Using PL400 for gluing hardwood is not normally the way to go. A membrane (as in craft paper) is common practice.

Glue+nailing normally requires REAL hardwood flooring adhesive - not a rigid construction adhesive. Using the wrong adhesive is almost guaranteed to create horrible noise/void warranty, etc.

I would clarify WHICH adhesive they suggest for a glue+nail install. If it really is PL400 then I suggest you find another installer. Gluing hardwood is not something you muck around with - it is an expensive mistake when it goes wrong.

How wide are the planks? Iv heard of glue down being recommended for wide plank floors (5"+). But in almost every other circumstance nail down over a felt/craft paper is the best option for normal 3/4 solid hardwood flooring.

Thanks for the answers, yeah it's 4.25"

I cannot picture gluing hardwood flooring. Stapling(superior to nailing) still allows the wood to move. Wood absorbs/loses moisture and expands/contracts. The wood will do one or the other, and if it is immobilized, it will still move and create a crack.

That membrane is simply a squeak prevention. I generally us either 15 pound roofing felt or red rosin paper. I also use one or the other under stair treads for squeak prevention.

Our contractor (who won't be installing it himself, he will be hiring professional installers) says the advantage of glue is for larger planks. He said at 4.25" there are chances some cheap quality wood would start bulging/peeling from its corners/ends if not glued. Our wood isn't cheap, it's North American maple wood which we shopped around for a lot.

Is 5" the threshold where glue should be used?

I generally recommend gluing for 5" and above, using a full spread adhesive. At 4-1/4" wide, it would depend on what kind of humidity fluctuations you expect. As to the membrane, what is the humidity situation coming from below? What is the moisture content of the subfloor?

I honestly don't know the moisture content of the subfloor. Below is another floor (NOT a basement, it's a ground floor). The end joists are insulated with Polyurethane Spray Foam (4 inches = R24), so we're safe there. The upper floor walls are R24, the lower floor walls are R18-R22.

As a matter of fact the whole house was insulated with SPF, the roof being R36.

Of course we do get humidity fluctuations in Montreal, with our winters being dry and our summers very humid. We will be installing an air exchanger and a central humidifier. I'm guessing the humidifier would help? Would you still use glue?

Thanks.

Thanks, I'll try to look into it.