Robert Borsch-Laaks
Bibliographic info:
9th International BUILDAIR Symposium, 8-9 May 2015 Kassel, Germany

Purpose of the work

Condensation risks in wooden building components are mainly caused by water vapor penetrating the cross section of the component through airflow. Even small pressure differentials result in a lot more vapor flowing through a joint of only a few millimeters than that which would migrate by diffusion through many more undisturbed square meters of area.

However, this does not mean we have to suspect potential damage caused by moisture behind every leakage we have found during a BlowerDoor test. Which driving force and which flow paths actually cause the gaseous water to condensate, depends on the location of the respective leakage.

Method of approach

This presentation provides a compact summary of the current findings from research and expert experience.

Content of the presentation

It all depends on the direction of the flow! Although we may perceive indoor winter air as “dry heating air”, in absolute terms, its water vapor content is higher than the concurrent outside air. For the risk of condensation in the case of vapor convection, this means two things:

Air flowing from the outside to the inside will never be able to condensate on its flow path, because it will always warm up on its way inside. Only a joint where the air flows from the inside out is actually able to cool the penetrated indoor air below its respective condensation temperature.

The force behind the airflow with the greatest moistening potential is the thermal power that creates excess pressure of a few Pascal in the upper part of the building section for a longer period of time. In addition, the amount of excess pressure depends on the height of the indoor air intake (chimney effect).

The risk of moisture damage also includes a structural element: Joints where air flows directly to the outside on a short path (e.g., connecting joints of windows and beam penetrations) usually do not correlate with falling below the condensation point. At fast flow speeds, because of the indoor air carried along, the joint faces may even show higher temperatures than they would have without flow.

The most critical leakages are those where the air can move through the hollow spaces on the cold side over longer distances.

Results and assessment

On the basis of building physics principles, the presentation analyzes the practical consequences for planning and implementation, the right diagnostics in the case of damage, and the problem-centered assessment of leakage detection during BlowerDoor tests.

To avoid moisture damage through convection, timber frames should also be free of hollow spaces and the necessary drying reserves for the structural component must be ensured.


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