Why Do Garage Floors Sweat? (Video)

“Hi, my name is Greg Batista here to answer another question from the internet.

SlipperySam asks: Why does my garage floor look wet when it’s humid?

That’s condensation. Concrete has high thermal mass, so it stays cooler than the surrounding air. When warm, moist air touches the slab, vapor condenses. The U.S. Department of Energy notes garages in humid states can experience surface moisture over 200 days per year. This isn’t just an inconvenience — constant moisture promotes mold, damages coatings, and accelerates corrosion inside reinforced slabs. Solutions include sealants, vapor barriers, and dehumidifiers. In Florida’s climate, “sweating slabs” are one of the most common issues owners report, and left untreated, they shorten a structure’s life.

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Authoritative references: U.S. Department of Energy – Moisture Control Guidance; ASHRAE humidity guidelines.

The author, Greg Batista, PE, CGC, SI is owner of G. Batista Engineering & Construction and is a nationally-recognized engineer and contractor with more than 35 years of experience and offices in Fort Lauderdale, Florida.