I have a client that wants to turn an unused garage on the 1925 house into a workshop area that is insulated and conditioned with a mini-split. 

I’m looking for recommendations on what product/approach to use for dealing with moisture from the old slab. It’s original to the house and definitely doesn’t have and vapor barrier or insulation under it. The ceilings are 10′ so there’s plenty of room to put rigid foam under the new subfloor but I’m not certain on the best product to use to deal with the slab’s moisture.

The slab will definitely get damp during heavy rainstorms so moisture management is a big concern.