Massive masonry outside walls get quickly wet due to driving rain

[Sami Roikonen]

Hello!

I'm modeling a massive masonry outside wall (600 mm, uncoated) and whenever I have driving rain on, the whole brick layer gets wet rather quickly. This is odd, since I've never encountered abnormally high moisture contents in massive masonry outside walls. Should I add a surface condition with a small Sd-value to the outside surface? Could I even switch off the driving rain altogether? The facade is an old bare brick wall so I think switching off driving rain might be a bit irresponsible...

I've encountered this problem with multiple different bricks (Fraunhofer: Solid brick masonry, Solid brick historical).

When I'm simulating massive masonry outside walls with rendering I have the same issue. The results seem more accurate when driving rain is switched off and when some Sd value is used in the outside surface.

I've enclosed some screenshots.

Edit1:
Could this effect be happening because the brick layer in the model is homogenous and the actual structure may have air pockets, vents, cracks etc. in the brick wall? Is there any good way of simulating them? Other than putting air layers in the model of course.

Edit2:
Bump. I'm hoping this gets answered.
I've tried to research this topic, but the most relevant article I could find was only about physical simulations [1]. It had no mentions of actual measured data from actual structures...

1. [G.B.A. Coelho, F.M.A. Henriques, Influence of driving rain on the hygrothermal behavior of solid brick walls, Journal of Building Engineering (2016), pp. 121-132. http://dx.doi.org/10.1016/j.jobe.2016.06.002]

[Christian Bludau]

Dear Sami,
sorry for the late answer.

I'm modeling a massive masonry outside wall (600 mm, uncoated) and whenever I have driving rain on, the whole brick layer gets wet rather quickly. This is odd, since I've never encountered abnormally high moisture contents in massive masonry outside walls.

This is depending on the a-value of the different bricks. They usually take up the driving rain quite fast. The tricky thing is to estimate the amount of rain for your facade. In my eyes it is not unrealistic, that the wall gets wet fast if it takes up all the driving rain. Please also keep in mind, that your initial conditions may include lower moisture than in reality. In this case the construction sucks up quite fast to the equilibrium state. Walls which are exposed to rain all the time can have a high water content over the whole section.

Should I add a surface condition with a small Sd-value to the outside surface? Could I even switch off the driving rain altogether? The facade is an old bare brick wall so I think switching off driving rain might be a bit irresponsible...

I would not do that. If you add a sd-value in the boundary condition, rain will be sucked in, but is drying out slower. If you place a membrane on you walls surface, no rain will be sucked in anymore. If the wall you are looking at, is exposed to rain, I would more try to adapt the rain coefficients.

Edit1:
Could this effect be happening because the brick layer in the model is homogenous and the actual structure may have air pockets, vents, cracks etc. in the brick wall? Is there any good way of simulating them? Other than putting air layers in the model of course.

That may be one reason, that the water uptake is lower in reality than in your calculation. If you insert an air gap parallel to your surface, that will break the capillarity. I´m not sure if this is what you like.

There are a couple of papers in German, but I guess they won´t help you much. Maybe ask Google Scholar: https://scholar.google.com/
German keywords would be: Mauerwerk, Schlagregen.

Hope that helps,
Christian