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Is Polyiso foam board a good option for sound proofing interior walls?

mikeysp | Posted in General Questions on

We are in zone 4a.

Is polyiso a good soundproofing material between interior rooms?

I have plenty of 1-1/2 trecycled polyiso on hand; and, I am getting ready to hang drywall.

The foam has a black felt like material on both sides.

Exterior walls already have 3-1/2″ of it outside of the house wrap. (2″ + 1-1/2″ layers).

Thank you.


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  1. Expert Member
    Akos | | #1

    Unfortunately it won't do squat.

    For sound control of walls you need three things:
    -mass (dry wall is the cheapst)
    -decoupling (double stud, resilient channel, hat channel with clips)
    -cavity insulation (anything fluffy works about the same for this)

    Polyiso doesn't fall into any of the above. You can't use it as cavity insulation as it is rigid and would add another resonance to the system which tends to increase sound transfer.

  2. Expert Member
    BILL WICHERS | | #2

    I would rank it around 0.0001% better than squat, but otherwise I pretty much agree with Akos here -- polyiso's only contribution to sound deading is it's mass, and it doesn't really offer much mass... The type of facing makes no difference at all in this case. I can't see adding that polyiso to the wall cavities making any noticeable difference at all in terms of sound deadening.

    I would, at a minimum, put mineral wool batts in those walls, then a second layer of 5/8" drywall (use 5/8" here, not 1/2", since the added mass and stiffer board make a BIG difference for sound proofing!). This assembly will be noticeably better than a single layer of drywall in terms of sound proofing.

    If you want the next step up, I would either go with resilient channel, or a staggered studwall. Either way cuts into your room a bit, but the staggered studwall might be a little easier to deal with if you haven't worked with resilient channel before. All you need to do here is tack a 2x2 along the inside edge of the top and bottom plates of your existing wall, then add another set of studs "in between" the original studs that are aligned with the front edge of that 2x2. This gives you a 1.5" isolation air gap between the exterior sheathing and the edge of the studs that support the interior drywall. That gap allows for "decoupling", which will make a BIG difference in sound transmission through the wall. Put mineral wool in between the studs, and use a double layer of 5/8" drywall on the interior and you'll have a relatively simple wall assembly that will perform pretty well for soundproofing purposes. Using green glue between the two sheets of drywall will also help, but won't make nearly as big of a difference as the staggered stud wall assembly will.


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