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Ceiling Sheetrock as Air Barrier — Sealing around Cassette-style Mini-Split HVAC

idahobuild | Posted in Energy Efficiency and Durability on


I am considering using the sheet rock on the ceiling as an air barrier for our new house (tied in to exterior walls).  I know they’ll be a lot of detailed sealing to do around fixtures; and I’m willing to methodically take on that challenge, however, I am wondering how to seal around the cassette-style, mini-split, HVAC system while maintaining accessibility to that equipment for maintenance down the road.

Any thoughts on this would be greatly appreciated.

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  1. mr_reference_Hugh | | #1


    It will really depend on the design of the ceiling cassette enclosure.

    Is the enclosure attached to any ducts or is it self-contained?

    You could take inspiration from the widely discussed topic of sealing recessed lighting.

    This article explains how to build a box and install it over the recessed light fixture from the attic.
    "Up in the attic, clean the ceiling surface around each fixture, slit the cover to accommodate the electrical cable, and place the cover over the fixture. Apply the sealant around the cable and along the edge of the cover where it rests on the ceiling. Now it will be safe to heap insulation onto that spot."

    You can see some example of boxes and smaller manufactured covers on this Google search results page (images)

    You should read about the specific unit you are installing to see if it needs a certain clearance around it. You could also call the manufacturer if necessary. Normally the installation manual will have quite a bit of details.

    If you can create a box like those used for recessed lighting, I would suggest using fire-rated OSB which will also be an air-barrier. I always like to go overkill on things and even if the fire rated OSB is more expensive, you don't need a great deal for this type of project. Once the box is in place and fully sealed, then you can ensure that it is well insulated.

    You could insulate with a rigid foam but I am not sure whether you would need to cover that with an "ignition barrier" (to protect the foam insulation from igniting and causing a fire). You could build a frame (an open type of box) that is larger than your air tight OSB box. You could place that frame over top of the OSB box. You could use the frame to hold some batt insulate securely in place around the OSB box.

    You could also place a secondary box (closed on all sides except the top and bottom). You could place it around the smaller OSB box. Then you could fill that larger box with loose fill insulation. I would leave the top off the larger box so it is able to "breath" and interact with the air in the rest of the attic.

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