FAQ:General:LiquidTransportCoefficentofaPaintLayer

(9): Liquid Transport Coefficent of a Paint Layer

I want to examine the effect of driving rain on a painted wall. What liquid transport coefficients Dws do I enter for the paint?

There are no measurements of transport coefficients or, equivalently, water absorption coefficients for paint layers themselves known to us.

What is measured sometimes is the water uptake for different paint layers by applying the paint on a standard substrate (such as cellular concrete or lime cement mortar) and measuring the water absorption for this composite material.

So the best thing you can do is probably the following:
Don't use a layer of rendering and a layer of paint; instead, use a layer of the 'hybrid' material for which you already know the combined water uptake from the measurements. Use the Dws from the hybrid water uptake (let it generate by WUFI from the measured water absorption coefficient) and use the Dww and other data from the original rendering.

The vapor diffusion resistance of the paint can then be included in the surface transfer coefficients (as long as it is not markedly moisture-dependent).

Please note some possible problems, though:

• The result of the measurement may (or may not) depend on the substrate material, the details of the application etc. So you should make sure that you are using a water absorption value that has been measured under the same circumstances as the case you consider in your calculations.
• The paint may slowly change its properties when it gets wet (e.g. by swelling). The mean properties over a rain period of two or three hours may be different than the mean properties during a measurement that takes many hours. Again, the measurement should be done close to natural conditions.