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Community and Q&A

How to Cap Ceiling Grilles to Prevent Air Leakage

alexwilson83 | Posted in General Questions on

I recently changed my hvac system from gas/ac forced air to a 3 zone heat pump and in so doing have a number of grilles that are now capped at the trunk lines (and the abandoned flex is tied shut, near the trunks).  I’m not sure how to go about capping the grilles so that I don’t have what I’d expect would be a decent leakage path to the attic.

My thought was to friction fit and caulk some pieces of rigid foam in the lower part of the boot (possibly with some loose fiberglass stuffed above it for good measure) followed by sticking a piece of foam on the back of the damper.  Getting to one of the boots from above would be fairly difficult.  For the bathroom grille I’d like to keep it reversible so that if an inspector gives me grief about the bathroom not having a supply I can reconnect it easily enough.

Does this seem like a reasonable approach?  House is in climate zone 4C (Portland).

Thank you for the help.

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  1. user-5946022 | | #1

    Can you access the attic?
    If so, the best solution seems to be:
    1. From the attic, remove the flex from the boot at the ceiling grill. If you can't get there, try to cut it out from below reaching through the boot.
    2. From the living space, remove the ceiling grill and see if you can also remove the boot. If not, either go to the attic and air seal around the perimeter of the now abandoned boot, or at least from the living space seal around the outside of the boot between the boot and the existing gyp.
    3. From the living space, install a piece of wood blocking on each side of the inside of the boot or boot opening, so you have a nailer, then install a piece of gyp board (drywall is one brand) to fit the opening. You could also try to cut the exiting gyp back a bit to expose the nailer to which the boot is attached, but you risk cutting past the edge of the diffuser which you don't want to do if you are reinstalling the diffuser. If you are good at finishing, you can try to tape and finish around this patch to make the entire diffuser go away, but ceilings are very difficult to finish. Alternatively, if you don't mind keeping the diffuser in the ceiling, install a piece of gyp to the above noted nailers into the opening, then caulk and air seal around the perimeter of the patch, paint the patch material black so it is less visible through the diffuser, and reinstall the diffuser to cover the caulk seam.
    4. Go back to the attic and air seal from above through the duct boot, then add insulation to fill the void left by the removed duct. If you really can't get there, your idea to stuff insulation in from below before you put in the patch at the duct boot is good, but put in as much as possible.
    5. Go back to the attic and pull the now unattached duct back toward your trunk line and out of your attic. Check the cap at the trunk line to be sure it is not leaking - if leaking, seal until it is not leaking.

    The reason to use gyp as the patch material is that it is fireproof. Since you don't need the hole anymore, you might as well seal it the best way possible.

    1. alexwilson83 | | #3

      Thank you, having a layer of fire-rated gyp makes sense.

      I can get above the ceiling and try to remove the boot as that does seem like the better way to do this. I doubt I'd be able to remove it from below as it is / should be spray foamed to the gyp (at least that was part of the scope when the attic floor was sealed / insulated, so I'm assuming it's there). It would give the opportunity to remove the old flex instead of abandoning it in place.

  2. Expert Member
    NICK KEENAN | | #2

    I would think leaving an un-fire-stopped hole in the ceiling is a code violation.

    The right way to do it is to take out the boot, insulate the ceiling to the standard of the rest of the house, patch and paint the drywall.

  3. Expert Member
    BILL WICHERS | | #4

    If you are OK with leaving the vents in place, get some 1/8" hardboard and cut it to fit into the inside of the grille. Use some weather stripping to seal the perimeter like a gasket. Reinstall the grille with the hardboard inside sealed against the drywall. Now you have a simple, removable air baffle inside the grilled and no visual change to the indoor space.

    If you want to remove the vent and grille permanently, you really need to patch the hole as you would any large hole in drywall (assuming these are all in drywall). If you're using drywall, you don't necessarily need special "fire rated" (which typical means 5/8" type X), you just need to match what the rest of the wall or ceiling is made of so that you end up at the same fire rating as the rest of the assembly.


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