What are you going over top of? How are you installing the hardwood? Let's start at the beginning.

Hi, hardwood will be floating on a click system. The base is concrete, then there is a little cement layer and on top of that is where the underlayment is meant to be.

So...you need a moisture barrier first and foremost. That means the industry standard is just fine: 6mil polyethylene sheeting = $0.06/sf at Home Depot. You will need moisture resistant tape such as "Red Duct Tape" = $9.95/roll = 75 linear feet of adhesion.

If you wish an underpad, you can look at 3mm ($0.40/sf) or 6mm ($0.80/sf) cork that sits over top of this moisture barrier. No glue. No tape. Nothing. It just sits there like a beach blanket.

Then you lay the floors over top. Done and done. The vapour barrier is not only "industry standard" it is also one of the lowest cost options we have on the market. Wow. Cheap and effective. Kinda cool. Which is why many installers use it day in and day out.

Cork is awesome underneath floating hardwood because the cork "floats" as well. The one thing you MUST do....make the concrete FLAT. That's the hardest part of this whole exercise.