Avoiding the risk of condensation

Prevention of condensed water

A suitably dimensioned thermal insulation ensures that the inner surface temperature is above the dew point temperature ϑT relevant for the room climate.


Prevention of condensation inside the construction

The diffusion resistance of different materials in a multilayer construction must generally reduce from the warm side to the cold side in each layer.


Vapour permeability

No water vapour must enter the warm side, which cannot escape on the cold side again.


Usage of vapour checks or seals

If these conditions are omitted through the normal design, then an appropriately designed vapour check or vapour seal must be installed on the warm side of the thermal insulation.

Rules of thumb for avoidance

Excessive water vapour condensation must definitely be prevented in order to avoid damage to the thermal insulation, or worse, to the load-bearing structure! There are some simple rules of thumb to get an insight into water vapour condensation.


Nice to know!

A consistent observance of these rules, as well as a proper execution of the work will ensure a structurally correct and thus durable construction.


Vapour checks and vapour seals

There are three basic rules for the choice between a vapour check and a vapour seal.


Rule 1

For a construction with existing ventilation or a diffusion-permeable external layer (e.g. timber formwork)

  • Vapour check with a low sd-value im Bereich von 5 bis 20 m

Merke: Innen so diffusionsdicht wie nötig, aussen so diffusionsoffen wie möglich

Rule 2

In a construction with no functioning ventilation between thermal insulation and diffusion-blocking outer covering

  • Vapour seal with a high sd - value between approx. 20 - 100 m

Notice: Inside apprix. 20-fold diffusion resistant than outside.

Rule 3

Rooms with a constantly high level of relative air humidity, such as indoor swimming pools, saunas, laundries, hotel kitchens, etc.

  • Vapour seal with a high sd - value from 1'500 m

Nice to know!

When choosing the definitive vapour check or vapour seal we generally go for the one that has the lowest possible diffusion resistance for our construction, since the diffusion of vapour should only be limited according to need and not as strongly as possible!


Potential problems with vapour checks and vapour seals

The vapour check or seal is not handled cleanly:

  • Joints are poorly glued or not at all.
  • Edge connections are badly designed or "forgotten".
  • Penetrations are not sealed.
  • Damage to the surface is not repaired.
  • Unsuitable, cheap adhesive strips which come off after a short time.

The ventilation installed at the time of construction becomes inoperable due to incorrect design:

  • Too small or even no local inlet/foul air holes at the eaves and ridge.
  • No air openings with large penetrations such as roof windows, chimneys, etc.
  • In the case of a ventilated flat roof, insulation pressed into the underroof.

The trapped moisture is not taken into account:

  • After installation of the wet layers (underlays, plaster, etc.), the windows are not opened or the rooms are insufficiently ventilated.
  • Insufficient drying times, construction too tightly scheduled.
  • Inclusion of components with too high a level of moisture (wet wooden beams, etc.)

Deficient workmanship

Despite use of vapour checks and vapour seals, damage due to moisture in the construction occurs time and time again, and we constantly encounter the same error. In most cases, deficient workmanship is the cause. We list some of the most common execution errors below.


Correct processing

Clean and correct processing is important for vapour checks and seals! Only qualified and dedicated professionals should be entrusted with the execution of these works.


Planning mistakes

  • The planner provides for a vapour check, although a vapour seal would be required for the construction concerned, or forgets it completely.
  • The planner leaves the choice of the vapour check to the skilled manual worker without elucidating the further build-up of layers and other major significant factors.
  • The planner fails to verify the correct installation of thermal insulation and vapour check, as well as the functionality of the ventilation.

Good coorperation is essential

Only through the cooperation of all involved planners and skilled manual workers will we succeed in eliminating these sources of defects. Please play an active part in this regard. We would like to assist you.


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