Insulating Hearth Question

[redbricknick]

Rebar too buku?

I was able to get a bunch of three quarter inch rebar for my hearth slab. Is that too big? We plan on pouring a six inch thick concrete slab..

[james]

Hey Nick,

If you are building your hearth to the Pompeii plans, with a good amount of insulation above the concrete and below the cooking floor, I think you should be OK. The hearth concrete slab won't get very hot, and the ability of the rebar to absorb heat, expanded and contract and eventully crack your slab won't be very high.

Would our concrete experts agree with that?

With the Bread Builder oven, where the concrete sits directly under the cooking floor and there is a layer of vermiculite concrete underneath that, I would worry. That concrete would get hot and the chunky rebar could become a problem.

I'm cooking pizza for a party of 13-year olds right now in my Scott oven (grudgingly), and I can see that concrete layer (through the ash drop that I never use).

My two cents.
James

[mrpbjnance]

Recommend the Super Isol

For ease sake i highly recommend the Super Isol from James.
I had it installed in about 20 minutes. I don't know what it would have been like to do an additinal layer with Vermiculite but it can't be easier than the super isol.

[TeeJay]

Asbestos in Vermiculite?

I live in an old house that is insulated with vermiculite that was poured into the walls and ceilings. This insulation turns out to be contaminated with asbestos, a known carcinogen. It was known by the manufacturer at the time that it was contaminated but they didn't care and sold the product anyway. I would assume that what is being sold at this time is safe but I've learned that assuming things can be dangerous.

[maver]

I read about that - there was a vermiculite mine (I think in montana) where asbestos contamination was a problem. That mine was shut down. I read somewhere that the vermiculite manufacturers have had to work to clear their image since. Regardless of asbestos I think the vermiculite packaging warns you to protect against inhaling the dust (at our local hardware store it does). Bear in mind that health risk with this stuff has a lot to do with cumulative exposure. We used asbestos as excellent hot pads in my college chemistry classes. If it's not in dust form it's inert and not a risk. If you are concerned, wear a dust mask. Once it's in place, don't disturb it. If you are concerned about inhaling something toxic be cautious cutting firebrick as the silica can cause lung disease - I found that just soaking before cutting dramatically cuts down the airborne dust. For me, I ended up using perlite instead of vermiculite as it was a bit cheaper.

[james]

Here is the link to another thread on Vermiculite. In California, you can buy "certified asbestos free" vermiculite. As a good general rule, I agree with everyone who has made postings on keeping down dust while working, and wearing good noise, eye and breathing protection. You should use them. This comes from someone who scratched an eye on building project -- I am now very strict on safety glasses.

http://fornobravo.com/forum/showthread.php?t=361

James