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Community and Q&A

Homeowner seeks joist insulation options for garage ceiling

kevek101 | Posted in Energy Efficiency and Durability on

Hi all. I know similar questions have been asked but I haven’t seen anything that quite hits my three concerns. Any experienced advice would be greatly appreciated.

We recently bought a small cape with a large attached two car garage in Vermont. I’m converting the garage into a semi-conditioned wood shop and we are also considering putting in a bed/bath in the attic over the garage (there is a lot of space up there). First thing will be to insulate and finish the ceiling of the garage (which will also be below the floor of the future bed bath). The joists, which are currently open to the garage side, were old solid 2×6’s 24 OC, but we’ve in preparation of these remodels we’ve added 2×6 LVL in between each to beef up the floor (per engineer specs). So now there is a joist 12 OC with tongue and groove floors on the attic side. There have been mice issues in the past from what I can tell. Though I know that they are hard to avoid in a garage in Vermont I at least would like to choose an insulation method that may not be easy for them to find a home. I’m also concerned about condensation forming between the two spaces. The garage shop should be pretty dry and cool, but the space above in the attic could be humid and warm if we decide to remodel for a bed and bath. There’s also the dust issue from the woodshop getting drawn into the space above. With those two considerations, what do you think are my best options for insulating between floor joists? One other caveat… I need to be able to do this myself so blown in anything is probably not going to be a viable option.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "What do you think are my best options for insulating between floor joists?"

    A. "Best" isn't always the same as "cheapest" or "easiest."

    As a first step, read this article: How to Insulate a Cold Floor.

    The article applies to bonus room floors (assuming that the room above is usually conditioned, and the room below usually isn't). In your case, the room below (the garage) may be heated before the bonus room is complete, but the advice still holds.

    If you are worried about mice, install 1/2-inch plywood above the existing tongue-and-groove subfloor boards before insulating.

    Considering the 12-inch-on center framing, with two kinds of joists, the best approach is either open-cell spray foam or blown-in cellulose. You can rent a cellulose blower -- with patience, you can achieve almost dense-pack density with a blower from Home Depot.

    After you insulate between the joists, consider installing a continuous layer of rigid foam on the underside of the joists, followed by 5/8-inch drywall.

  2. kevek101 | | #2

    Thanks Martin. I've read that article in the past and just re-read it now. I'll look into dense pack. Though I've read about all the pitfalls of cut and cobble on GBA I did wonder if that might be an option since this would be in an above grade floor and not on a cathedral ceiling.

  3. GBA Editor
    Martin Holladay | | #3

    If you buy your cellulose from Home Depot, they will usually lend you the cellulose blower at no charge.

    Personally, I would find it much easier and faster to blow cellulose into these joist bays than to attempt a cut-and-cobble job in joist bays that are 12 inches on center.

  4. kevek101 | | #4

    Good point. Very good point. Wasn't looking forward to cutting and notching all that rigid. Plus I've heard cellulose is typically impregnated with borax which supposedly helps prevent mice friends. I've got to admit I'm questioning my ability to get a really dense fill especially at the ceiling, then again I've never tried blowing in cellulose so it's more like ignorance driving my decisions. One detail-y thing though...I'm assuming I'd seal with rigid or use wood blocking at ends of joists which rest on top plate of garage walls. In the attic there will eventually be an unconditioned knee wall space 5 feet back from top hopefully, it makes this area of detailing a little easier?

  5. kevek101 | | #5

    see attached

  6. GBA Editor
    Martin Holladay | | #6

    In most cases, it's best to bring the triangular space behind your kneewalls into the conditioned envelope. For more information, see this article: “Two Ways to Insulate Attic Kneewalls.”

  7. kevek101 | | #7

    Good to know. Have not seen this article. Thanks for your time Martin.

  8. Expert Member
    Dana Dorsett | | #8

    Since the garage isn't exactly outdoors, the floor doesn't have to be dense packed to 3.5-4lbs density the way walls in VT should be. A single stage rental blower is good enough for moderate dense packing, snaking the 2.5" hose all the way to the end of the bay from a 3.5-4" hole located in the middle. When the hose hits the end, back it up about 1.5-2' and blow until the compressor stalls, pull the hose back out a foot or so to get the flow started again, repeat. You probably won't hit 3lbs per cubic foo density that way, but it'll hit better than 2.5, you won't have a blow-out, and the amount of settling of the cellulose over the following few decades will be negligible to none.

    When turning around to pack the other it might be difficult to get the hose all the way to the end of the bay if you've already dense packed the first end right up to to your blowing hole. To deal with that, mark the hose with a wrap of tape at about the 5' mark, and again at the 1' mark. If you stop the initial blow 4- 5' from the end, the amount of blow-back that has mostly-filled the rest of the bay should be sufficiently low density that with the blower set to "air only" it would be able to air-drill it's way through the fluff down to the end. ( Worst-case you'd have to make another hole near the far end to fully pack it out.)

    If the blowing holes are cut with a hole saw, saving the plugs, patching the hole is pretty easy.

    Another tip: Take an old sweatshirt sleeve and slip the hose through it to use as a blow-back seal at the blowing hole. You'll still end up with a face and hair full of fire retardent and cellulose dust by the time you're done (you'll still need goggles, and decent filter mask), but it'll be orders of magnitude less.

  9. GBA Editor
    Martin Holladay | | #9

    For more advice on blowing cellulose, see these two articles:

    How to Install Cellulose Insulation

    Borrowing a Cellulose Blower From a Big Box Store

  10. kevek101 | | #10

    Thanks for the details Dana. I’ve been looking for proper way to do this in a floor system. Sounds like you are recommending drilling holes through t&g floor in attic rather than from the mesh fabric that’s stapled to the bottom side of joists? The ceiling is open in the garage.

    One other question. It’s a 2 car garage and the joists span 12 feet from top plate to carrying beam in center of garage. No hangers here and no blocking so if you add up both sides of garage the bays I’d be packing are 24 feet long. Would that change your approach?

    Haven’t yet read those two articles Martin posted so not sure if there’s an answer there.

  11. GBA Editor
    Martin Holladay | | #11

    You'll need to install the ceiling drywall first -- generally that would be 5/8 inch drywall for a garage.

    Once the ceiling is drywalled, you could either insert the cellulose hose through holes in the ceiling, or through holes in the subfloor above -- either way would work.

  12. kevek101 | | #12

    So continuous layer of rigid plus drywall, then drill holes through both and shoot cellulose back in. Is the thinking that the cloth just wouldn’t hold the cellulose up?

  13. kevek101 | | #13

    Or how about fabric + strapping nailed into bottom of joists. Idea being that the strapping, say every 2ft would be enough to hold weight of fabric up. After cellulose is in then I’ve got strapping in place I can hang rigid and drywall from. Don’t mean to get into the weeds here, Ive just never seen cellulose applied horizontally so I have no idea how much that muslin fabric can hold.

  14. Expert Member
    Dana Dorsett | | #14

    I was envisioning drilling through the garage ceiling drywall (+ rigid foam, if rigid foam is being used), and NOT the floor.

    Dense packing a ceiling in mesh will be very difficult to make flat compared to dense packing walls in mesh. Most box stores don't rent the rollers for flattening out the pillowing of mesh-blown dense-pack either.

  15. kevek101 | | #15

    Thanks for clarifying Dana.

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