Condensation forms when hot and humid indoor air meets a cold surface (e.g. a window pane), producing water vapour. When the moisture cannot dry, this can lead to fungus or rot.
Condensation through water vapour diffusion
Water vapour diffusion is a slow and natural process that occurs in any construction. Water vapour moves from the indoor environment with a higher water vapour pressure to the drier outside environment. Only a limited amount of water vapour is transported. Condensation only occurs when the water vapour comes into contact with a cold, vapour-proof layer. When working from vapour-proof on the inside to vapour-permeable on the outside, the amount of vapour is limited and it can dry out easily.

Condensation through convection
In practice, condensation issues are usually due to a poorly executed construction, mainly in terms of airtightness. In case of air leaks or when the insulation material is not properly attached to the structure, a convection current may form. This is the natural process in which warm, moist air travels until it hits the cold outer surface of the structure and bounces back. When this air cools, condensation forms against the outer surface. The amount of condensation is many times greater than with water vapour diffusion and as a result, it damages the structure instead of simply drying out.

Importance of airtighthness
Good airtightness is essential in avoiding air leaks (and therefore convection).
Rules of thumb to keep condensation from forming in the construction
  • Make sure the construction is airtight to exclude convection currents that may give rise to condensation.
  • Always use a vapour barrier on the inside of the construction. The outside of the construction is preferably as vapour-permeable as possible, to allow any condensation water to evaporate.
  • Condensation caused by water vapour diffusion doesn’t have to be a major issue as long as the total amount is limited (< 150g/m²) and the construction dries out naturally every year during the warmer months.
  • When a vapour barrier on the inside is not an option, it is possible to install the barrier within the structure under certain conditions. This frequently occurs when an existing roof construction with wool on the outside is insulated additionally in accordance with the Sarking principle. Sarking insulation must always be installed with a continuous vapour barrier. The rule of thumb implemented by the WTCB (NL)/CSTC (FR) provides a safe assessment: the R value of the outer layer must be at least 1.5 x the R value of the inside layer. This keeps the temperature on the inside of the vapour barrier high enough so no condensation can form.
    Importance of ventilation
    The natural transport of vapour from the inside to the outside can never replace a ventilation system. A well-dimensioned ventilation system contributes to the creation of a healthy indoor climate and limits the amount of vapour in the inside air, for instance by evacuating the moist air after a shower.

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