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Sound isolation with an unvented attic?

Andrew Shu | Posted in General Questions on

Hi,
We’ve insulated the roofline and exterior walls with spray foam insulation and put fiberglass in the interior wall studs, and double drywalled with green glue on bedroom interior walls, but there’s still sound transmission between rooms since there’s no insulation above the ceilings. Would adding roxul above the bedroom ceilings work? Would that cause any problems? Dust?
Thanks!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Andrew,
    Q. "Would adding roxul above the bedroom ceilings work?"

    A. It cant hurt. But you probably should include resilient channel when you install a new ceiling.

    Q. "Would that cause any problems? Dust?"

    A. Demolishing a ceiling and installing a new ceiling always creates dust. But once you have cleaned up the construction zone, there isn't any reason why the Roxul between your ceiling joists should cause any dust problems.

  2. Expert Member
    Dana Dorsett | | #2

    If the attic space has a floor rather than open joists to make installation from the top easy, it's often possible to insert a sound-dampening block of blown fiber in joist bays above partition walls without demo-ing the ceiling, using a ~1.5-2" hole- cut with a hole saw, inserting a woven polymer feed-bag, and blowing insulation into the bag with only a small portion of the bag below the gypsum level. When the bag is stuffed, push it all up into the joist bay, and repair the hole using the hole-saw plugs.

    This is often done at "dense-pack" densities for the purpose of blocking air movement, but for sound abatement you don't want to go quite that high, since over some density the tightness of the packed fiber makes it more mechanically rigid, and starts to transmit more sound than at some density (but nothing like an open joist-bay would.) I've only seen this done with cellulose, but there's no reason it could achieve similar results with rock wool or fiberglass.

    Most insulation contractors who do dense-packing would be familiar with the feed-bag method.

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