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Community and Q&A

Sound dampening floor cavities

kevinkeegan | Posted in GBA Pro Help on

This is for sound dampening purposes only? I have a second floor area which has a room on the second floor with exposed floor  joist cavities and I want to improve the sound transmission from the second floor to first floor. The ceiling is finished on the first floor so, I only have access to the floor cavities from the second floor.  The sub flooring has been removed from the second floor floor joists. The depth of the cavities are about 10 inches deep. There is fiberglass insulation laying directly on the back surface of the first floor sheet rocked  finished ceiling  so the 10 inch cavity is above this layer of fiberglass insulation. I was thinking of either adding cellulose  insulation to fill the 10 inch voids, unless fiberglass is less expensive and does just as good a job. The other question I have is should I fill the entire void to the top of the floor joists, so the insulation contacts the underside of the new plywood sub flooring? Just a clarification, many years ago the first floor ceiling was leveled using a 2″ x 4″ frame work below the existing second floor, floor joists. This 2″ x 4″ framing is attached to the first floor perimeter walls and in the center of this framing it is held up with 5/4″ strapping nailed to the second floor floor joists and it is in this dropped ceiling area where the fiberglass insulation is installed. I added another layer of fiberglass insulation on top of the existing fiberglass insulation, so the void I have now is from the bottom of the second floor joists to the top of the second floor joist, approximately 10 inches. For sound damping purposes only should I fill that 10 inch void to the top of the second floor, floor joists? and if so what would be the best and most economical insulation to use? Once this is determined I plan to install Rugapol for foot fall deadening.
Thank you, Kevin

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  1. Expert Member
    BILL WICHERS | | #1

    You want an air gap between the insulation and one of the two "sides" of the assembly for best sound attenuation performance. You do NOT want to completely fill the cavity. Density of the insulation does make some difference, with denser insulation generally being better (mineral wool is better than fiberglass for example), but it's not usually a massive difference. I don't think you'll have any problems with cellulose here.

    Note that you'll still get impact sounds (footsteps) coming through this assembly. You could try a layer of homasote board under your flooring for a bit better overall performance, but you'd really need to decouple the first floor ceiling (hat channel, etc.) for best performance.


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