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Looking for ceiling solutions with 2 feet of cellulose

Brad Abernethy | Posted in Energy Efficiency and Durability on

I want to put a solid 2′ of cellulose in my attic of a new construction house and am thinking of running one layer of 1/2 drywall followed by 1 by 2 s run 1′ o.c., then another layer of 1/2″ drywall to carry the load.

I know some people use 1/2″ OSB or plywood covered in drywall or something else, but I ran the numbers and my detail is cheaper and it gives me a space to run wires without compromising my ceiling envelope.

Does anyone foresee issues with my detail? I don’t know where to look for loads of the cellulose and if maybe 5/8″ drywall with proper penetration sealant or some other cheaper solution can be used.

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  1. GBA Editor
    Martin Holladay | | #1

    I think you are worrying unnecessarily. According to Bill Hulstrunk, a cellulose insulation expert, "We have never seen a sagging issue due to the weight of the cellulose installed above a ceiling. That may be because some of the weight of the cellulose is being redistributed onto the ceiling joists. We have blown very high R-values, up to R-100, and never had any issues with the ceiling sagging."

    For more information on this issue, see Is 5/8 drywall sufficient to support R-60 cellulose in a ceiling?

  2. Chris Brown | | #2

    I would agree with Martin. I currently work for a manufacturer of glass wool (fiber glass) insulation and have over 45 years experience in the insulation industry, including manufacturing and contracting. In all those years of contracting and through countless field studies and testing protocols, I can safely say that I have never seen a 5/8" drywall ceiling sag from the weight of any insulation, including blown rockwool, provided of course that the ceiling joists are framed on no greater than 24" O.C.

  3. Expert Member
    Dana Dorsett | | #3

    Open blown cellulose has a settled density of about 1.4lbs per cubic foot, which can vary a bit with seasonal humidity.

    The sixth commandment of the gypsum construction bible states:

    Thou shalt not exceed 1.3lbs per square foot dead loading on half-inch gypsum with 24" o.c. timber spacing, nor 2.4lbs per square foot dead loading at 16"o.c. spacing. With 5/8" gypsum and 24" o.c. spacing thou shalt not exceed 2.2lbs per square foot.

    See the limitations section, starting on page 3 (p19 in PDF pagination), and in particular section 6 on page 4 (p.20 in PDF pagination.)

    So, if supported by 24" o.c. trusses with 2' of cellulose you're at 2.8lbs per square foot dead-load WAY over the dead weight limits for half-inch gypsum and are also over the limits of 5/8".

    A box store pricing 7/16" OSB is substantially cheaper than half-inch gypsum in my neighborhood, and far more rugged in almost every respect. Using OSB as the mechanical support layer the spacing doesn't much matter- it has PLENTY of capacity to spare, and you can then fur-out for the gypsum layer 24" o.c. without reservation. That's half the furring you were considering. Ring shank nails (quickest, if you have nail gun) or bugle-head screws (most mechanically secure- even with wider spacing) recommended for hanging the OSB.

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