Plain outdated fiberglass will not be nearly as good as rock wool in that perception. I'd personally go ahead and take Roxul fellas tips and put in flush to subfloor, the joists could even be tall enough for just a 2nd layer of batts. TrustATrader.com - a Listing of tradespeople https://chiefe306ete8.jts-blog.com/31557705/the-best-side-of-window-retrofit