AAc blocks instead of Vermiculite insulation under slab

[Les]

Re: AAc blocks instead of Vermiculite insulation under slab

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]

Re: AAc blocks instead of Vermiculite insulation under slab

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/new-under-oven-insulation-board-754.html

[carioca]

Re: AAc blocks instead of Vermiculite insulation under slab

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

AAC SuperIsol

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

Re: AAc blocks instead of Vermiculite insulation under slab

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]

Re: AAc blocks instead of Vermiculite insulation under slab

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.

[vtbread]

Re: AAc blocks instead of Vermiculite insulation under slab

here is how my math plays out:
500kg = 1100 lbs US
10 cm x 60 cm bearing surface = 96 sqin
1100 / 96 = 11.5 psi
AAc block bearing capacity = 348 psi

ton of concrete on top add 2200 lbs
3300 lbs / 96 sqin = 34.4 psi

These blocks are plenty strong to take the weight. I have a wall of them directly behind a steel wood cookstove (less than 6") for about 9 years now. There is a crack in the gypsum plaster but nothing else and I can tell you that stove gets fired hot quite often.

Not sure where the vermiculite panels snuck in, I am talking about Autoclaved aerated concrete (AAC) in Europe marketed under Ytong and Hebel as I am sure a few others.

[vtbread]

Re: AAc blocks instead of Vermiculite insulation under slab

thanks dmun,

I like the idea of the sandwich oven. maybe you could just cast your firebrick layer into the AAC block invert, then flip it over. Not sure how the expansion coefficients would work. Possibly needs a separation/slip layer.
done. apply plaster finish and get baking.

I like this idea.

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

here is how my math plays out:
500kg = 1100 lbs US
10 cm x 60 cm bearing surface = 96 sqin
1100 / 96 = 11.5 psi

Could you just clarify what the 10cm x 60 cm dimensions refer to?

Not sure where the vermiculite panels snuck in, I am talking about Autoclaved aerated concrete (AAC) in Europe marketed under Ytong and Hebel as I am sure a few others.

Sorry, I jumped a paragraph.

Here is the explanation: Ytong AAC blocks thermal conductivity varies between 0.26-0.56 W/(m K) (depending on size and type : see http://www.ecostein.net/Therm_isol_eng.pdf).

For a few other materials the conductivity is:
Skamolex Superisol 0.08
Skamolex Vermiculite slabs: 0.14
(from Skamolex web site)
Insulating firebrick (Grade 23): 0.18
(Googled somewhere ... sorry)

AAC blocks have much higher conductivity than any other materials recommended by FB experts, which is why I looked at a more fragile material which is in a similar ballpark in terms of thermal conductivity to the recommended ones. And the more fragile material is strong enough. It also has known properties when subjected to high temperatures, as it is designed for hight temperature insulations, unlike AAC.

The price of AAC blocks is much lower, of course, but so is the cost of self-mixed vermiculite concrete. So if I was feeling lazy and wanted to spend more, I would go with better insulating blocks.

Does this make sense?

Best wishes,

W.

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

Hi Wlodek,

I've just looked at that PDF from your link on the Ytong insulation, and it looks like those figures are for a completed wall (with bricks or stucco), not just the blocks themselves, and they're U-values in W (m2.K). The same site lists the thermal conductivity of the Ytong blocks as 0.10, 0.12 or 0.15 (that's the value at the top of the table, which is in W (m.K) - these are the values you should compare with the other insulation products.

I'm using Thermalite blocks for my hearth insulation (probably very similar stuff - another brand of AAC). Their website gives a value of 0.11 W (m.K) - the value I found on the web for vermiculite concrete was 0.094 W (m.K) - all pretty close to the numbers you quoted. I've got links to the original documents with these figures in on my blog at Forno Economico if it's of interest.

AAC bricks actually have a lower conductivity than insulating firebrick, and since the blocks have a very similar conductivity to vermiculite concrete, I've decided to use them. I've got a load of thermocouples under and over the blocks, and when I've finished I'll be reporting back here if they work. They were really easy to use, cheap, and I could cut them like they were polystyrene. There's more on my blog on all this if you want more info. If it's an enormous disaster I will also post the results here and look sheepish

Carl

[Wlodek]

Re: AAc blocks instead of Vermiculite insulation under slab

Oops, thanks Carl, not for the first time I mis-read something, not for the last either. So this is another non-argument of mine.

I'll probably still go with a specialist insulation - not sure which one yet, probably the cheapest I can find.

Thanks for putting me right,
W.

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

No worries - glad if I can help. I'm only using the blocks because I'm trying to save some pennies, plus I'm too lazy to mess about with vermiculite concrete I'm probably going to use them for my enclosure too, because they're easy to build with and will act as handy insulation too. Watch this space!

[vtbread]

Re: AAc blocks instead of Vermiculite insulation under slab

No problem, glad the insulating properties were cleard up.

I have a pallet full of the stuff left over from a house building project most are 8"x12"x 24" should make a pretty nice insulation layer.

I just poured my slab for the oven and shed and am working on the superstucture now. Get a roof on it first, tent in and then I can focus on the oven. It's a bit nippy here right now.

I'll post some pics as I progress.

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

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!

Carl

[jengineer]

Re: AAc blocks instead of Vermiculite insulation under slab

That is very good indeed Very little loss from the bottom - any chance of getting more insulation on the outside of the dome? oh yeah, its a done deal...never mind

[Carl]

Re: AAc blocks instead of Vermiculite insulation under slab

I'm hoping the outside of the dome will hold more heat once everything's dried out a bit - it's got a minimum of 6" loose vermiculite plus 4" of AAC block around the dome at the bottom - and more like 2 foot of loose vermiculite on the top... I'll post another graph once I've got a door and it's dried out a bit.