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Drywall under dense-packed cellulose: reinforce with OSB?

charlie_sullivan | Posted in Green Building Techniques on

We are planning on dense packing the joist bays under bedrooms that are over a garage. With the netting bowing down, the drywall would likely crack when screwed up (meant in both senses). Conventional solutions I know of would be to add strapping crossways to the joists to help hold back the netting and to allow space for it to bow down, or to staple the netting to the sides of the joists an inch or so in to provide similar space. A new idea that was suggested is to put a layer of OSB up first, before the drywall, with the idea that it would be strong enough to push the cellulose flat, followed by the drywall. I’m wondering whether anyone has done that, has other solutions, or has thoughts about the advantages and disadvantages of those approaches.

It seems like the OSB has several advantages: It would eliminate horizontal air channels above the drywall, which in theory are not a problem, but might become part of an air leakage path given that this is retrofit work and there will surely be places where things that shouldn’t connect do connect. It could serve as a redundant and robust air barrier, if we tape the seams, helping energy efficiency and keeping bad garage air out of the bedrooms above. It would compress the insulation a little more, enhancing the density to help avoid any future settling. And it could also provide an easy way to attach things to the ceiling in the future, though with some risk of violating the integrity of the air barrier.

Details: Zone 6. The garage has an uninsulated slab, but decently insulated walls. It is not heated. It’s rarely used for cars–it’s mostly a bicycle garage and workshop–but we are keeping the option of bringing a car in partly thinking that it might be a good place to charge an electric car sometime in the future.

More details:two walls are 50% below grade, where they have 2″ EPS, soon to be infested with carpenter ants, I presume. Above grade, the walls are 2×4 with fiberglass plus 2″ EPS on the exterior. The biggest heat and air leak is the garage door, which we will likely replace soon, but it will still be the major weak point in the envelope.

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  1. GBA Editor
    Martin Holladay | | #1

    There's no reason you can't install OSB is you want, but it's an expensive solution compared to the usual solution -- strapping the ceiling with 1x4s, 16 inches on center.

    Another typical approach is to install a continuous layer of polyisocyanurate foam on the underside of the ceiling joists, followed by 1x4 strapping, 16 inches on center. Then you can make a few holes in the polyiso so that you can blow cellulose into the joist bays.

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