Hardwood floor install over concrete

This client wanted to get rid of the carpet, and install hardwood over concrete. Special attention must be given to the moisture content of the concrete floor. Any wood product directly on the concrete would be ruined with no vapor barrier. We agreed the proper installation is to use a 12-15 mil vapor barrier, then two layers of 1/2" plywood (glued and stapled together) as a subfloor. The hardwood floor installed on top of that, essentially making a floating floor.