Condensation precipitates when the dew point is undercut. There can be various reasons for this.
A distinction is made between the following cases of condensation in the façade and roof area:
- Primary condensation by diffusion: Water vapour can enter the roof structure through diffusion. A functioning wall or roof structure allows the moist air to diffuse to the outside without condensation. However, if the outer layers are too dense, the moisture in the component will precipitate.
- Primary condensation by convection: Warm, moist air can penetrate the roof structure through small openings (damage) in the airtight layer and precipitate in the form of condensation by falling below the dew point. Convection can transport ten times more water vapor than diffusion. Therefore, the amount of condensation is particularly high in this case.
- Secondary condensation: The so-called summer condensate is formed when the air heated during the day under a roof covering cools down again at night in summer. This condensation can be ventilated by sufficient rear ventilation without causing damage.
- Reverse diffusion: Further condensation can also occur in the inner layers of the roof structure due to reverse diffusion, if the outside air is warmer in summer and has a higher humidity than inside. The water vapour flowing inwards (reverse diffusion) can lead to condensation if the inner component layers are diffusion-tight, e.g. in vapour barriers.
- Thermal bridges: Thermal bridges lead to a low surface temperature on the side of the room of exterior components. As a result, the dew point can be undercut, condensation precipitates and leads to mold in the interior.
All exterior components such as wall structures or roof structures must therefore be proven with regard to condensation water failure, which is usually done in the thermal insulation verification. A mathematical proof is not necessary if certain requirements are met, such as compliance with the sequence of layers specified in DIN 4108-3 with regard to the Sd value. Other factors that allow execution without mathematical proof are mentioned in section 5.2.2 of the ZVDH's "Leaflet on Thermal Insulation for Roofs and Walls".
Source: bauwion
