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I live in a 1970 house and I am looking to replace the attic insulation. The current insulation is old fiberglass batts. I want to remove everything, foam seal it, and then blow in cellulose to get it to R-49 or R-60. My joists are 24" o.c. and I am guessing the ceiling drywall is 1/2" thick. My concern is sagging or even collapsing the drywall due to the weight.

As I understand it, homes from this time period used nails and glue instead of screws for fastening drywall ceilings. If there is too much weight, or the drywall has started warping due to moisture, or other reasons, the extra weight of the insulation can bow or collapse the ceilings.

Does anyone have any recommendations or experience? It seems like there is no real way to know if there will be a problem or not unless there is already visual separation. When I remove the current insulation I will inspect the drywall to see if any is separating, and if so I will drill it in, but otherwise is it basically up to chance? Should I go ahead and screw in all the drywall first just to be safe? Or maybe put in chicken wire or netting attached to the joists to support the weight of the insulation?

I want new insulation but definitely do not want to collapse my ceilings! Thank you

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    Well, start by rectifying your guess. 5/8" is far more common on any ceiling of the era, but especially at 24" centers. You'll need to verify before we can continue. Also, glue is rarely used for drywall, even on ceilings. Ring-shank nails did fine, so try to verify what you have in that regard in case it's something odd.
    – isherwood
    Commented Jul 27, 2023 at 19:10

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There are two concerns: sag and detachment. Both are covered if:

  • The drywall is actually 5/8" thick and not the 1/2" you suspect.
  • It's fastened well. This would've been with four or five pairs of 1-5/8" ring-shank drywall nails per joist in the field and more at the butts.

If the former isn't true I'd expect sag under 18" of cellulose, though maybe not immediately. Over time you'd start to see the joists when light is at a long angle to the ceiling. Actually I'd expect to see that already due to its own weight.

If the latter isn't true you know what can happen. That said, I've never actually seen that happen in many years of remodel work. It's probably not a real concern.

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