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Community and Q&A

Zone 7 garage wall insulation

DavidJ7 | Posted in Energy Efficiency and Durability on

Zone 7
1000sf detached garage
No hvac at present, will be added and run intermittently.
1960’s build

Current assembly:
Steel siding
~1/4″ rigid exterior sheeting
Shiplap (possibly salvaged)
2×4 @ 16″

The shiplap has large gaps (up to 1×12 inches). Short of spray foaming/caulking the shiplap prior to putting bat insulation in I do not see any practical way to seal it. Is there any reason not to just go with R-13 batt insulation (faced or unfaced) with a vapor barrier? Interior will be unfinished.

Attic will be sealed and ~R-60 cellulose.

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  1. Walter Ahlgrim | | #1

    If you are not going to heat it now it makes no financial sense to insulate the building now.

    From a comfort point of view cars do not care if they are hot or cold, so unless you plan on spending a lot of time with the cars I see no point in heating or cooling a garage.

    I see no point in insulating any building without air sealing it.

    If you are not going to cover the insulation you will be making a very nice home for the local field mice.

    When you are ready to heat the garage your air barrier should be the new drywall covering your insulation.


  2. User avatar GBA Editor
    Martin Holladay | | #2

    I agree with Walter. Hold off on insulation until it's time to install heating equipment.

    If you ever insulate, you should make a stab at air sealing -- using either the flash-and-batt method, or the cut-and-cobble method, perhaps. If you don't want to invest in an exterior air barrier (using flash-and-batt or cut-and-cobble), at least install an interior air barrier -- carefully sealed drywall.

    Walter is also correct that the interior drywall is essential to keep down the rodent population.

  3. DavidJ7 | | #3

    I probably should have added a few more details. The garage is also used as a workshop. Using a miter saw when it is 10 below is both dangerous and a bit miserable. The point of the insulation is to extend the usable days for the garage. I do have space heaters and such but will not have a more extensive system in until later.

    Unfortunately, I have yet to find a way to add any interior sheeting that can cope with the foundation conditions. The slab lacks both footings and rebar and sits on highly expansive clays. This means that there is significant movement throughout the year. Any drywall added will crack extensively.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    The building you describe is going to be hard to insulate. If the building moves enough to crack drywall, you're going to have lots of problems.

    I guess I would advise you to install closed-cell spray foam under the circumstances you describe, followed by drywall for fire safety. Even if the drywall cracks, you still need it for fire safety.

  5. Lance Peters | | #5

    Digging down and insulating the edge of the slab to below the frost line might pay off nicely in two ways:

    1. If the soil below the slab remains more thermally stable the building should move less season to season.

    2. If the edge of the slab is insulated and the building is insulated, the slab will conduct heat from the ground (which is warmer than the outdoor temp in winter) and keep the building warmer than it would otherwise be. Of course, if the building is poorly insulated and/or leaky this won't make much of a difference. Garage doors are notoriously leaky and poorly insulated.

    I'm not sure what your local CZ7 ground temperatures are, but after doing a bit of research I decided against insulating below the slab in our new build in CZ6 (Ottawa) for this very reason. I'll have both parking and workshop areas in our garage and I would like it to stay above freezing without supplemental heat, a goal that seems reasonable.

    In my opinion, R60 in the attic may be a waste with only R13 in the walls. How much of a waste likely depends on many factors including the ceiling to wall area ratio. Cellulose is cheap, but keeping it to R40 could free up some cash to put into better insulating/sealing elsewhere.

  6. Jon R | | #6

    Horizontal wings of foam insulation will decrease the amount of frozen soil outside the slab edge (with or without building heat). With a little slope and flashing it can also help move surface water away from the outside edge, reducing another cause of movement.

    Consider lots of radiant heat to get a quick warm-up.

    Sounds like a film would provide a more durable air barrier than gypsum. I don't know what your code requires, but there may be various more flexible options for wall coverings. And in a non-visually critical garage, there might be ways to mount non-air-barrier gypsum that are less prone to cracking.

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