Installing 10" wide ¾ solid oak planks in a room that previously had multiple additions, some of which had particleboard underlayment. The original room had 1 ½ " subfloor. Due to serious uneven and level concerns, the entire room was leveled and ⅝ ply put down over the subfloor that previously had particle board (which was removed). Due to the original subfloor needing leveling (builder's felt buildup, and sanding down high spots) part of the room is ⅝ ply and the rest ply and felt.

I've decided to put another layer of ½ ply underlayment over the entire room so that there is a strong, consistent, even surface for the planks. As the planks are so wide, I plan on nailing with glue assist and also screwing them down.

Which brings me to my question, can I put the vapor barrier below the top underlayment level so that it will not interfere with the glue down? The floor is over a dry crawlspace covered in heavy duty plastic, with good ventilation. However, being that this is in the Pacific Northwest it does get humid.

Thank you

You need to use a roll on moisture retarder that is compatible with the glue you are using. BONA and Wakol both make great systems just for this application.

Thank you!
These look like great systems. Would using the glue obviate the need for screws?

With full trowel glue you will not need screws.