Should we glue above and below our rubber underlayment for wooden floors?

Question

We have received our delivery of 3/4 inch thick engineered wood planks.

They are 8 inches wide, random lengths, and the white oak layer on top is about 3/16 thick. Subfloor is very good and solid and is 3/4 plywood.

We have decided to install a 1/4 inch thick rubber underlayment to reduce sound movement through the house.

The manufacturer of the flooring is happy with that (rubber) decision and has no issues, but they insist that we first glue the rubber to the subfloor and then glue the planks to the rubber.

Further, they have specifically recommended Bostik greenforce which is not only a glue, but a VAPOR BARRIER as well.

So, some very specific (glue both sides, use this very specific glue) recommendations from the manufacturer.

My question:

Isn't it ill-advised to install two vapor barriers (the two glue layers) with a gap between them ? I thought we always wanted to avoid that situation, wherein vapor can get between the two glue layers and can never get out because they are vapor barriers.

I don't want to go against the manufacturers advice, but this was a red flag because I know, for instance, you don't want to stack two layers of densshield on top of one another because you create a vapor "sandwich" between the two barriers ...

Thanks.

Answer 1

You’re right...we avoid two layers of vapor barriers, but that is for walls, ceilings, etc. with material in between the vapor barriers that can have moisture in the lumber or trap moisture within the void space as heat transfer the walls, ceilings and reaches it Dew Point.

In this instance you are “sealing” the DRY rubber underlayment. (Emphasis on “dry”.) You’ll have minimal moisture movement downward into the rubber underlayment compared to air movement up into ceilings or sideways into walls. (Most of the moisture will come up from below.)

However, I’d contact customer service (in writing) and ask them to clarify...and keep a copy of their response.

There seems to be two conflicts 1) sealing the rubber underlayment to the plywood subfloor, and 2) sealing the rubber underlayment to the engineered flooring. I like the idea of sealing the rubber underlayment to the floor, but we generally allow the wood flooring to move, depending on seasonal humidity, etc.

I’d re-read the instructions to be certain and then I’d follow the manufacturer’s instructions...after they clarify...for your location.