Hi,

I read the sticky post about the underlayment. It seems it's only used on the first floor, and most of the time with subfloor. I wonder if I need underlayment on the second floor with nailed down solid wood. The lowes guy told me that usually I don't need underlayment on a second floor. Any suggestions? Thank you.

If I need underlayment, can I use the not-so-old carpet pad? It seems they are of the same material. Thanks.

I'm assuming by underlayment, you are referring to foam/cork/rubber underlay pad? Those are only used under floating installations. For a nail down install, you need to use 15 lb asphalt saturated felt paper between the hardwood and plywood as a vapor retarder.

Thank you so much for your reply. I found one link about the felt paper:

http://www.hardwoodinstaller.com/hardwo ... -paper.htm

It seems it's mainly used "to keep plaster dust from getting in their eyes or blasting everywhere when a board was niled in the old fashioned way," Thus, I guess the installer will take care of it, and I don't need to worry. right? Thank you.

The underlay is primarily meant to reduce moisture penetration from basement areas into the hardwood from underneath. Not so crucial for second floor areas but still a good idea from an installers point to use it for ease of installation.
If you are floating a floor, on the other hand, it is a completely different underlay and is absolutely necessary under the floor regardless of location in the house. The carpet underpad is not a substitute.