This message board is great! I have learned so much. I only wish I would have known about it before I bought my prefinished hardwood flooring. I purchased Bruce(bad word) 2 1/4x3/4 oak and am researching the best underlayment. I am installing over existing maple and pine T&G over T&G subfloor. There is small areas that are slightly unlevel. A high traffic area that has sagged about 1/8 inch in a 2x4 ft area and another area along outside wall that dropped about 1/8 in. This an older house and I dont think there will be anymore settling/sagging. I plan to place a layer of felt in those areas, but am wondering if foam underlayment would also help to fill any gaps I might have missed. How is it with moisture? Does it breathe and let moisture escape? I have not seen any info re: this type and apogolze if this is a repeat question. Thank you for your wonderful help w/ my 1st major DIY project.

I'd forget the foam and layer up the asphalt felt paper as needed. The foam will only compress anyway. 1/8" isn't very much. Consider yourself fortunate!

An 1/8 is actually within spec. but I have seen an overlapped seam in the felt make a ripple.

Thanx alot. I might be back w/ more questions if the need arises!