Avoiding the risk of condensation
Prevention of condensed water
![](/fileadmin/_processed_/0/2/csm_Dampfdiffusion_32_72701910ae.png)
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
![](/fileadmin/_processed_/c/0/csm_Dampfdiffusion_33_d20af34259.png)
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 checks and vapour seals
![](/fileadmin/_processed_/f/7/csm_Dampfdiffusion_36_ef3533f6ed.png)
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
Potential problems with vapour checks and vapour seals
![](/fileadmin/_processed_/b/b/csm_Dampfdiffusion_39_87f1200b13.png)
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.
![](/fileadmin/user_upload/600_Bauphysik/Dampfdiffusion/Dampfdiffusion_40.png)
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.
![](/fileadmin/user_upload/600_Bauphysik/Dampfdiffusion/Dampfdiffusion_41.png)
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.)
Planning mistakes
![](/fileadmin/_processed_/1/1/csm_pic_Planungsfehler_Dampfdiffusion_5cf5a1f370.jpg)
- 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.