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Evets 06-04-2010 09:26 AM

Under the Hearthslab Insulation
Hi folks, I'm Steve and I just finished the block stand for my 42" pompei style oven. I'm about to start framing up for the slab pour and was wondering how hot the underside gets. I've not yet decided which to use under the floor, either perlcrete or foamglas. I was thinking about adding a layer of rigid polystyrene insulation under the concrete. I figured I could lay that on the form with lag bolts pushed up through the bottom and then the concrete poured directly onto the foam. I'm just not sure how much heat it can take. Any thoughts?

Evets 06-09-2010 06:06 PM

Re: Under the Hearthslab Insulation
I received an email from Owens Corning stating that the foam panels would start to break down and melt at only 150 degrees. I had already poured the slab without the foam anyway, but it was nice to know I probably made the right decision. I still don't know how hot it gets under the slab but I guess I'll find out soon enough.

BackyardPermaculture 06-09-2010 09:12 PM

Re: Under the Hearthslab Insulation
If you've got decent insulation above the slab, the slab itself should not heat up - it should remain very close to ambient temperature.

RTflorida 06-09-2010 10:43 PM

Re: Under the Hearthslab Insulation
A little food for thought - I have 4" of vermicrete under my hearth (on top of the 3 1/2" support slab). I have taken readings with my IR a couple of times. By no means a scientific project (just curiosity). Just after pizza (maybe 2 hrs total firing) I think the slab reading was around 110-115 degrees (15-20 above ambient); after a much longer fire of 3-4hrs the tem rose to abount 125 degrees. But really not much more that that contained space acheives on a typical hot summer day,


Raffy 06-10-2010 12:24 AM

Re: Under the Hearthslab Insulation
The slab does get warmer than usual (ambient temp) with 4 inches of perlcrete for mine. For residential ovens, wherein you fire up the oven every so often, maybe 3 to 4 times in a month, its not much of an issue. It takes a while for the firebrick to achieve heat equilibrium so before the bricks get totally heat soaked, you are probably done with cooking.

Whereas in a commercial oven running all day, it is suggested that a thicker layer of insulation should be used to keep most of the heat in. Like everyone says more insulation never hurts. What is in the plans is sufficient but it is up to you if you want to use vermicrete then ceramic boards to increase the insulation efficiency.

Oh and polystyrene sounds like a bad idea. Stick to materials recommended in the plans or ask suppliers regarding service temperature and melting point of the materials before considering them as insulation. To my knowledge anything below 650 C for service temp is a bad idea.

SteveP 06-11-2010 10:52 AM

Re: Under the Hearthslab Insulation
I only have 2" ceramic fiber board for my insulation under my hearth and I get about the same temp reading as RT does.

BackyardPermaculture 06-12-2010 02:30 AM

Re: Under the Hearthslab Insulation
Mine has 4 inches of AAC/Hebel, and a quarter inch cement sheet under that, and when I feel underneath it I really can't feel much difference from ambient temp. I don't have any gear to measure it more accurately though.

... Although a few hours later and I can feel that it is warm underneath; certainly not hot, but warm.

Neil2 06-12-2010 11:38 AM

Re: Under the Hearthslab Insulation
It is best not to insulate under the suspended slab. There is no advantage to trapping heat in the slab.

Eric Pfeifer 06-13-2010 08:02 AM

Re: Under the Hearthslab Insulation
I have 1 inch of percrete and two inches of FB board under my hearth. At equilibrium the structural slab gets to about 100 F. We have summers here in Minnesota which are (sometimes!) warmer!

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