July 18, 2025

How does condensation occur? Causes and examples

Découvrez comment se forme la condensation, ses causes et ses conséquences dans l’air, sur les murs ou les fenêtres. Explications claires.

Condensation results from an increase in relative humidity up to 100%. There are two main causes that can be at the origin of this phenomenon:

  • Temperature variations
  • The contributions of liquid water into the ambient air

This physical phenomenon is very common, especially in humid areas, poorly ventilated homes or places subject to significant thermal variations.

1. Temperature variations, a key factor in condensation

The amount of water vapor that the air can contain depends directly on the ambient temperature. The hotter the air, the more steam it can hold. As the temperature falls, that ability decreases. If the air is already saturated with water vapor, a drop in temperature can cause condensation to appear.

a. Condensation due to day/night variations

During the day, the temperature rises, the saturation threshold increases, which decreases the relative humidity.
At night, the temperature falls, the saturation threshold falls and the relative humidity rises. If it reaches 100%, condensation appears in the form of dew, fog or droplets on cold walls.

💡 This explains, for example, the fog up on the windows in the morning or on the cars parked outside.

Dew formation at sunrise

B. Cold spots: a localized cause

A cold spot is a surface whose temperature is lower than that of the ambient air. The humidity in the air will then condense at this precise location.

Common examples:

  • Condensation on a window in winter
  • Glasses fogged up when entering a heated room
  • Humidity on the cold walls of a poorly insulated house

👉 This type of localized condensation can lead to mold, bad odors, and even structural damage in the long term.

 

2. The contributions of liquid water into the environment

When liquid water evaporates in an enclosed environment, it causes the ambient humidity to rise to a critical level. If there is insufficient ventilation, condensation is unavoidable.

a. Occasional humidity inputs

  • Showers and baths
  • Boiling stoves and pots
  • Indoor laundry drying

These inputs rapidly increase the humidity level in the air, especially if no ventilation is planned.

B. Continuous humidity inputs

  • Water infiltrations in the walls
  • Capillary rises from the ground (presence of saltpeter)
  • Industrial or humid environments

These situations create a permanent humid climate, encouraging not only condensation but also the development of fungi and pathogens.

3. How can condensation be prevented?

a. Improving ventilation

Regular ventilation or the installation of a VMC makes it possible to maintain the correct ambient humidity and to evacuate water vapor.

B. Use an absorber or dehumidifier

  • Desiccant bag or silica gel in small spaces
  • Electric moisture absorber in large rooms

c. Limiting thermal bridges

  • Reinforcing wall insulation
  • Limiting thermal bridges in windows and doors
  • Monitor surface temperatures


4. Things to remember about condensation

  • Condensation is linked to a relative humidity of 100%
  • It occurs due to air cooling or excessive evaporation
  • It occurs on cold walls, windows, ceilings, etc.
  • It is avoidable thanks to good ventilation and insulation

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