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Community and Q&A

Mysterious moisture in garage

truthbox | Posted in General Questions on

I’m trying to diagnose a mysterious moisture issue in a garage. This is an attached garage with living space above (I know, I know – no cars are stored in the garage) located in Mass. Whenever there’s a large rain or it’s particularly humid – including during the winter – there’s an excessive amount of moisture on the garage slab. It can be wet to the touch. It seems to spread from the garage doors back into the garage and follows a pattern that roughly approximates the tire tracks where somebody would have pulled their cars in. It’s a problem because the humidity is way to high in the garage. Things are getting moldy and I worry about the moisture moving into the house.

At first, I assumed the moisture was moving up through the slab (i.e. no vapor barrier). But if there’s something impermeable on the slab like a garbage bag or styrofoam, then the slab will be dry beneath that. I assume this means the moisture is not moving through the slab.

So I’m thinking it’s either condensation or that the water is somehow being wicked back from the edge of the garage (if the door is closed, there’s still an exposed strip of concrete). Either way, it seems odd.

Any thoughts on what would explain this? And any recommendations on how to remedy this situation? Would a sealer or epoxy coating help here? Would installing an exhaust fan in the garage help dry things out more quickly and prevent mold?


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  1. GBA Editor
    Martin Holladay | | #1

    Your two suggested theories -- condensation or wicking -- are certainly possible. If you closely observe the weather conditions, you should be able to determine the mechanism involved.

    The most common time for condensation is when warm (especially warm and humid) weather follows a cold spell.

    The most common time for wicking is immediately after a rain.

    To reduce wicking, you need (a) to keep the exterior grade below the top of the slab, and (b) to make sure that the grade slopes away from the garage on all four sides, and (c) to maintain (if possible) wide roof overhangs.

  2. truthbox | | #2

    Thanks Martin.
    It may be both, at different times. The exterior grade at the garage doors is about 1" below the slab, but there's not a lot of slope away from the doors. It doesn't seem like bulk water is hitting the slab (or driveway in front) and draining back into the garage. So it's surprising to me that the water would wick so far back into the garage (10'-15'). In addition to ensuring better drainage away from the garage, is there anything that can be done to the concrete to limit this wicking?


  3. Expert Member
    Peter Engle | | #3

    It's more likely condensation than wicking. The hints are in your posted observations.

    It happens when there is rain and it is humid, even (or especially) in winter. In winter in MA, it gets cold and the floor of an unheated garage gets cold, too. If the weather warms up outside and it rains, then you are going to have dewpoints above freezing and a slab temperature near freezing. Air leaking into the garage condenses on the slab. In summer, ground temperatures are lower than the outside dewpoint, and this also points to condensation issues.

    The tire tracks are attracting extra moisture because of the road salt that is soaked into the floor. Salt is hygroscopic and it attracts moisture, accelerating the time it takes to saturate the surface.

    The pattern starts at the garage doors because the cold air is colder there and the warm and humid air is warmer and more humid - the closer you are to the doors, the closer conditions are to outside conditions.

    Unfortunately, there's not much to be done about it. Improving the weatherseals on the garage doors or replacing the door with an insulated and sealed door can help by keeping outside air out of the garage. This will limit chilling in cold weather and also limit the ingress of humidity with warm and humid air. Heating the garage in winter would help, but at a big energy penalty. Dehumidification in summer would also help, again with an energy cost. I dehumidify my detached garage in summer just to keep my lumber in good enough condition to use for furniture.

  4. kjmass1 | | #4

    I noticed this just today as well (outside of Boston). Temps in the low 40s from overnight, rising to mid 60s with 90 percent humidity today and chance of rain. I had actually just water sealed my detached garage yesterday, full brick exterior on concrete slab, with parged mortar coat on the inside. The inside mortar was weeping moisture, and exterior brick was dripping wet (but sealed!). Windows were all fogged up- I imagine it takes forever for the concrete to rise in temperature compared to the outside air.

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