The flooring store where I bought it recommended a "closed cell" 3 mil product branded "whisper walk". I researched out and it seems that something thicker would be better. There is a product called TuTuff but it seems as though the only flooring application is in crawl spaces. It seems almost too good to be true when you read the website.
I would probably steer away from hardwood as I hear any moisture can cause the ends to curl. This is especially true for the wider planks from what I hear.
Its funny bc they do not recommend normal hardwood below grade but you can use a non engineered bamboo below grade. I think its because the bamboo is essentially manufactured from the grass through a special process. Looking for a good product that works especially well. Hopefully I can find something soon..........
let me know thoughts.........
Thanks for the reply
Mark