Insulating an existing basement slab with spray foam and a thin slab pour? – GreenBuildingAdvisor

I would be interested to hear if anyone has any experience insulating an existing (old) basement slab with spray foam, then topped with a new thin-slab pour (about 1.5″)?

In my particular situation, there are some complications that may steer me away from the traditional guidance shown here and on fine homebuilding of rigid foam and plywood layers.

In particular, 1/3 of the slab used to be old garage with the slab graded to a floor drain. Another 1/3 of the slab slopes lower than the rest of the slab, but most notably the first 6′ of a hall where a substantial point load was placed on an area of slab without a footer, has sunk 3/4+” over 20 years, and the rest of that end of the house sits at least 1/2″ lower.

To further complicate things, I believe I have high ground water (maybe 4-8′ below slab) that keeps soil damp and the old slab at 70-95% RH year round. So a slab grind to CSP3 and a fluid applied vapor barrier will likely be necessary, but there is a lot of uncertainty about how the existing structural framing that touches the slab will respond. There has not been any bulk moisture on top the slab, and there is a footer drain that stays dry.

I figure 1-1/2″ of spray foam topped with 1-1/2 to 2-1/4″ of concrete pour would remove the need to do any leveling and provide a higher r-value per height.