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Combining dense pack cellulose and mineral wool in double-stud wall

Tyler Keniston | Posted in Green Building Techniques on

Hi all.

I’m planning to build a double stud wall around 10″ thick. My initial plan was to use dense pack cellulose with an interior smart vapor retarder (intello +).

For a few reasons I am considering adding some mineral wool to the equation. I wanted to bounce the idea off others and see if it make sense.

1) This is for a wood shop and potential music studio (combined) so reducing noise escape is very important. I am aware that air leaks are vital for sound containment, but I have also heard great things about mineral wool. Would there be a discernible difference between 3.5″ of mineral wool coupled with dense pack vs just 10″ of dense pack?

2) I am planning on dense packing myself and am concerned about having 10″ deep cavities as far as difficulty of controlling density (not experienced with it). I also am not sure that there are many professional folks around me experienced with it either.

3) I would prefer not to have walls deeper than 10″ and the mineral wool would add some R-value for the given depth

4) If I do use mineral wool, is there an ideal location for it in the assembly. My initial idea was, from the exterior sheathing in: DP cellulose, insulweb on backside of interior studs, mineral wool, intello (or membrane could be used in this case), 2×2 service cavity battens, drywall. Then I thought perhaps it’d be best against the sheathing in between the exterior studs, followed by dp cellulose. I am tempted by an integrated service cavity approach, but worry that with only a 10″ wall, the vapor retarder needs to be all the way to the inside to follow the 1/3 to 2/3 insulation rule.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    In order

    >Would there be a discernible difference between 3.5″ of mineral wool coupled with dense pack vs just 10″ of dense pack?

    Not really.

    >I am planning on dense packing myself and am concerned about having 10″ deep cavities as far as difficulty of controlling density

    That's a legitimate concern. Have you ever DIY dense packed anything?

    >I would prefer not to have walls deeper than 10″ and the mineral wool would add some R-value for the given depth

    At 10" the performance difference of replacing a few or even several inches of R3.7/inch stuff with R4.3/inch stuff is VERY small. At 10" of dense packed cellulose you'd also have to model the thermal mass aspects of the cellulose to see if swapping out some of it with higher-R/lower-thermal mass rock wool increases rather than decreases energy use. Cellulose has more thermal mass than brick of equal thickness, and a correspondingly longer thermal lag. Modeling this stuff accurately gets complicated, but it's real.

    >If I do use mineral wool, is there an ideal location for it in the assembly.

    Put the rock wool on the interior side of the assembly, where the risk of a fire getting started is higher. It's completely fireproof. Put the cellulose on the exterior side, in contact with the structural sheathing, where it will wick & share the seasonal moisture burden of the sheathing- it's protective.

  2. Expert Member
    Zephyr7 | | #2

    For #1, I doubt there would be much, if any, benefit to adding mineral wool for sound attenuation. The dense pack cellulose should do a pretty good job too, especially in a double stud wall. For sound attenuation, you want to stop air leaks, but there is something new to think about: decoupling. You want heavy, squishy things in the way of the sound. Hard materials transfer sound better. Use 5/8” type X (not ultralight — you want the mass) drywall. It’s heavier, stiffer, and damps sound better than 1/2”. Think about using resilient channel to isolate the drywall from the studs. While expensive, using green glue in a sandwich of two drywall panels makes a big difference in sound proofing.

    Best for soundproofing would be resilient channel to hang 5/8” double layer drywall with green glue between the layers. Build the walls and ceiling like this (if the ceiling is a roof, you could probably skip that part, I’m thinking you want to isolate your studio from the rest of the occupied spaces in the house). Use a heavy, gasketed door with a double rubber sweep. Put some layers of sound board under your flooring (unless you’re on a slab, then there is no point). You may want to consider double stud walls for the interior walls of your studio too, just for their soundproofing ability. Fill them with dense pack cellulose too. Definitely use at least the resilient channel throughout the room.

    There are lots of forums out there discussing building home theaters and soundproofing them. I’ve seen some detailed threads on AVSforum. You might want to check those out.

    Waaay back in college I worked in a recording studio. They used double concrete walls with some kind of dampening material between. Interior Walls were maybe 18” thick with massive doors. The key was to decouple the inside wall layers from the outside. A double stud wall with offset studs is similar.

    Don’t forget to seal any electrical boxes with putty sheets. They’re a classic point of sound leakage. Make sure they are in their own stud cavities with nothing on the opposing side of the wall.

    Bill

  3. Matt F | | #3

    The mineral wool probably won’t be any different for sound than the cellulose. The sound damping properties of fluffy insulation are generally exaggerated. For sound you just need insulation to fill the air space. Adding mass makes the biggest difference. Thick walls and double studs are good. This wall should be pretty quiet as is. Build it, see if it works for you with the plan on of putting up a additional layers of drywall if needed. Use heavy 5/8 for your first layer at least. Greenglue between layers has some benefit for sound.

    Any penetrations are going to be be what you need to focus on for sound. Windows, doors, outlets.

  4. Tyler Keniston | | #4

    Thanks all for the thoughts! It's nice to hear that the mineral wool wouldn't add much seeing as its more expensive. It seems like it just comes down to my confidence in tackling a dense pack project of 10" deep. I think I might get some insulweb to span between the studs, creating individual bays i can keep better track of.

    "Cellulose has more thermal mass than brick of equal thickness, and a correspondingly longer thermal lag." wow that's surprising. I am not too familiar with the thermal mass issues in regard to modeling energy use, but I'll take your word that it gets complicated. Things are always more complicated when the magnifying glass comes out.

    1. Jon R | | #6

      > Cellulose has more thermal mass than brick of equal thickness

      Brick has much more (thermal mass/volume = specific heat x density):

      cellulose: 131 KJ/m3.K
      brick: 684 KJ/m3.K

      1. Expert Member
        Dana Dorsett | | #7

        OK, that's correct. It's really thermal diffusivity, a function of both R and mass, not strictly thermal mass. The R-value of brick is squat relative to cellulose, and with walls of equal thickness will have a much longer thermal time lag. The cellulose wall has comparable to (even denser and higher thermal mass, but lower R) insulating fiberboard sheathing.

        According some marketing fluff from Gutex (a manufacturer of fiberboard insulation) a 180mm (7") thick fiberboard wall has a thermal lag of about 12 hours, compared to a bit more than 7 hours for a comparable layer of cellulose, and about 4 hours for a comparable layer of brick, and about 2 hours for a comparable layer of rock wool.

        At 10" of cellulose would have a time lag a bit more than 10+ hours, making it pretty ideal for averaging diurnal load all year, and nearly zeroing the wall losses during the shoulder seasons (and zeroing summertime gains through walls in cooler climates.)

        When you start modeling the mass effects and thermal bridging of the rest of the assembly it does get complicated fast, but I suspect trading 3.5" or 5.5" of dense pack cellulose for rock wool in this assembly would be at best a wash on annual energy use, and possibly higher energy use despite modestly higher R.

  5. Tyler Keniston | | #5

    Not sure about cost here... but i'm still scratching my head and weighing options on the interior finish material... and the thought of using some 1/2" ply under the drywall popped up. The reason being for being able to attach objects solidly anywhere.

    Could that function similarly and without other problems (moisture etc) as double drywall? I'm imagining the major drawback is buying all that extra plywood...

    1. Expert Member
      Zephyr7 | | #8

      I don’t think you’d gain much with the plywood. 5/8” drywall using the proper anchor is actually pretty strong. A double layer of 5/8” is going to be tough. I have a 3 hour rated wall assembly at work (3 layers of 5/8” on each side, for a generator room) and it might as well be a concrete wall.

      I’ve had good luck with EZ anchors for light loads, mollybolts for light/medium loads, toggle bolts for medium/heavy loads, and “toggler” anchors (they look like a little bar with a long plastic handle, you poke them through the hole, pull them flat against the back of the drywall with the plastic handle, thread in a machine bolt and then cut off the plastic handle) for heavy loads.

      Bill

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