I got approval from my HOA to install hardwood flooring on my 2nd floor condo. I told them that I would install an underlayment to reduce noise. It's a 1/2" engineered floor (tongue and groove), and I would like to do a floating installation. The subfloor is gypcrete, so I cannot nail it. I was originally planning to use cork, because I've heard it has good acoustic properties. I had intended to buy 1/8", but it appears that I really shouldn't get anything less than 1/4". The 1/4" cork is pretty expensive, and it looks like the specs aren't as good as competing options. For half the price, I can get MP Global QuietWalk with an IIC of 66 and an STC of 71. There is also Eco Cork Foam from HD that seems comparable. The 1/4" cork has an IIC of 51 and an STC of 54. From what I've read, these numbers can be manipulated, so I don't know what to believe. My HOA doesn't have specific requirements which gives me some flexibility. My main concern is noise for my downstairs neighbor. I would also like to go as ecofriendly as possible and would prefer an underlayment with no formaldehyde, etc. That's what originally drew me to the cork.

Honestly, if your HOA or condo board doesn't have a specific IIC rating, go with a rubber underlay. Cork is great, but not always necessary for the cost, plus it should be glued down, and wouldn't be my first choice for a floating install. Mind you, I also wouldn't float T+G material - floating floors work best with a click locking system. While you can do it, you'll want to put T+G glue to keep the boards from separating, and be ready for visible gaps in the dry winter months.

As will all floating installs, the subfloor should be FLAT as possible. 1/8 over 10ft or better, otherwise you'll feel all the deflection and movement.

And remember, if your neighbors complain about noise, direct them to the HOA board and their lack of IIC requirement in a multi-dwelling space. It's their problem.