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Soundproofing Under Hardwood Floor?

Renovations102 | Posted in General Questions on

Hi folks,

I’m finishing my bonus room above the garage. The subfloor is Advantech, the floor will be 3/4″ solid oak. The bottom of the subfloor (ceiling of the garage) is sprayed with 2″ of 2# closed cell spray foam.

Do any of you or anyone you know use soundproofing under their wood floor?

The room is for music & movies, & so I am just trying to isolate it from the house as much as possible.

If this is recommended, can you all provide some good quality manufacturers of soundproofing for this application?

Thank you.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    The usual material for this sort of this is homasote board. Homasote board is a dense but still somewhat squishy fiberboard that helps with sound control when used under hardwood floors. It helps a little, but it’s far from being sound proof. If you really want to be soundproof, you need to isolate the ceiling drywall underneath the floor using hat channel, and ideally going with a double layer of 5/8” drywall too with green glue in between the layers. You can get a lot more complex as you work towards really soundproofing the room.

    Since the room is over a garage, I wouldn’t worry too much about soundproofing the floor — your cars won’t care when there are explosion sounds in your movies. I would put some effort into the wall between your theater room and the rest of the house though. Probably the best combination of good and cheap here is to use 2x6 top and bottom plates for that wall and staggered 2x4 studs (“staggered” here means every other stud is aligned with the opposite side of the too and bottom plates so that there is a separate set of studs for each side of the wall to prevent acoustic coupling through the studs). Fill the stud bays with mineral wool. The next step up is a double layer of 5/8” drywall on one, or better yet, both sides of that wall with green glue between the drywall layers.

    If the wall already exists, the simplest way to add some soundproofing is to add a second layer of 5/8” drywall on at least one side of the wall, ideally with green glue between the existing and new drywall layers. You can also hang the drywall on resilient channel to isolate one side without reframing the wall into a staggered stud wall.

    With sound proofing, you need mass (which is why you want to use the heavier 5/8” drywall here and not the usual 1/2” residential drywall), and you need decoupling (staggered or double stud walls, resilient and/or hat channel, etc), and you use some absorbing materials (green glue, sometimes MLV — mass loaded vinyl — sheet). Every step up in complexity and cost gets you more sound blocking performance. Even the simplest step of just doubling up the drywall on one side helps though.

    Note that to do a really good job, you need to close off any leak paths too, so no electrical boxes on opposite sides of the wall in the same stud bay, stuff like that. You also need a good, heavy door that is gasketed all the way around.

    Remember that every little bit helps, so you don’t need to go all out to get a noticeable benefit. I would at least double up the drywall on one side, and using a solid wood door with weather stripping and some type of threshold seal will help a lot.

    Bill

  2. Expert Member
    Malcolm Taylor | | #2

    Renovations102,

    There are a number of sound attenuation underlayments for use with hardwood floors which all work fine as long as you understand their limitations. They will slightly reduce both structure borne and air-borne sound, but for any significant difference they need to be used as part of a complete floor assembly with other things like resilient channels, multiple layers of drywall, batt insulation in the joist bays, etc. If you decide to go that route I would suggest using a complete tested assembly so you know what reduction you can expect.

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