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Cellulose over 1/2″ drywall and 24″ o.c. furring strips

Le_Marquis | Posted in Energy Efficiency and Durability on


I want to put between R30-R40 cellulose insulation over my poorly performing loose fill fiberglass. I am concerned about the weight of cellulose and sagging, and to be on the safe side I am considering two ways to mitigate that: one, I put more furring strips (the drywall is attached to strips, not joists) and attach them to the joists using angle brackets. My reasoning is that they should help absorb some of the weight of cellulose. The other one would be to put Great Stuff Pro Wall & Floor adhesive between the drywall and joists, hoping that it will help the drywall support the weight. I know that materials are usually pressed to make a solid bond, but I am wondering if just squeezing adhesive under the joists will make a strong enough bond for my purpose.

Is the Great Stuff idea feasible?

Thank you so much for your help!

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  1. GBA Editor
    Martin Holladay | | #1

    Squeezing Great Stuff into a 3/4-inch gap between your drywall and your joists isn't going to help.

    If you have an existing ceiling, then adding more furring strips would require you to remove the drywall, wouldn't it?

    My guess is that you don't need to do anything. Adding insulation almost never leads to problems with sagging ceiling drywall, as Bill Hulstrunk noted in this article: How to Install Cellulose Insulation.

  2. Le_Marquis | | #2

    Hi Martin - Thanks for your post. Actually I meant strips of wood 1X4 that would run perpendicular to the joists, so no need to take drywall down.

    Great Stuff adhesive would not work because the gap is too big or because the two surfaces are not compressed? What if I use PL Premium instead?

    I had read this article from Bill, but some people have reported sagging with 1/2 drywall with 24". Just want to be on the safe side...

  3. charlie_sullivan | | #3

    Other options might be:
    a) Add more loose-fill fiberglass, and only do cellulose for the top 3-4 inches, or
    b) Add a lot more loose-fill fiberglass, perhaps nominally R60, and you'd still get better than R50 actual performance.

    b) requires the space available, and a) requires two visits by an insulator, but it might still be less labor than installing the reinforcements with angle brackets which sounds like a lot of work to me.

    The Great Stuff idea might add just enough extra support to make the difference if the situation is marginal, but it would be hard to count on it for any significant help.

  4. Expert Member
    Dana Dorsett | | #4

    An additional R40 of cellulose (about 10.5-11") would only adding about 1.25lbs per square foot to whatever is already there.

    How much is already there?

    Old school blown fiberglass is well under 0.5lbs per square foot at 10" (0.75" of furring + 9.25" of 2x10 joist= 10" ). Even better class new-school higher density stuff would wouldn't be heavier than that, but could be close. (Owens Corning AtticCat is only 0.44lbs per square foot @ 10.25"./R30.) Measure the depth of the fiberglass- you're probably looking at about 0.03lbs per square foot per inch of depth, and not more than 0.04lbs per square foot per inch. (eg: If you have 7" of fluff, it'll be between 7 x .03= 0.21lbs per square foot and 7 x .04= 0.28lbs per square foot.)

    Half-inch gypsum is good for loads up to 2lbs per square foot with 24" o.c. joist spacing, odds are you're going to be way under that.

  5. Le_Marquis | | #5

    Charlie - thanks for your answer. I think I will stick to adding cellulose. You are probably right about Great Stuff not giving much support, so I won't go there.

    Dana - I find multiple sources of information regarding lbs per foot for 1/2 drywall. How much can 1/2 drywall with 24" SAFELY support? I really don't want to risk sagging...

    I have maybe 7-8 inches of loose fill fiberglass. Do you think adding strips of wood to provide greater support makes sense or are you really convinced that R40 blown in + 7-8 inches of loose fill cellulose is safe for my configuration?

    Thanks a lot for your help. I really appreciate!!

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