Old 04-19-2013, 04:30 AM
adm adm is offline
Join Date: Apr 2013
Location: UK - Surrey
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Default Re: Roxul RHT80 Hearth Floor

Originally Posted by vtsteve View Post
If you can get Foamglas, the insulation value is better (and no water can wick through from below). I capped mine with cal/sil, but I believe other members have put their bricks right on the Foamglas.
I am very interested in this....currently trying to figure out whether a 4" layer of Foamglas on top of a 6" reinforced concrete slab will work to sit my firebrick hearth directly on, or whether I should add a 2" layer of CalSil board as you did....

(I want to use the Foamglas instead of Thermalite/Cecon blocks due to the resistivity to water)
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Old 04-19-2013, 05:57 AM
UtahBeehiver's Avatar
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Join Date: May 2012
Location: Salt Lake City, Utah
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Default Re: Roxul RHT80 Hearth Floor


I used 2" of FoamGlas with 1.5" of Ca Sil, my logic was Foamglas does not absorb water but is is fragile and does not take abrasion well. So you have to careful when laying the floor, Ca Sil was more forgiving. All Foamglas or a combo works, up to you. One thing I would do if you use FoamGlas is slather the edge with leftover mortar which acts as protection buffer from your toes as you build your dome or work on your oven. Good luck
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Old 04-19-2013, 06:45 AM
adm adm is offline
Join Date: Apr 2013
Location: UK - Surrey
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Default Re: Roxul RHT80 Hearth Floor

Thanks for the swift reply Russell,

Good point on the fragility of the Foamglas. I think I will plan to use 2" Foamglas and 2" CalSil under my firebricks.

I am just reading through your build diary now - very inspirational as I am pretty much set on a corner build like yours.


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Old 04-19-2013, 07:11 AM
Join Date: Jan 2010
Location: Wallingford, Vermont
Posts: 108
Default Re: Roxul RHT80 Hearth Floor

Yes, I also parged the exposed edges of the Foamglas with left-over mortar, leaving a gap so it couldn't wick water through the parging. Before I did, every time I brushed it there was a stink of sulfur and a dusting of powdered glass (it's foamed with carbon monoxide and hydrogen sulfide - don't cut a lot of it in a confined space!).
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