For the oven base slab (not the pedestal / shelf) do I need to insulate between the slab and sand layer? If so, what should be used?
Thanks very much.
Hi GT, and welcome to the BrickWood forum!
I know the terminology is a little novel, but I’m still not sure I understand your question. So I’ll answer both:
- The ground slab, which is what the concrete blocks rest on, does not need any insulation. It is poured on top of base rock and, if done according to the instructions that BrickWood provides, it “floats” on top of four concrete footings.
- The hearth slab, which rests on top of the concrete blocks, does not have any insulation between the slab and the blocks. The void (or open area) on top of the slab gets a layer of insulation consisting of either commercial insulating castable concrete or BrickWood’s homemade (and excellent) mix of Portland cement and Perlite. Sand goes on top of that, and it has little insulating effect, and finally low- or medium-duty firebrick on top of the sand. The firebrick act as a refractory surface, soaking up heat and refracting most of it back into the oven. Any heat that conducts into the sand layer then hits the thermal break provided by the insulation layer, keeping it from being sucked up by concrete. Plain concrete is thermophilic and will pull heat out of your oven without that insulating layer.
I hope one of those two bullets answers the question, and let’s talk more!
Yes, this is exactly what I needed to know (was asking about the hearth slab). Thank you very much for the clear and detailed explanation!