Why Metal Buildings “Sweat” (and How to Spot It)

The problem you’re seeing

Walk into a pole barn or shop on a cool morning and find drip lines on purlins, damp floors, and a musty smell? That’s usually condensation, not a roof leak. Warm, humid air touches a colder metal surface and crosses the dew point, turning vapor into water.

Condensation in plain English

• Bare metal gets cold fast—especially overnight.
• Warm, moist indoor air hits that cold metal.
• If the metal is below the dew point, water forms and drips.

How to confirm it’s condensation (not a leak)

• Weather/timing pattern: Drips show up after sharp night-to-day temp swings; may dry up on warmer afternoons.
• Location pattern: Moisture collects under roof panels, along fasteners, and at wall-to-roof transitions—often in streaks.
• Material clues: Rust or staining around purlins/fasteners without a single obvious “hole” path.

Quick 30–60 minute checks

• Light leaks at doors: Add sweeps and weather-strip; adjust tracks so seals meet evenly.
• Penetrations: Seal around conduit, hose bibs, vents, and fan housings.
• Indoor moisture sources: Wet equipment, animal watering areas, green lumber, unvented heaters.
• Air movement: A small circulation fan keeps surfaces closer to room temp.
• Wet floors: Squeegee instead of letting puddles evaporate indoors.

When to ask for help

• Drips persist after sealing obvious leaks and improving airflow
• Returning rust, staining, or musty odor
• Hard-to-reach transitions and fasteners keep sweating