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Without going into details on the, well, lets call them 'domestic political restraints' inhibiting me from tearing down the existing ceiling or blowing in insulation, I'd be curious to get some feedback on a 'novel' approach to insulate an existing ceiling.

Granted, this approach would require a fair amount of drywall patchwork, but this is the best I can come with at the moment...

I'd start roughly in the middle of the room and cut a 1 foot wide hole in the ceiling perpendicular to the joists. Effectively just take a drywall knife and cut across the entire ceiling.

Within the hole, begin stuffing insulation batts (perhaps mineral wool) down each side of the joints until I can't press any more in. Granted this wouldn't be air-tight but would drive some r-value and create soundproofing.

I was also thinking of using an extendable stick (e.g. pointer stick or hiking stick) that I can insert into ceiling small and then lengthen within the joist to give some extra pushing power if needed.

Is this just ridiculous?

(Per a question from a commenter, note that primary goal is sound isolation over adding r-value. Would be curious if someone would recommend just adding a 1/2 drywall w/ Green Glue sheet on top of existing rock for a reasonable STC gain)

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  • Ask a spray-foam contractor if they can insulate the ceiling with a minimally-destructive procedure like that. Might save you hours of hard work and not cost any more than the materials you're already considering.
    – Jeff Wheeler
    Commented Dec 15, 2020 at 1:22
  • Thanks @JeffWheeler. I've considered that but hate the flashover time on foam, the voc's, and its not really the best sound absorber in my limited research
    – I_hate_noise
    Commented Dec 15, 2020 at 1:31
  • If there are any wires or pipes you'll get hung up on them. I presume this is a ceiling with a floor above preventing access from above?
    – Ecnerwal
    Commented Dec 15, 2020 at 1:35
  • Thanks @Ecnerwal. Correct -- It is a multi-family in NYC from prolly 1905. We bought it gut renovated and the fix & flippers decided to save a few hundred dollars by not insulating the ceilings in 60% of the unit. I do no own the unit above. Right now we are at an STC rating of 33ish is my guess. I've drilled a few holes with a boroscope and for the most part seems like fairly empty cavities (same some lighting wires)
    – I_hate_noise
    Commented Dec 15, 2020 at 1:44
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    Hold up. If there's a unit above you, you shouldn't even need insulation in the ceiling, since that unit should be at a similar temperature. Are you doing this for sound isolation? That has completely different considerations.
    – user3757614
    Commented Dec 15, 2020 at 7:38

5 Answers 5

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Check out 4x8 sheets of "sound board" available in the lumber department of your big box stores. Using thermal insulation to muddle sound doesn't add up. My neighbor, a drywaller, put it up throughout his condo unit then drywalled over it. You could just screw it directly into your ceiling then add drywall on top of it. Electrical boxes would need box extensions but this is a lot easier and more to the point than trying to shove thermal insulation into narrow spaces filled with pipes, vents, conduit, romex, and maybe some speaker or other low voltage wires....and which only mitigates heat loss with a bit of sound insulation. Go for the real thing and don't let those cloggers get you down (sorry Geico, or whosever ad that is on tv)...

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Fiberglass batts will not push very well. They'll go in a few inches, then get hung up and start compressing. I think Corning used to make plastic wrapped ones that might have slid through your cavity better, but I'm not sure how much that would have helped or if they even make those any more.

Instead of trying to push fairly solid, coherent batts into the cavity, you should look into blown in insulation. You cut a smaller hole (though that's not necessarily easier to patch, there is less joint to have to tape & mud - search here for how to patch a small hole), stick the hose in and blow. It'll move around many/most obstructions like smaller pipes and wiring.

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  • Thank you @FreeMan -- hoping to avoid blown at the moment as doing so from a living space throws off a fair amount of boric acid (assuming cellulose). I wouldn't be all that concerned about that in normal scenarios but my wife is pregnant and its not ideal. I think in short though, you've answered the question in the sense that my 'shove down the joist' install method probably isn't practical
    – I_hate_noise
    Commented Dec 15, 2020 at 3:34
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How about cutting TWO holes? Use a long fishtape and shove in a couple strings from the first hole to the second hole then tie those to a couple sticks like paint stirrers that you can drill some holes in and fashion a sort of clamp. Clamp it on the insulation batting, start pulling the strings while feeding it in from the other hole and voila, you got your insulation batting in.

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  • I love the creativity here. I follow you couple sets of fishtape to paint stirrers, though having trouble with the vision beyond that. How/Where are you imagining the clamp? I think this beats my idea for using a powerful RC car with an attached plow to 'drive' the batt down the joist. If I wasn't good at mudding ceilings before, I'd def be great at it after if I go this route.
    – I_hate_noise
    Commented Dec 15, 2020 at 13:59
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I have seen this technique done for walls, using a sheet of cardboard either side of the batt to push it in. Also, some sound-rated polyester batts are firmer and so push easier.

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  • Thanks @Polypipe - I"m unfamiliar with polyestter batts -- will check it out
    – I_hate_noise
    Commented Dec 15, 2020 at 14:04
  • Common polyester is the same material that is used inside pillows.
    – Polypipe Wrangler
    Commented Dec 15, 2020 at 20:51
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If I undersand correctly you are going to take the drywall off a 1 foot strip across the entire ceiling?

And that this is a between-floor, not an attic space?

Try this:

Use cellulose blown in. For each side of the open space, run your hose into the end, then start it up. Add insulation slowly. Pull the hose out.

Issues:

  • If the joists are braced, getting the hose past them will be tricky.
  • If you have pot lights, you must replace them with ones that don't overheat when insulated. If you do this, just go to LED lights.
  • It's tricky to pull out the hose at just the right rate to fill the space. Pull too fast, and you have half filled space. Pull too slowly, and the hose fills up and clogs.
  • Blown insulation always makes a mess. This may create issues with She Who Must Be Obeyed. At the very least, put sheets over all the furniture, and seal off the room entrances with tarps. Open a window and run an exhaust fan so hat the room is over all under negative pressure.
  • You need a partner on the machine.

You won't be able to do the foot gap. Use fiberglass bats for that.

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  • Hey! That's what I said! You did add more details...
    – FreeMan
    Commented Dec 15, 2020 at 20:16
  • Thank you @Sherwood My original plan was to go this route and was all ready for it (even had the 36 bags / 1000 lbs of GreenFiber cellulose delivered and was ready to go), but at the moment isn't feasible. I might explore again in the spring if can't ID a different, albeit, more taxing solution.
    – I_hate_noise
    Commented Dec 15, 2020 at 22:01
  • @I_hate_noise, if I were you, I'd wait and go with the cellulose -- it's a much better sound absorber than fiberglass anyway, and it's way more likely to get good coverage in to any tricky spots that are hard to reach.
    – Nate S.
    Commented Jan 14, 2021 at 21:45

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