Hello I have to make a decision on flooring installation method, and my research so far has indicated that this may be a 50/50 decision, unless there is something that I have missed.

This floor is above a 2-car garage, structure is AdvanTech 3/4 subfloor over 16″ wood I-joist filled with dense pack cellulose insulation. The garage underneath is unheated but will be insulated.  The room is currently heated/cooled by a mini-split system, and will have a gas fireplace as well in the future. Location is Maine, area code 04240.

Flooring material is 5″, 6″ and 6.5″ wide T&G oak boards, 5/8″ thick, and reclaimed from an old tobacco mill from the south. Very unique and characteristic, and they look awesome.

Right now I think there are only there are two options:
Method #1; Install a vapor barrier underlayment, either felt paper or Aquabar, and nail the flooring into place.  The vapor barrier is beneficial because the room is above the unheated garage.
Method #2; Nail and glue the flooring, which would eliminate the underlayment. The glue is beneficial because the boards are wide and is preferred for solid boards greater than 5″ wide.

I’ve talked with the floor miller, flooring installers, and flooring sellers, and all have offered almost the same advice, which is either method would work, but no one can recommend which solution is better.

How do I decide what is the most important thing to have – a better vapor barrier or better board attachment?  I am not sure how much vapor protection there is with the AdvanTech flooring, and if that is acceptable enough without an additional layer.  Or, will a better vapor barrier really offset the chance that boards will cup and become a hazard in the future.  One guy told me that I’m on the edge of both situations, and if I had a lucky coin, that might be the best way to come to a decision.

Any additional information or insight that might push me in either direction would be greatly appreciated.  Thanks.