaning you are losing more oven heat into the insulation, therefore more wood to fire and longer heatup times. AAC is three times as dense, and poorer K value (higher conductivity) than superisol. By comparison, vermiculite concrete has a K value of 0.7 (see jengineer's review below)

http://www.fornobravo.com/forum/f6/n...board-754.html (New Under Oven Insulation Board)

[carioca]

Originally Posted by vtbread

<SNIP>
Based on the data here presented as a direct comparison:

AAC SuperIsol

Density 0.03 0.008 lb/in³
compressive strength 609 377 PSI
specif. heat capacity 0.24 0.2 BTU/lb-°F
Thermal conductivity 1.7 0.5 BTU-in/hr-ft²-°F

<SNIP>

Hi guys, this comparison worries me a lot: according to the tables referred to on mattweb, CalSil has a compressive strength of just one-tenth or so of that of 25 MPa concrete -- does this mean I can't put it right under the dome wall itself?

I've seen the boards cut to fit the inner curve of a dome, but would have thought you'd need it to extend under the wall itself to prevent heat from leaking out under the wall?!

Cheers,

Carioca

[Wlodek]

Are there any longer term issues with repeated high-ish temperatures? These blocks would be just under the oven floor, so there would be some 200C or more repeatedly applied to these blocks from above. I have little knowledge in this area, but cutting and laying the blocks sound like an easier (and hence neater) solution for a newbie like me than pouring the vermiculite cement.

Also, I am sure someone has worked out the pressures that the dome walls exert on the insulating layer - plus the possible chmney weight in some cases. Do we know how they relate to the compressive strength of these new materials?

Yours rather interested,
W.

[Wlodek]

Will someone correct me if I am wrong, please?

Surface pressure of a prefab dome with an average outside radius (for an eliptic dome) of 0.6m, 10cm wall thickness and 500kg weight, exerted by its lower surface is only 0.01MPa (2.06psi).

The most fragile Vermiculite panel has Compressive strength of 1.3MPa or 189psi.

I think we are OK with these panels, especially if we do not stand the components on edges or drop them. Brick domes will have the weight in similar ballpark, and bigger bottom surface. What if you put a ton of concrete on them to increase their thermal mass? With the extra ton of concrete and the same bottom area the pressure would be 0.04MPa or 6.17psi. Still OK.

Or did I get badly wrong?

Still unsure about the long term thermal properties of concrete and aircrete.

I think I'll go with the Vermiculite panels.


W.