Thread: Sweating Concrete

I had a home (1999) a few days ago that had an corner of the concrete in the garage that was "sweating" according to the homeowner. The area was dry when I was there, but obvious efflorescence and appeared to have been wet previously. This area is in the same corner as the water heater and furnace. When the garage door was down I tested the water heater for backdraft, but nothing. The furnace is a power vented system. Any ideas?

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Check the outside area for excessive moisture (detached/clogged gutter, bad grade, etc). If your seeing efflouresence there's a good chance water is passing through the foundation in addition to any that is condensing on the surface.

Any chance this is on the North side of the house or in an area with little or no sun?

Also, check for any other sources of moisture for the garage. Bath fans, laundry equipment, etc. The furnace and water heater could contribute too, so you're on the right track.

Moisture source under the slab.
Could be missing vapor barrier, improper grading-sprinkler-etc. as an external source, plumbing leak under the slab.
An abnormal draft or cooling causing condensation in a local area could be a possibility, but the efflorescence kind of rules that out.