I'm in Oregon where the ground is very moist. I'm considering installing 1/4" plywood underlayment on top of my 2"x6" T&G decking (for height purposes), and going with 9/16" engineered Kempas.

Should I lay down a vapor barrier between the decking and the underlayment, and another vapor barrier between the underlayment and the Kempas?

Also, should I use roofing paper, or is there a more recommended vapor barrier?

Thanks.

Craig

Roofing paper is not a vapor barrier. 6 mill poly paper should be used as a vapor barrier. Roofing paper only slows vapor transmission and works as a sound insulator. I don’t know about sandwiching the plywood with moisture barriers it may trap moisture. Most installer use a 6 mill poly paper then the plywood and finally 15 weight or higher roofing felt directly under the wood floor.