Simple old fiberglass will not be nearly as good as rock wool in that sense. I'd take the Roxul guys information and put in flush to subfloor, the joists may even be tall enough for just a next layer of batts. They fit effortlessly in the cavities by using a https://simpleeyeshadow43210.bloggip.com/34728329/the-basic-principles-of-cost-effective-glazing