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Foam board or Rockwool for insulation/sound insulation

Benneaf | Posted in General Questions on

It’s time once again for home upgrades.  We’ll be expanding the house and installing new siding and windows.  It’s not all about energy efficiency, its about creating better space, aesthetics, energy efficiency AND minimizing outdoor noise.  We live very, very close to the interstate.

I have two questions.  One…when it comes to exterior insulation will foam board or rockwool minimize sound better?  Second, when it comes to windows will a triple pane or laminated pane window cut sound more?

This is the same house as in

Thank you in Advance
Andrew Bennett

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  1. bigred | | #1

    I recently completed residing and energy upgrade to my home (zone 2A). We pulled everything down to the studs, foamed all internal to external penetrations, sealed the sill plate to slab, installed rockwool insulation and then R-3 R-zip. For windows we went high efficiency Pella fiberglass and wood windows. Where sound was critical (master bedroom), went STC rated glass. Overall, everyone who comes in our house notices how quiet it is. In the bedroom we have a neighbor's heat pump that sounds like a 747 taking off when it kicks on. With the STC rated glass you can barely hear it, it's just a whisper. Open the window, you can't hear the TV. It is a double pane STC 31 (meaning a 31 db sound reduction). STC is achieved by different thickness of glass, in my case of double pane, a 3 mm and a 5 mm pane instead of 2, 2.5 mm panes. Believe it or not, a triple pane, with three 2.5 mm panes would actually have a lower STC rating that the double pane I have. So it's not just thickness of glass but the pane thickness plus air gap. I did all of the work on my house myself (with hired carpenters as assistance) and was very happy with the outcome. Even shocked a few inspectors who came expecting to shit all over a "homeowner" job, but ending up telling me this was the best siding job they had seen. I did get a little anal, with liquid flash on ever nail, using 316 ss siding nails, galvanized sheathing nails. "excessive" flashing, and frankly I doubt any contractor could have made money with the details I did. I paid by assistants by the hour as I wanted high quality and was willing to spend the time to get it right. I am a serious woodworker so had a professional level shop to pull tools from, so very few contractors have access to a 3 HP table saw, track saws, band saws, etc.

    1. Benneaf | | #4

      I'll definitely take a look at Pella's windows. I know the detailing will be important then it comes to sound reduction.

  2. Robert Opaluch | | #2

    Roxul/Rockwool "Safe'N'Sound" is made to reduce noise in stud walls, but is not rated for insulation value, unlike their less dense ComfortBatt product for wall insulation. Safe'n'Sound has a density of 2.5 lb/ft³, an STC (sound transmission class) of 45 on 16” center wood studs, 52 with 24” center steel studs, and 50 when used between floor joists.

    Roxul/Rockwool makes ComfortBoard rigid board insulation for exterior insulation that is much heavier and attenuate sound better than foam. I used 4'x8'x1" polyiso foam as exterior insulation, and can attest that it doesn't do much for sound attenuation. I don't have STC values.

    They are changing the name from Roxul to Rockwool. I think in Canada they used Rockwool as the brand name before using it in the USA, but not sure.

    Note that there are plenty of other ways to reduce sound transmission through walls. Use type X fire-rated 5/8" thick drywall, or double layer drywall might have STC of 40 from what I've read. Second layer of drywall can be mounted mostly using glue to the first layer, to avoid screws or nails through both layers. That worked great for soundproofing rooms in the home I built. I also split (non-load bearing) partition studs except for the top and bottom 1.5' to reduce transmission, but can't do that on any exterior or load-bearing walls. Caulking the space below drywall at the bottom plate helped too.

    1. Benneaf | | #5

      I'd like to do some exterior insulation just to beef up what little the house has. I want that insulation to do double duty to cut down sound. It sounds like Rockwool will be the way to go. I'll think about adding another layer of sheetrock on the inside as well. That's not something I had even considered.

  3. Expert Member
    BILL WICHERS | | #3

    You want two things to block sound: mass, and squishiness. Mass helps to block the sound, squishiness helps to absorb the sound. Mineral wool will work much better than rigid foam due to more squishiness. For mass, a double layer of 5/8" drywall on the inside will help a lot.

    If you're going to be completely stripping the exterior, and you really want to cut down on sound, see if you can build a staggered stud wall. A staggered stud wall has every other stud aligned with the opposite wall face. This means there is no stud that is rigidly connected to both the inner and outer surfaces of the wall, which greatly reduces sound transmission through the wall. If you can build this way, use mineral wool batts inside the wall, and then a double layer of 5/8" drywall on the interior, you will notice a BIG reduction in sound coming through the wall.

    Let's assume for a moment that you build that double stud wall as described. All your windows are now weak spots, as are your doors. You can get sound attenuating windows. There are two main ways to do this: one is to use special IGUs that have different thicknesses of glass in the two panes. The other is to use an IGU that uses laminated glass in at least one of the panes. Laminated glass works wonders for sound reduction. Don't forget to use windows that will help cut down on sound too! Don't believe the info out there about triple pane windows helping with sound -- they're better than double panes, but not nearly as good as purpose-built sound reducing windows.

    For a door, you want a very heavy, solid door. A big, thick wood slab is probably your best bet, along with good weather stripping (which will also help with air tightness). The usual foam filled steel or fiberglass doors aren't as good about blocking sound as a thick, solid wood door. Note that by "thick" I mean thicker than the usual 1-1/2" or so. Hammer and Hand has some examples of "thick" doors, but they aren't cheap. Use the best that you can afford here for best results.


    1. Benneaf | | #6

      I've been looking at the Provia Embarq line for a door. They are thicker than normal...but are foam filled. For an "American" door they seem to seal up very well. Are you familiar with their product?

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