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Community and Q&A

Insulation for interior walls

Westhawk39 | Posted in Energy Efficiency and Durability on

We are in the process of building a “pretty good house” in Manitoba Canada. D.P. Cellulose insulated 12” thick walls, triple pane windows, no gas appliances. Also working hard to limit noise transfer between bedrooms. We have decoupled walls with staggered studs, and framed them either 4”
Or 4.5” wide (bottom plates) with 2×4 studs.

These wall thicknesses seemed great at the time but now I’m not sure how to insulate them. Do we use 3.5” roxul bats and leave 1/2” void or do we use 5.5” Fiberglass and compress it? I’m wondering if the Fiberglass will negatively affect the indoor air quality? Does anybody have insight on this?

From reading on green glue website they say Fiberglass will work equally as well as roxul but that air spaces can work against you.

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  1. Robert Opaluch | | #1

    Air spaces both help (stop bridging) and hurt (compared to insulated spaces). When you stagger studs, the direct connection between the stud and drywall (on one side) is replaced by an air gap, which helps reduce noise transmission through the stud. However, if you allow an air gap through the wall from drywall in one room to drywall in the other room, sound will travel easier than if that space were stuffed with fiberglass or Roxul batts (or cellulose). A layer of air on one side of a batt (between the batt and drywall) won't hurt.

    However, be sure to isolate any electrical boxes in the wall. Either cover the outside of the box inside the wall, with a sound-deadening product, or stuff the batt behind it to eliminate a path from the electrical outlet through the wall to the drywall on the other side of the wall.

    Roxul SafeNSound is designed to reduce noise and fire hazard, but doesn't seem very different from Roxul ComfortBatt. I tried to get a Roxul rep to tell me the R-value of SafeNSound, or STC value difference of the two products, but they don't have the data or won't tell. Either would be better than fiberglass, but you are paying more too.

    To soundproof interior non-loadbearing walls between bedrooms, I used 3.5" fiberglass batts, cut 2x4 studs up the middle (except the bottom and top 1.5 feet, where the stress is highest on the stud) rather than stagger (which uses more studs), and used double-layers of drywall (to add thermal mass in a passive solar home, but it also reduces noise transmission). I caulked the bottom plate in the gap between the drywall and the subfloor. (Otherwise, noise can travel through the bottom plate directly from one room to the other.) After carpet was installed, the walls were very soundproofed. We could not hear our son crying in the adjoining bedroom, until we opened the (solid core pine) door into the hallway. Yelling or even pounding the wall was barely heard in the other room. I was impressed. This was cheap to do, and apartment buildings, hotels and motels should do something like this to reduce noise problems among units.

    There are new sound-deadening drywall products with a noise deadening material sandwiched in an inner layer of the drywall, which have great STC values.

  2. Westhawk39 | | #2

    Thanks Robert, very helpful advise. I like your method of cutting up the center of the studs a lot, and great to hear your having success with this method. Did you use acoustical sealant for the edges you air sealed?

    I liked what I read on green glue’s website and have decided to use their product between my layers of double drywall. It’s a little pricey however I seem to be able to justify using 23 tubes for the bedroom and bathroom separations. My new build consists of a equivalent 1080 sq ft footprint of the house as shop below which I’d also like to have good sound separation between for when I’m using a table saw below and somebody is sleeping above. I don’t know if I can justify $1000 (Canadian) of green glue here... thinking sound channel and double 5/8 as a minimum.

  3. Robert Opaluch | | #3


    I built those walls in 1983 when lots of high performance building products weren't available, some not even imagined yet. For the second layer of drywall, I used glue plus fewer screws or nails, but I don't know remember what glue. (Glue doesn't need to cover the entire surface of the drywall, just spread around and use some fasteners to hold drywall in place as glue dries.) I used caulk for covering the bottom plate. Base trim and carpet were added, which also reduced sound transmission at the bottom of the wall. Acoustical sealant can smell, so be sure whatever you use will not leave an odor in the bedrooms.

    You could look up STC (sound transmission coefficient) values for each of the layers of your wall (or ceiling/floor). Add them up to get the STC value of the wall. Charts online will tell you how STC translates into suppression of typical noise sources. Lower frequencies are hardest to suppress, and usually require heavier materials (like drywall or concrete). Typical wall assemblies are shown online in various research reports to compare effectiveness of different options.

    Floors and ceilings are harder to suppress noise due to their inherent construction (e.g., can't stagger joists like studs). Some thicker carpet pads or wood flooring underlayment are better at sound suppression than others (e.g., cork). Floating wood floors would suppress sound much better than older style hardwood nailed to the subfloor. Resilient sound channel is a very effective product, but I haven't used it, but good idea for the shop ceiling, but pricey. A ceiling separate from the floor joists, or even just strapping between joists and drywall ceiling would help. Roxul SafeNSound might be worth the expense if the shop will operate while someone tries to sleep or doesn't appreciate the sweet sound of a tablesaw!

    QuietPutty or similar products can wrap electrical boxes to reduce sound transmission through any ceiling light fixture electrical boxes. Avoiding any holes in the ceiling is a better idea if you can surface-mount lights (e.g., fluorescent fixtures).

    Aside from suppressing sound, you could use a sound-generating machine in a bedroom to mask other sounds. Some will increase their volume when outside noise is detected to mask noise dynamically, and are reasonably priced.

    Shops, saws and hammers are big noise generators! I have hearing loss from working in mills, construction and loud music. All of you readers that work in construction should use hearing protection when exposed to loud sounds, or you'll regret it later in life. With hearing loss from noise exposure, you will hear a high frequency hissing sound continuously instead of hearing higher frequencies, as your hearing deteriorates with age. And you'll probably live longer than you figured!

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