After the demo and getting the columns out of the way, we were ready to prep for the slab. Much of the prep involves the plumber, as he has to put in both the waste lines and the hydronic heating tubes before the slab is poured.
In my county, all waste lines below grade have to be cast iron pipe, which you can see here. I think PVC is probably bullet proof, but the cast iron is really bullet proof. I strongly expect to never have to worry about those waste lines in my lifetime.
My plumber, who is otherwise an outstanding guy, talked me into using a reflective, quilted insulating blanket below my slab. It costs about as much as 2″ rigid foam and it serves as an insulating layer and a vapor barrier. It’s a good vapor barrier. But, the reflective surface is irrelevant buried under concrete with no air space. Furthermore, it has an R-value of about 1.5. That’s better than nothing, but I regret not having used the rigid foam. The most important bit of insulation is at the edges of the slab, which I did do correctly with 2″ foam. But, if you do not insulate below the slab you end up heating the ground beneath the house to a depth of 10′ or so. Once that heating is done, you don’t lose much heat to the earth (as actually the earth gets warmer as you go down), but it does dramatically increase the thermal mass of the system. That means that the week or two at the beginning and end of the heating season can be a bit wacky as the slab and earth come back to equilibrium.
Once the slab is poured, the place felt much better. Before it was a dark, dank dungeon. After, it was a light, airy skate park.