I’m in the process of building a house, and all of the interior walls are 2x6s, I’m looking at doing insulation in the interior walls, mainly for noise mitigation, but also to limit thermal transfer between rooms (each room is zoned separately).

I was looking at doing Rockwool for interior insulation, but the 2×6″ rockwool is 3X the price of fiberglass.  I can’t justify the extra expense given all the data on performance differences.  However, I’m not a fan of fiberglass, due to slightly worse noise performance, and sagging potential over time.  So I was thinking about using denim insulation whcih is only 20% more expensive than fiberglass, the problem is I can only find it in 3.5″ locally.

So the question I have for everyone is, can I use 3.5″ of denim in a 2×6 cavity and not have degraded performance or sagging?  is a 2″ gap too much?  I know that some folks say a small airgap helps with sound, but 2″ is a lot, and I’m a bit worried about cold air pockets, but since it’s interior only, maybe thats not a problem?  Denim is a lot denser and more rigid than fiberglass, so seems like it would be fine from a friction stand-point, but wanted to get feedback from the experts.