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Insulation in wall between house and garage

bdubb1979 | Posted in Construction Techniques on October 19, 2021 10:51pm

Moved into a new house back in August and am currently in the process of replacing drywall on the exterior side (garage side) of the wall between my living space and garage. Current insulation (faced fiberglass batts, with facing towards living space) in the wall is original to the house (1967) and clearly needs to be replaced. There also doesn’t appear any sort of air seal or vapor barrier. A couple of questions:
1) Does this wall need to be air sealed (or vapor barrier)? I assume yes to prevent garage fumes from entering the living space.
2) If an air seal (or vapor barrier) is needed, what is the best approach to do this &  insulation? I am not removing/replacing the drywall on the interior side of the wall, so sealing would need to be performed from the exposed exterior side. Thus, I can’t install a continuous poly sheet. I also plan on performing myself (and don’t want to spend a lot of money), so spray foam is not an option.

Additional Info:
– 2 x 4 framed wall, 16 inch on center studs
– 5/8″ drywall will be installed on garage side
– Home is in Massachusetts
– Garage is not heated

Thanks

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Replies

  1. andy_engel | Oct 20, 2021 06:35am | #1

    Great questions! I'd use mineral wool for insulation as it's easier to install well than fiberglass. Use canned foam to seal up holes in the horizontal bits of framing, particularly top and bottom. You don't need a vapor barrier here. The drywall will serve as an air barrier if you install it well. Use acoustical sealant on the framing along the perimeter of the drywall sheets for a good air seal (Kamco and other drywall suppliers will have acoustical sealant - PL will work too, but the sealant always stays flexible). Gasket the electrical boxes for good measure.

  2. User avater
    unclemike42 | Oct 20, 2021 07:50am | #2

    I second the idea of using rock wool.

    The wall and/or ceiling between the garage and occupied space needs to be a fire-separation by code. (not vapor proof)

    Remember to think about any common walls above the ceiling level. If there is not a complete sealed drywall ceiling, it may be better to run drywall on the wall to the house all the way to the roof.

    Vapor barriers are chosen and installed primarily to keep the insulation dry so that it maintains its R value, and avoids growth of mold and related ills. (assuming hot air would meet some insulation at temperatures below the dew point on the way to the outside)

    It also is a good idea to look at ventilation of the garage.

  3. jwmurray.home@gmail.com | Oct 20, 2021 12:30pm | #3

    When you say there is no existing vapor barrier, isn't the facing on the existing batts some sort of vapor retarder? I am not sure why others would suggest you do not need a vapor barrier here. If the garage is cold, and the house is warm with no vapor barrier ... I think you could get moisture in the walls. So if you take the faced batts out, you would need to put some kind of vapor retarder back between the studs. Perhaps cut rigid insulation to fit between the studs (on the warm side) or strips of poly between the studs. Or a vapor retarding paint on the inside?

    Air sealing will also keep moist air from moving through the wall and also fumes from the garage out of the house. Air sealing the drywall could be the way to go if working from the garage side.

    1. andy_engel | Oct 20, 2021 06:23pm | #4

      The vast majority of moisture that makes its way into walls does so on air currents, not through the drywall as vapor. Most of Mass. is Zone 5, where a vapor retarder, not a vapor barrier, is called for. Do a good job air sealing either side of the wall, and the interior wall paint serves just fine as a vapor retarder.

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