Can I substitute Insulating Castable with Castable Refractory

Hello, I am starting the build of the base for a 28" Mattone Cupola. I see that on step 49 I need to add insulating castable or a mix of perlite and cement.
My question is, I did not buy a 50 lb bag of the insulating castable when I picked up the other materials at Harbison Walker last week, but I did buy extra castable refractory. Can I substitute one for the other and would I still add the 4cf of perlite into the castable refractory for it to work?

Hello, Andrew, and welcome to the BrickWood forum!

I’m a bit leery about the castable refractory, but that’s because I’m more of a Mattone Barile guy. My sense is that it would be a very expensive way to cast your insulated base. If you have one or more bags of spare castable refractory I’d recommend you sell it rather than pour it into your void.

I would definitely not buy the insulating castable. Portland cement and Perlite are much less expensive, and will do the job of providing a thermal break between your hearth and the concrete that wants to be a heat-sink.

Would love to see your progress whenever you can post pictures!

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Thanks for the advice, I also noticed an update that it would take 100 lbs of insulatable castable, double what the installation guide shows. So I think I’m going to go with perlite and portland cement.


That’s a big negatory!

You do NOT want to substitute the Perlite / Vermiculite & Portland Cement insulation (or Insulating Refractory) with a dense Castable Refractory. The Castable Refractory will absorb the heat from the hearth firebrick… and the concrete slab will get super hot as it absorbs heat from the castable refractory. Nobody wins!

Insulating Castable will work (Qty 2 / 55lb bags @ $60 per bag)… or the surefire Vermiculite / Perlite & Portland blend (Qty 1 / 4cf bag of Vermiculite or Perlite & Qty 1 / Portland Cement = about $25 total).

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