Found a lot of good info around here and wanted to ask some questions about my scenario. I’m putting in a floor over at at-grade slab that’s about 40 years old. It was originally a patio slab so probably thinner than a house slab. I doubt there is any insulation or vapor barrier under the slab. The slab is also pitched down (used to be outside the house now it’s enclosed) so I’m leveling the floor with framing. I’m still determining best way to frame everything. I’m in climate zone 7 so cold winters and hot summers.

My initial thought is poly sheeting over the slab then joists/sleepers over top, and OSB subfloor. I’m going to use the same flooring I used in the adjoining room which is Coretec LVP with “FloorComfort 100-sq ft Premium 2 Milli-m Flooring Underlayment” beneath. If it would help, I was going to lay some batts or foam board between the joists for a little extra warmth.

Another idea was thinner pressure-treated sleepers attached to the slab, then a layer of foam board, then my subfloor. I don’t think I can lay foam board on the slab since it’s not poured flat and I’m worried about it breaking with pressure on it.

I’m gotten confused reading about barriers vs. retarders, which side they should be on, if I’m trapping moisture, etc.

Please let me know what you think, and if you need more information I’d be happy to provide it. Thank you!