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Community and Q&A

Garage Humidity Source?

David Pahl | Posted in General Questions on

David Meiland asked for the source of the humidity in my garage that might cause condensation. Must say I’m uncertain. The floor is concrete. There is a floor drain and sink that connects to the sewer. The garage is closed for the winter as we are in Florida except for Christmas so that would limit dryer winter air from entering. Otherwise don’t have a clue.

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  1. John Klingel | | #1

    Where is the garage when you are off to Florida? Is there a vapor barrier under the slab? Are any of the walls below grade? How wet is the ground around the perimeter of the garage? Frozen, or wet?

  2. David Pahl | | #2

    I don't know if they used a vapor barrier under the slab or not, but this is an area of concern. Our two car garage is attached to another as this is a town home (my neighbor has noticed condensation on his door in the winter but no mildew yet). The back wall sets on the poured concrete wall of the basement. The other side wall has about one half on a poured basement wall and the other half on a partial wall that also supports the brick porch at the entrance. The front of both garages has a connected black top drive about 25 ft. long with about a two foot drop in elevation. Winter humidity in the basement is about 50% and in the summer I run a dehumidifier. Drainage away from the foundation seems adequate. While I don't know the actual foundation construction, it is typical to have footer tile with a stone backfill as the soil is clay and we do have a sump pump that cycles frequently during wet weather.

  3. David Meiland | | #3

    Interesting puzzle. Since you are not there in the winter, presumably you are not putting the normal moisture from living into the space, but is your neighbor living there, and what's the chance that humidity from their space is entering yours? There is a good chance that soil moisture is migrating through the foundation and slab and creating part of the moisture load. Sorting out problems like this requires some detective work, visual inspection of the building for other clues, checking the slab for moisture content, maybe doing air diagnostics to see how well connected the houses and garages are. As for the garage door, I would ask a door installer in your area what is typical and whether the performance you are seeing can be improved somehow.

  4. David Pahl | | #4

    I doubt if the neighbor would be the source of the moisture. The wall between the garages is insulated and the wall even extends to the roof in the attic area. The builder tried to achieve as much sound insulation between units as possible. The two weeks we spend here at Christmas is when we witness the water dripping off the door. Would a heavy (6 mil) plastic sheet covering the floor while we are gone help the condition or just trap the water under the sheet?

  5. David Meiland | | #5

    You might try taping down a section of plastic sheeting (or maybe a few spread out around the room) and see if condensation develops under it. A foot square taped around all edges, leave it a day or two. It's a very crude test but can be useful. Obviously it would be more useful in the winter.

    Plastic laid on the floor might help somewhat, I suppose. That's what's usually done in crawl spaces.

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