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I have a detached, at or above grade garage with a concrete slab floor and sheetrocked, insulated walls.

I noticed some moisture on the drywall during pre-purchase inspection last fall, but it's gotten worse since winter (midwest US) started. I know the drywall needs to be replaced at this point, but I'm still uncertain on the original cause.

I believed it to be runoff from our wet cars and melting snow chunks behind the tires, but the folks at my hardware store didn't think it would account for the quantity of moisture I'm seeing. The roof was recently inspected and found to be solid. I have good visibility to the underside of the garage roof and have never noticed moisture there. The problem seems to stop about 3-4' up from the garage floor.

Main question:

  • Does dripping from cars (and a mis-graded slab) account for this moisture damage, or could it be something else?

Bonus question:

  • What changes would you make to prevent this problem with new drywall? I've considered putting in a treated lumber baseboard to separate slab and wall, adding gutters to the roof, and/or replacing the bottom 4'of sheetrock with concrete board.

Holding up the drywall fabric showing degradation below. One foot in height. Four foot section of drywall cut away to explore the problem Representative of floor height drywall, wet and peeling Garage, left side, outside, above grade Garage, right side, at grade

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    Moisture can wick upwards so drywall should never be touching the floor but I'd guess that there is another source somewhere.
    – matt.
    Jan 28 at 20:25
  • You mentioned snow. Under the eave looks like a nice gutter for snow melt accumulation. That door with the insulation beside it is the same exterior door shown in the outside image, right? So the garage is conditioned? In that case, was there lots of water from your car(s) at that wall before the snow melt came? Do you recall the problem noticeably intensifying only after the snow melt?
    – popham
    Jan 28 at 21:01
  • What sort of ventilation, if any, does this (heated or unheated?) garage have?
    – Ecnerwal
    Jan 28 at 21:09
  • Thanks all -- the garage is unheated and has no ventilation. The exterior and interior door are the same. My suspicion is exactly what matt mentioned above. I've definitely seen water running off from the cars towards the walls. We've had an unusually warm winter so there hasn't been much snow accumulation or melt this year, and I wasn't the owner for prior years so it is tough to say.
    – John
    Jan 28 at 21:15

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Cementboard would certainly stand up to the water better, but a key thing here would be to provide vents both low and high so that moisture can get out of the garage.

For an unheated garage that would normally be simple, inexpensive vents allowing for passive airflow. I can't see the cost to purchase, install and run mechanical ventilation or an HRV paying off in an unheated space.

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