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Blown mineral wool retrofit interior walls?

CMOD | Posted in Green Products and Materials on

I have a client who is interested in getting loose fill mineral wool blown into the attic and interior wall and ceiling cavities (‘drill and fill’) of her 1986 builder-grade house, primarily for insulation in the attic and sound attenuation (noise between rooms and from plumbing). Absolutely does not want to pull off crown moulding/base/trim to add another layer of gyp. board or isolation clips.

I have suggested that this is probably a large cost for a fairly minimal STC improvement, and I’m unfamiliar with any modern blown mineral wool products. I can find one or two on google (roxul’s is discontinued, Rockwool, insul-fill). I’m also a bit concerned about weight on the gyp. when blowing it into a ceiling cavity.

I’m about to start calling representatives for their take on this, but I’m curious – has anyone done drill-and-fill with anything other than cellulose or FG? how did it go?

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  1. Expert Member
    Dana Dorsett | | #1

    A guy in my office had rock wool blown into his 1840s vintage antique 30+ years ago. It went reasonably smoothly, and has held up quite well. The only issue he had during the installation had to do with the framing anomalies, where the wall cavities were open to the floor joist cavities between floors, and a LOT of rock wool got blown between floor/ceiling joists before the contractor figured it out. The ceilings didn't sag from the weight, but the contractor spent a day figuring out how to block the joist bays without ruining the antique plaster ceilings. A 1980s house won't have those issues.

    The technique doesn't vary too much from cellulose, and it's somewhat less dusty due to the fact that it doesn't come with or need fire retardents.

    The density of open blown attic rock wool isn't dramatically heavier than fiberglass, and quite a bit lighter than cellulose, typically still well under 1lb per square foot @ R49. Open blown cellulose would roughly 2x that of open blown rock wool at any given R-value. Take a peek at this document:

    If sound abatement is the goal, do NOT dense pack it into the wall cavities, since that will put it under dynamic tension, with more mechanical coupling from one side of the assembly to the other. New-school 1lb density fiberglass in the partition walls is probably quicker, but there is a bit more risk of airborne fiber fragments floating about the house until the walls are sealed.

  2. KeithH | | #2

    Hi, DIYer here.

    You mention plumbing noise. I'd think if you have exposed piping in the attic or crawl space it would be faster, cheaper, and more effective to insulate the piping with one of the thicker rubber peel and stick pipe noodles. Awhile back, I replaced the failing 1" piece of poly tube (I kid you not) that was my plumbing main with 1" copper and covered it with rubber pipe insulation. Well pump related noise dropped from reverberating throughout the house to a very modest hum. Bonus points for less hot water temperature loss or less cold water line condensation. For maximum benefit, pull one at a time each 1/2 hole pipe clamp so you can run the pipe insulation over the whole run and upsize the clip accordingly.

    Another idea: does her water heater have an expansion tank? A larger enough one? A new enough one? How about the pressure reducing valve? My previous house had problems with both that caused a lot of noise, especially when irrigation would run. Replacing the out of service life PRV really cut down on the noise. They only have a ~10 year life. Same for the expansion tank. Those are cheap and easy items that might help a lot.

    What's the insulation up in the attic look like in general? I can't believe that a modern depth of cellulose wouldn't address enough of ceiling transmission under the attic. Blow more cellulose?

    What's the heating system? Forced air? How about jump ducts? Are there any? I'd blame those first. How about back to back supplies? More culprits. Direct cut-in to the supply trunk line? That would spread noise as well. Are you certain there are no stud bay returns? I'd certainly be sure to identify those before blowing any stud bays? My old house had these. They also don't help with noise transmission but wall stacking after the fact is pretty destructive.

    How about back to back outlets or switch boxes between hallways and bedrooms? Trying to address those might be inexpensive.

    Can you tell if the interior door framing has a large void? Popping the door trim and using canned spray foam can really help reduce the noise coming around the door through the trim.

    I think Roxul does an amazing job of reducing high frequency noise like voices or road noise (and not a ton for low frequency like bass etc) but it doesn't sound like you have any real opportunity to use Safe N Sound or Comfortbatt.

    Instead of noise dampening the whole house, what about the central bedroom hallway (if it has such a thing)? Pop the trim and door trims in the hallway, comfortbatt all the bays, dampen around penetrations or piping. You wouldn't be addressing room-room but maybe that would provide common area-room noise.

    Last idea in this riff. In my last home, I converted carpet and linoleum on the first floor to hardwood flooring and tile (with cement board). I was surprised to find that all that mass and strength significantly dampened noise transmission. I was especially surprised that getting rid of the carpet improved the sound situation. Sure some noises bounce around now but the general creakiness and paper thin noise transmission of that builder grade 1960s house went away. I didn't even use a cork underlayment, green glue, or ditra. Mass layers really do help with certain sounds.

    I don't really address your question but I hope those ideas help figure out what will really help your client.

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