EKS-HUM-020 / Condensation, measurement and control
Condensation
凝露
Condensation is the formation of liquid water when water vapour reaches a condition at which the vapour phase can no longer be maintained.
- Aliases
- surface condensation, water-vapour condensation
QUANTITY, SYMBOL AND UNIT
Quantity / objectCondensation of water vapour
UnitNot applicable
Definition
Condensation is the formation of liquid water when water vapour reaches a condition at which the vapour phase can no longer be maintained.
For moist air contacting a surface, condensation is thermodynamically possible when the local surface temperature falls below the local air dew point. Actual wetting also depends on local air contact, transient temperatures, surface condition and the time available for mass transfer.
Relations and distinctions
Condensation and high RH
High RH indicates proximity to saturation in the air. Surface condensation additionally requires a local surface condition that reaches or crosses the relevant dew point.
Condensation and moisture load
Condensation is a phase-change event; moisture load is the rate at which water enters or must be removed from a space or air stream.
Engineering meaning
- Compare the highest credible local dew point with the lowest credible surface temperature, not only room-average values.
- Inspect drainage, corrosion exposure and electrical clearances where condensate can form or migrate.
Common misconceptions
MisconceptionCondensation can be ruled out whenever the room RH is below 100%.
Accurate explanationA surface can be colder than the room air and locally cool adjacent air to saturation even when room-average RH is well below 100%.
Application boundaries
- A nominal dew-point comparison does not capture thermal bridges, door openings, night cooling or start-up transients.
- The onset condition does not by itself predict condensate mass, droplet transport, drying time or equipment damage.
Supporting sources
-
ASHRAE / SRC-ASHRAE-F25-CH01
ASHRAE Handbook - Fundamentals, Chapter 1: Psychrometrics ↗
2025 SI edition / Chapter 1, moist-air cooling and dehumidification, lines 867–876
Supports: Moisture condensation occurs when moist air is cooled below its initial dew point; Condensation removes water from the air stream; The dew-point relation supplies the thermodynamic onset condition -
International Organization for Standardization / SRC-ISO-10211-2017-SCOPE
ISO 10211:2017 - Thermal bridges in building construction - Heat flows and surface temperatures - Detailed calculations ↗
2017 / Public abstract and scope
Supports: Minimum surface temperatures are calculated to assess surface-condensation risk; Thermal boundary conditions and geometry affect the surface result
Maintenance record
Condensation
- Maintained by
- Yakeclimate Engineering Team
- Authoritative sources verified
- August 3, 2026
- Terminology checked
- August 3, 2026
- Last updated
- August 3, 2026