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Blown-in insulation on open vaulted ceiling

Marcus DuFrane | Posted in Green Building Techniques on

I am had built with the plan to blow in insulation, and now that Im about to do it my mind is struggling with some physics.

I ceiling joist to the roof rafters with the help of an engineer to vault my upstairs bedrooms. This leaves me with a void of half the difference shown in the diagram attached. My question is will 5/8 drywall secured at 16″ on center handle the load of blown in cellulose? If the insulation blows in as shown by the shaded area there shouldn’t be any more than 18-20″ over an given area. My biggest concern of course is how this would settle over time.

Any comments on:
A) a better solution given that the drywall is installed and finished
B) reasons why this is or isn’t OK?
C) ways to aid in making a blown in installation most effective.

Thanks in advance.
-MD

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Marcus,
    You have scissors trusses. There are two issues here:

    Q. Will the drywall bear the weight of the cellulose?

    A. Yes. As cellulose expert Bill Hulstrunk says, "We have never seen a sagging issue due to the weight of the cellulose installed above a ceiling." For more information, see How to Install Cellulose Insulation.

    Q. Will cellulose insulation installed above a scissors truss slide downhill?

    A. Probably not, as long as the slope isn't too steep, and as long as the insulation is installed carefully. I've gone back and forth on this issue for the last few years; the answer to this question depends on part on the steepness of the ceiling slope. Most builders who insulate scissors trusses with cellulose say that the technique works, as long as you plan a method for establishing a ventilation channel.

    The issue has been discussed a lot on these Q&A pages. See, for example:

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/32621/how-best-insulate-new-truss-roof

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/36373/insulating-scissor-truss

    https://www.greenbuildingadvisor.com/community/forum/general-questions/22469/vaulted-ceiling-insulation

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