concrete blocks on top of vermiculite/concrete hearth

[project1]

Hi all,

a quick question about the size and strength of the insulating hearth. I have poured the 4 inch vermiculite/concrete mix on top of 4 inch structural slab for hearth and the hearth extends to the outside of the concrete block stand.

My question is related to constructing the oven enclosure: can i build the walls of the enclosure (which will serve as the support of my roof and all the stone material i plan to clad the oven with) with 4 inch concrete blocks or will this be too much weight for the "cork-like" hearth material to bear? ie. will it compress under the weight and cause structural issues for the oven enclosure, roof, and cladding construction?

Thanks in advance,

CR

[dmun]

Yes you can build your 4 inch block structure on top of your vermiculite concrete pad. It has tremendous compressive strength. You will want to consider some kind of exterior coating to keep it dry. It's not something you want exposed to the weather.

[karl]

Originally Posted by dmun

Yes you can build your 4 inch block structure on top of your vermiculite concrete pad. It has tremendous compressive strength. You will want to consider some kind of exterior coating to keep it dry. It's not something you want exposed to the weather.

I have no experience with Vermiculite, but I have just installed a 2" layer of cement/perlite as a top-layer under my oven-to-be (underway). This is not something I would recommend to build upon. Certainly the oven itself is heavy, but it fortunately distributes its weight over a fairly large area. To me the cement/perlite layer looks (and feels) suspicious and I would not have installed it if I knew the properties of it.
But Vermiculite may be different (better). I did not have access to it and the perlite was mentioned in the FB instruction as an alternative.

regards from karl

[dmun]

No, vermiculite and perlite behave almost exactly the same. It looks weird, and you can easily stick a pointed object into it, but it does have high compressive strength.

Is it the best thing to build on? Maybe not, but you can certainly build on it.

[david s]

Could you trim off 4" of the insulation and then build off the concrete supporting slab ? The insulating vermicrete/perlcrete is easy to chip away or cut neatly with an angle grinder.

[project1]

Thanks for the replies.

I was actually considering cutting a section of it out in order to build the blocks on top. The blocks don't concern me as much as the hip roof design constructed of granite stones that i am planning to build on top of it.

i read in another thread that the vermicrete layer has very high compressive strength (as dmum states) and was satisfied by that. Any ideas on what type of mortar I should use to bond the blocks to the vermicrete - refractory or regular? Thanks.

[egalecki]

I'd use regular. If it doesn't need to heat up, (and I'd say you wouldn't want it to) don't use refractory.

Are you planning to have perlite or vermiculite between your insulating blanket layer and the block enclosure? That would ensure no heat transfer to the outside of your enclosure.

[project1]

Would a 4-5 inch layer of vermicrete be sufficient or should i definitely use the ceramic blanket and if so, does anyone know where to buy some in the Toronto area - shipping from FB was very expensive.

[egalecki]

I'm using both. It's overkill, but I want to keep as much heat in as possible. I googled "ceramic blanket" and checked ebay for the same, and found out that for me, at least, it was either pay shipping or pay more for the blanket. I ended up spending it on shipping (yes, I think it was a little bit more than it might otherwise have been) to support James and the Forno Bravo community he's set up.

So, I'm going to have 2 layers of blanket and a couple of inches of vermicrete. The vermicrete will basically serve to give me a surface to do finish work. If you aren't going with an igloo, you can get by with just blanket, and maybe some loose perlite or vermiculite around that for safety's sake on the heat transfer to the outside of your enclosure.

I don't know for sure how many inches of vermicrete you'd need without blanket, but I think the plans call for 6 inches if you don't use blanket.

[tdibratt]

Project1, call Elvis at Alphatherm in Concord, Ontario. He sells a blanket.

Regards, Tony.