I poured the insulating slab today. 16 cubic feet of perlite to a little more than one bag of Portland cement. I think that ratio is in the neighborhood of 8 to 1, just from eyeballing the size of the bags. We’ll see how well it turns out. It’s a good thing I was prepared by seeing several posts saying, “Don’t worry if it looks like it won’t hold together. It will hold.” My mix certainly looks like it won’t hold together. I wetted it until it made a slushy sound while mixing, and I could squeeze together a snowball of material and it would hold its shape.
Super Isol is the way to go for the insulating hearth. It would have taken 30 minutes instead of four hours (I ran out of material and had to go back to the store). It also would have taken $250 (delivered) instead of $125. I’m trying to keep the costs of this one as low as possible. The main goal is to have fun and accumulate knowledge. When we build our dream house in 10 to 15 years, I’ll know what to tell our contractor.