So, are you saying you DO have moisture in your existing sub-floor? Putting a moisture barrier of any kind over existing problems is probably not a good idea, speaking as an old architectural woodworker.
If there is no moisture/rot in the existing floor, a good sub-floor made with moisture resistant adhesive could be a good solution. How would you plan to secure the new layer of 3/8 ply?
Greg and Linda Heuer
'99 Excella 30 w/HAHA - The Silver Otter
'08 Chevy LTZ 2500HD 6.6 DuraMax
| WBCCI 1927 - Unit 149
| AIR 53869