If you want to add a capillary break to a basement or crawl space floor in an existing building, it is difficult to determine if there already is a capillary break (poly) beneath the existing slab. If you tape a piece of poly on the surface of the existing slab and moisture condenses on the underside of the poly, can you assume that there is no existing capillary break? Would it be destructive to add another layer of poly to the top of the slab if there is an existing capillary break under the slab?
Existing homes are a challenge!
Place dimple / drainage mat down - insulation - plywood...
The plastic test for concrete doesn't tell you what is under the concrete - poly or not, just if there is moisture in the concrete which will cause issues
Plastic is placed on top of concrete all the time (with or without plastic underneath) for vinyl plank installs, etc... & no it is not destructive. With that if water starts ponding under it, it will find a way out & that doesn't mean going down
Thanks for the input.