Insulating castable for hearth insulation layer

I posted a version of this in the wrong forum the other day, I think. What am I looking for in an insulating castable as a substitute for the perlite/vermiculite and Portland cement mix (for step 49 of the base)? My local refractory has tons of different options and I’m not sure what I need in terms of density, etc. Thanks for any help!

Here’s the rundown on insulating castable refractory.

And this is a link on BrickWood’s store to an example product.

But note well: there is only a slight performance improvement gained by using the commercial product. And the real world cost differential is breathtaking. A bag of Portland Cement is about 5 bucks, and a bag of Perlite about 25. That and water is all you need to make an excellent insulated base that will be fully cured in 48 hours. (Vermiculite is a different material with similar properties, but it will take nearly a week to cure. I’d suggest you stay away from it.)

The insulating castable refractory costs anywhere from $90 to $120 per bag. It may set a little more quickly, and insulate a little more effectively, but you pay a huge premium for that.

Here’s a post about my experience with making the insulating material from scratch. It was one of the easiest parts of my build.

I should also say no one will judge you if you decide to go with the commercial product. Check out HarbisonWalker and use the terms “Food Grade Lightweight Castable Refractory Cement” with the sales rep—they’ll set you up right away.

Hello. I’m glad I seen this post. I’m currently on the insulation part for the slab and used the Harbison Walker insulating cement and it cracked on day 3 after pouring. I emailed them and still waiting on a reply. Anyone else have an issue with this product?

@EfranC, I wish your seen my post first! I just poured my oven on Saturday and did the insulation last week. I’m just a few steps ahead of you. Harbison walker insulating cement is the WRONG product. It’s basically rockwool fiber mixed with cement. It will NEVER cure rock hard and will always have some compression to it, NOT what you want under 1,000 pounds of oven. You need greenlite 45, that’s the stuff they recommend for sub hearth insulation. I’d you call them and ask for sub hearth insulation, that’s what they’ll recommend IF the sales guys knows what he’s doing. However, after doing exactly what you did, I ended up going with the perlite/cement mixture and it worked perfectly. Please learn from our mistakes and just go that route. Another complication for me is that my local harbison closed due to covid so I could get the correct product the second time around anyway.

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Thank You for the information. They did get back to me. They recommend pouring the insulating concrete in 1” thickness increments waiting for the first layer to dry before adding the second. They are pretty far from where I live so I’ll be doing the Portland cement perlite mixture this week. What Oven are you building I noticed you wrote that you poured your oven.
I’m building the 28” Cupola.

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This conversation is making me a little nervous! My local place is recommending Kast-o-lite 22, and I’ve asked them to double check that it cures rock hard and can bear 1000 pounds. I’m having a hard time finding perlite as a backup, though - all our box stores are sold out, as are the nurseries.

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I Removed the cracked insulating cement and did the perlite Portland mix. It’s pretty light weight stuff. The insulating layer in the void itself doesn’t hold a ton of weight besides the sand and firebrick.


Contact Uline. They will deliver as much as you need.

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Looks like a perfect pour! It will lighten up overnight and you might see a few pellets of perlite poking up from the surface. Don’t worry about those. The Perlite material has an interesting combination of properties, as it both attracts and repels water. Technically water gets trapped in the microscopic holes in each pellet, but the material itself is volcanic glass so it doesn’t retain the water.

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I’m building the 28" cupola as well. Part of the oven’s rim will sit on the firebrick, most of it actually. Only an inch or two on the sides and back sit on the red brick perimeter. Or in your case the concrete perimeter you made. Looks good.