Quick answer: A warehouse slab at 12 °C gets wet beneath air at 22 °C / 70 % RH because the air’s dew point is about 16.3 °C. Keep humid outdoor air out and dry indoor air until its dew point stays below the slab temperature; first confirm condensation rather than leaks or moisture rising from below.
A warehouse floor "sweats" when the concrete slab is colder than the dew point of the air above it. This happens most in spring and after cold spells: the slab is still cold from winter while warm, humid air comes in through open doors or ventilation. Cooling the building makes it worse. Fixes: keep humid outdoor air out (close dock doors, stop ventilating during humid weather), dry the air with dehumidifiers so its dew point stays below the slab temperature, move air across the floor, and in persistent cases warm the slab.
Quick answer
- Condensation when slab temperature ≤ air dew point.
- Warm, humid air on a cold slab: spring is the peak.
- Keep humid air out, dehumidify, move air; confirm it isn't a leak or slab moisture.
Confirm the mechanism
| Observation | Check | Suggests |
|---|---|---|
| Wide damp area after warm, humid weather | Near-floor air T/RH and slab surface temperature | Slab below dew point → condensation |
| Local wet spot | Look above for drips, pipes, roof leaks | Leak |
| Persistent damp regardless of weather | Construction, vapour retarder, drainage | Moisture from the slab or ground; building specialist |
ASHRAE describes surface condensation on non-porous surfaces below the air's dew point (ASHRAE Handbook—Fundamentals, Chapter 25).
Why air and slab cross
| Driver | Effect |
|---|---|
| Slab thermal mass | Stays cold for days or weeks after cold weather |
| Warm humid air entering | Dock doors, ventilation, roof vents |
| Night ventilation in humid weather | Brings in moist air |
| Cooling | Lowers air temperature but not necessarily dew point; can chill the slab further |
Worked example
| Condition | Dew point | Slab at 12 °C |
|---|---|---|
| Outdoor air 22 °C / 70 % RH | about 16.3 °C | Condenses |
| Dehumidified air 18 °C / 50 % RH | about 7.4 °C | Dry |
To keep a 12 °C slab dry with a margin, hold the air dew point at about 10 °C or below: at 18 °C that's about 60 % RH or lower (calculated).
Test each remedy against the cause
| Remedy | Works when | Limits |
|---|---|---|
| Close doors; air curtains; dock seals | Humid outdoor air is the source | Operations may need doors open |
| Stop ventilating in humid weather | Ventilation brings in moisture | Air quality needs |
| Dehumidification | Air dew point is too high | Must cover the floor area and the door load |
| Fans moving air over the floor | Still air near the floor | Doesn't lower dew point by itself |
| Warming the slab (heating) | Slab is the cold surface | Energy cost |
| Cooling the building | Rarely; can make it worse | Lower slab temperature |
See our warehouse dehumidifiers and large commercial dehumidifier sizing.
Short site record before specifying equipment
- Floor area, doors, ventilation, heating and cooling
- Wet areas on a plan, with times
- Near-floor air T/RH and slab surface temperatures, wet and dry spots
- Outdoor conditions on wet days
- Goods stored, forklift traffic (slip risk)
FAQ
Frequently asked questions
Why does my warehouse floor get wet in spring?
The slab is still cold from winter while warm, humid air comes in; the slab is below the air's dew point.
Will air conditioning stop a sweating floor?
Usually not; cooling can chill the slab further. Lowering the air's dew point is what helps.
How do I stop condensation on a concrete floor?
Keep humid air out, dehumidify so the dew point is below the slab temperature, move air, or warm the slab.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics; Heat, air and moisture control in building assemblies.
