Moisture Is Always Incredibly Tenacious

"But I keep my garage door closed!" you say. Well, that doesn't always matter. Humidity in the air can still get to your garage floor, even if your garage has been kept closed up. No garage is waterproof, so any garage floor is susceptible to the effects of moisture, even those protected by doors and walls. Not only that, but moisture can also creep in to the concrete from the sides and underneath, though that depends largely on your area, just how much rain you've had, how long water sat against your garage, and how high your water table is.