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Community and Q&A

Confused about vapor barriers in attached garage

jay443 | Posted in General Questions on

I live in MN and am insulating my garage ceiling in an attached garage. I’m so confused about vapor barriers. I’ve read the articles here and on other sites, and it seems like I can’t get a “final” decision.

On the one hand, I’ve read about how air leakage is the main culprit of transporting moisture, so I plan to air seal as best as I can with spray foam and caulk. Thus, I shouldn’t worry too much about moisture passing through drywall because it only does a little.

On the other hand, I will be conditioning this space in the winter so that I can work in my shop. Also, I’ll be driving snow-covered vehicles into the shop. So there’s a lot of snow melting and the temperature is about 45 degrees when heated. I feel like that’s a lot of moisture that I don’t want passing through to the cold sheathing.

What’s a guy to do?


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  1. jay443 | | #1

    I should clarify that I'm talking about the garage ceiling only, as the walls are already insulated with kraft-faced fiberglass and covered with drywall.

    For the ceiling I have been using unfaced fiberglass batts, as I was given nearly all of it for free. I know it's not as good as cellulose or foam but it's what I've got.

  2. GBA Editor
    Martin Holladay | | #2

    You don't mention whether there is a bonus room or living space above the garage.

    The classic answer is that the vapor barrier goes on the warm-in-winter side of the assembly. If your garage is 45 degrees, and you have a bonus room heated to 72 degrees above the garage, the vapor retarder faces the upstairs room. Your bonus room subfloor (usually OSB or plywood) is a perfectly adequate vapor retarder, so no polyethylene is needed -- even in the 1970s scenario (before building science made us smarter).

    If there is no bonus room above the garage, the 1970s answer was to put the poly on the garage side of the ceiling, because that is the warm-in-winter side. Do you really need it? No. As you correctly point out, pay attention to air sealing and don't worry about vapor diffusion.

    If you want to read more about these issues, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

  3. jay443 | | #3

    Martin, there is no living space above the garage. Sounds like the "2016 answer" is to not use a barrier at all.


  4. GBA Editor
    Martin Holladay | | #4

    Since 2007, the IECC has required (in section 402.5) that walls and ceilings in climate zones 5, 6, 7, 8, and marine zone 4 -- a range of zones that includes Minnesota -- have a Class I or Class II vapor retarder — in other words, kraft facing, vapor retarder paint, or polyethylene -- on the interior side of the assembly.

    I recommend using vapor retarder paint, since this approach is less likely to trap moisture in your ceiling.

  5. Jon_R | | #5

    You will have a higher than usual dew point. All partitions leak some air, so you will have some condensation in the walls/ceiling and on any external sheathing. A solution is to maintain the garage at a slight negative pressure with an exhaust fan.

  6. GBA Editor
    Martin Holladay | | #6

    The situation is not as simple as you described. You wrote, "you will have some condensation in the walls/ceiling and on any external sheathing." Really?

    What part of the ceiling? Will this condensation occur on the drywall ceiling if the garage is heated to 45 degrees? Why?

    It's certainly possible that there might be condensation on the wall sheathing if the walls are insulated with fiberglass batts -- that's common enough. But if Jay did a good job of air sealing the drywall installed on the walls, the sheathing may stay dry.

  7. Jon_R | | #7

    You switched "in" to "on", but OK, make it "some condensation in the insulated partitions". Does that leave any questions?

  8. Expert Member
    Dana Dorsett | | #8

    The air tightness of garage doors is VERY poor, and the inherent ventilation rate in garages is pretty high. Unless there is something other than the occasional snow/ rain drip onto the garage floor for an interior moisture source, the dew point of garage air is going to track the outdoor dew points pretty well. There is likely to be nearly 100% air exchange with the outdoors every time the garage door opens, purging the high-humidity air.

    Even if the indoor humidity is elevated for several hours at a time, it's not the same as say, storing 4 cords of green firewood in there.

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