Underlayment for above grade wood flooring

[Patrick43]

I recently installed oak, nailed down flooring in a room on the second story of my home. I used a 3 in 1 foam underlayment instead of red rosin or asphalt paper thinking that it would provide more sound proofing. Was I incorrect to do this and what are the consequences? I've read that moisture/condensation may be an issue between the underlayment and subfloor. Thanks.

[HayZee518]

shouln't be a problem because its on a second floor - if it were the first, then yes.

[pushkins]

The only problem that I can see that "might" happen would be compression of the foam underlay. Typically the foam underlay is for engineered flooring which is usually 5" wide planks, this allows a greater distribution of compression as well, the floor is "floating" it doesn't have the same pressure points. As you have nailed down the tongue and groove you might find down the track that the walking paths compress the foam allowing the tongue and groove flooring to move as you walk, sometimes heard as squeaks. For example when you nail it down, it pulls the plank down onto the foam and firm to the subfloor, if the foam compresses at all, the nails will remain where they are and the planks can move.
Like HayZee says, it's on a second floor so moisture shouldn't be a problem.