I have a question regarding the preferred placement of the moisture barrier in a nail down engineered floor. Here is the existing 1965 floor assembly:
- Pier and beam over crawlspace
- 3/4" x 4" subfloor (not T&G) laid at a 45 degree angle over the joists
- 15" roofing felt moisture retarder
- 3/4" particle board underlayment

In preparation for the new floor, I will be removing the existing particle board underlayment and 15# felt, re-screwing the subfloor to the joists, and then installing 3/4" T&G Sturdi-Floor. This leads me to two questions:

1. Should the moisture retarder (likely 15# felt again) be placed between the subfloor and the underlayment, or should it be between the underlayment and the new wood floor?
2. If the retarder is between the subfloor and the underlayment, is there a need for anything between the underlayment and the new wood floor?

From top to bottom , you should have :
-Flooring
-15 lb felt paper retarder
-Plywood sub floor
-3/4" x 4" pine substrate
-Joists

There's not really a need to add an additional retarder layer and the general school of though on that is that the extra layer can partially trap moisture around the plywood instead of letting it move up or down.

It's better to control moisture in the home mechanically than to sandwich extra layers of moisture retarding paper.