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Best insulation for the ceiling in an attached basement garage?

Amanda Pomeroy | Posted in Energy Efficiency and Durability on

My dad and I both have the same problem: not enough insulation in our attached basement garages. I at least have the advantage of living in CT (Zone 5) and having some poorly sealed drywall and some fiberglass insulation on the garage ceiling. The floors are a little cold, but it could be worse. He owns a log home in northern VT (Zone 6) and has zero insulation on the garage ceiling, which makes his living room floor more than a bit chilly.

I’ll be ripping down the drywall in my garage for a number of reasons (the walls currently have fiberglass right up against the concrete, with no pressure treated wood at the bottom of the stud wall — it’s been providing a home to mice and growing some lovely species of mold in other sections of the basement). In both cases, I’m thinking of installing the following on each garage ceiling:
2″ of polyiso between the joists, well sealed at the joints and edges (R-13)
Roxul to fill the joists (R-23)
2″ of polyiso attached to the bottom of the joists to eliminate the thermal bridging (R-13), also well sealed at the joints and edges
1″ furring strips below the polyiso
That should bring the total value to about R-50.

Should I be worried about the foam sandwich creating a double vapor barrier? I don’t want to trap any moisture in the assembly, especially if I use foil-faced polyiso. We also both have our electrical panels located in the garage, so I’m concerned about sealing well enough around all of the wires/pipes.

Thanks in advance for any suggestions!

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that will provide answers to your questions: How to Insulate a Cold Floor.

    You don't want to cut any polyiso into thin strips and insert the polyiso between your floor joists. (That method is called the cut-and-cobble method, and I don't recommend it in this case.) The best way to use polyiso is as a continuous sheet on the underside of the joists, and that's what you plan to do. That's where the polyiso belongs. The joist bays can be filled with mineral wool, cellulose, or fiberglass.

    At all stages of the work, pay attention to airtightness. It's usually easy to seal the polyiso seams with a high-quality tape; at the perimeter of the ceiling, you may need to use some canned spray foam.

  2. Reid Baldwin | | #2

    In this application, do the furring strips accomplish anything? On an exterior wall, they create a rainscreen that allows the cladding to dry from both sides, but that wouldn't be applicable to the drywall on a partition wall or garage ceiling.

  3. Expert Member
    Malcolm Taylor | | #3

    I think a couple of things may be at play.
    North America appears to be divided almost equally into two camps: the east ceiling strappers and the west leave it be-ers. There is no convincing either side that the other has a valid argument for their preference.
    At two inches of insulation, it's right in the grey zone as to whether it's easier to use long drywall screws or to strap first.

  4. Amanda Pomeroy | | #4

    Hi Martin,
    I've read that article and several others you've written and found them incredibly helpful. I understand that air sealing at every step is important. I'm not sure how well I'll be able to air seal the existing subfloor, which is why I was thinking about that first layer of polyiso. I could probably manage it in my stick frame house by taping the seams of the existing subfloor and caulking at the points indicated in your article, but I have no idea how to accomplish it in a log home. There's no 2x4 framing or drywall to caulk.

    To complicate my own house, I have a cantilevered floor with the pipes for my baseboard heat running through the cantilevered space (yes, the builder was an idiot). At some point the previous owner had shoved some fiberglass in that space, but it wasn't doing anything except providing a comfy home for the mice. I've pulled all of that out and I'm ready to airseal and insulate. Would I use rigid foam along the outside wall and bottom of the cantilever, with fiberglass or Roxul to fill the space? I'm assuming since there are heating pipes running through there I'm trying to keep that space as part of the conditioned basement. Would I eliminate the blocking above the bearing wall as well?

    Thanks so much for your help!

  5. GBA Editor
    Martin Holladay | | #5

    If you have heating pipes running through cantilevered joist bays, you need to do a very careful job air air sealing. Ideally, all air leaks from the exterior into the joist bays need to be carefully sealed. Then you need an air barrier (rigid foam will work) on the exterior side of the pipe, without any insulation on the interior side of this air barrier. You can add as much insulation as you want on the exterior side of this air barrier.

    In short, the existence of pipes changes everything. In cases like the one you describe, the use of rigid foam on both sides of the fiberglass batts or mineral wool makes sense.

    If you have a leaky subfloor, that might be another reason to consider installing rigid foam on both sides of the assembly. If you want one side of the assembly to be more vapor permeable, you can use unfaced EPS as the air barrier up against the subfloor instead of foil-faced polyiso.

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