Flooring – How to install vapor-retarder underlayment paper under vinyl floor planks

The vapor barrier for a crawl space belongs above the floor joists and below the sub floor. You pretty much have three alternatives here.

1) Remove all the flooring - including the subfloor - install the vapor barrier - and replace the flooring, except you'll pretty much have to destroy the flooring in the process.

2) From underneath, take 6 mil plastic and set it into each space between the floor joists. You can then either wrap the plastic tightly around each floor joist, stapling it in place and then taping over the staples, or you can cut the plastic so that it doesn't wrap the floor joists, and tape each cut edge.

3) Buy spray foam and use it as both the vapor barrier and insulation. For this application you would want the fire rated open cavity foam like what they sell here: http://www.tigerfoam.com/products.php

The reason the floor materials aren't as effective is because this problem is best solved with an air-gap and a flexible connection spanning that air-gap. But you have a hard time supporting a floor with that air-gap. Therefore, solving this problem via the ceiling below this floor is better, using resilient or sound attenuation channel there.

That said, on your floor, carpeting over Homasote, QuietWood, or some other material will be as good as you'll get on the floor. Since I don't have experience with MLV or GG, I'll defer to others on that. However, the cement backer board seems like the weak link here since that will transmit sound rather than absorb it.