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Community and Q&A

Detached garage in the Pacific Northwest – Insulation recommenations?

Amy DiMarco | Posted in Energy Efficiency and Durability on

I am considering insulating my detached garage design to protect it from mold and to keep the temperature a little more even for storage. It will not be heated. What roof insulation/barriers do you recommend for a plywood substrate and metal roof? Also, what do you recommend for the walls. As I am not heating, I was thinking of not going to current code with R-values. Any advice would be appreciated. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    It's unclear whether installing insulation in your garage walls or roof will reduce the chance of mold, increase the chance of mold, or make no difference.

    Is your concrete slab insulated?

  2. Expert Member
    Dana Dorsett | | #2

    Metal roofing manufacturers usually specify the underlayment- read the specs, and don't cheat.

    With the goal of protecting it from mold in mind...

    Use a back-ventilated type of siding (vinyl) or a rainscreened assembly with a vented air gap to keep the structural sheathing from direct wetting from the exterior. Use a polyethylene or EPS vapor barrier under the slab to limit ground moisture wicking. Use EPDM sill gaskets or 10mil polyethylene + a cheap foamy sill gasket between the foundation wall and the bottom plate of the framing as a capillary break to limit moisture wicking up from the footings.

    Caulk the wall sheathing to the framing in every stud bay, and use unfaced batts between the studs (partition-wall soundproofing batts are fine, if 2x4 construction, unfaced R20s if 2x6), carefully fitted & trimmed, split rather than stuffed behind power runs, etc. Use an moisture-tolerant wallboard (eg. GP DensGlass) on the interior side of the walls to enhance drying toward the interior. Latex paint finish is sufficiently vapor open to allow drying.

    If you have 2x8 rafters make it a vented cathedralized ceiling rather than a mini-attic for racoons to turn into a condo. Vent it at the eaves and install R23 rock wool or R21 high density fiberglass in there, which would leave more than an inch between the top of the batt and the roof deck. If 2x6, go with high density R15 batts, which will give you 2" of vent space. Use moisture tolerant gypsum board on the ceilings too. It's more expensive, but the HD batts work better and are somewhat less likely to become mouse nesting material. (Use metal screening over the vent openings.)

    Roof overhangs of at least an inch for every foot of height on both the rake & eaves limits direct wetting of the siding, which in turn keeps the sheathing and studs dry.

    There is usually at least some solar gain, and the temperature inside the garage should average above the average outdoor temps if you keep the doors closed and it's reasonably tight. In the PNW the outdoor dew point temps are rarely above the deep subsoil temps for extended periods, so you won't have much of an issue with air leakage causing a condensation issue on the interior of the garage. Ground moisture and direct wetting of exterior surfaces are the primary enemies. If you take a rainscreen approach and leave it vapor-open to the interior, with good capillary breaks and ground vapor barriers, it should look and smell new inside the stud bays pretty much forever.

  3. Expert Member
    Malcolm Taylor | | #3

    I swing towards Martin's take on this one. Unoccupied, unheated houses here in the PNW don't do very well. Uninsulated sheds and barns fare a bit better.

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