Driving rain and wooden facade panels

[Sami Roikonen]

Hello!

I've been modeling a outer wall's inside corner in a passive house. The house has wooden frame. Model enclosed.
In the first model I made I removed the wooden facade paneling so I could ignore rain, which always seems to mess up the calculations. The inner thermal insulation (aluminum coated PU) was simulated as 3 separate layers: Pu in the middle and vapor retarder layers (Sd 100 m) on each side.

The calculations worked out fine and the results seemed to be normal.

I tried additional calculations with the wooden facade boards included. This meant that I had to include driving rain and radiation. I also added an air exchange factor of 5 (perhaps too little?) to the ventilation air layer behind the facade boards. I used a thin weather resistive barrier (Sd 0,5 m) to simulate the paint so that the rain doesn't immediately saturate the model.

Now it seems that the construction is absorbing more and more moisture by the year. I only calculated for 2 years, so the trend might reverse... I understand that the outermost wooden parts are going to absorb a some moisture from the initial 0.7 RH, but my results seem far too high. Normal wooden walls aren't wet all year round, especially the inner wooden pillars.

I think the driving rain and modeled thin weather resistive barrier don't work together well. I remember reading, that one method of simulating outer envelope structures is actually turning driving rain absorption to 0 and simulate the waterproof coating by using a vapor diffusion resistance factor (Sd) for the surface. I added the driving rain, however, just to see it's effect and to compare different cardinal directions (south gets more rain and radiation than north in Finland). Otherwise the the only difference between north and south would be the radiation and I could immediately say, that since south gets more radiation, it's going to be warmer and drier than north. When rain is included, it's not easy to say which direction is drier. Have I overlooked something with simulating driving rain (0,7 absorption) with the 1 mm weather resistive barrier?

Also I'm not sure about the modeling of the aluminum coated PU thermal insulation. I did it using 3 different layers (vapor retarder-PU-vapor retarder). Now there seems to be elevated moisture INSIDE the PU-layer right next to the aluminum layer. Is this reasonable? I'd say it's pretty counter intuitive to have elevated moisture inside the insulation board. Does this happen in reality. Would a better way be to simulate the insulation board as a single layer that has the effective vapor resistance factor as the former model?

I also got an error message after the calculation saying "out of memory". Saving amoung other options was greyed out, but I could have a look at the results. After I manually terminated the calculation (despite the iteration section stating "calculation finished") I could save the results. Afterwards I've been having a lot of slowness and some crashes on my computer. Weird. Cache directory seems to be a temp folder in the computers main C-disk with nearly 300 GB of free space.

I'm grateful for any feedback.

-Sami Roikonen

EDIT: I enclosed the project data, sans result files.

[Christian Bludau]

Dear Sami,

generally the calculation of membranes on a surface as sd-value in the boundary condition instead of building them in as a layer leads to a faster calculation in WUFI. But doing that you have to switch of the rain, as the sd-value does only affect the diffusion.

Further with a membrane on the surface you also could switch off the rain absorption, as membranes in the most cases do not contain liquid transport coefficients and so they will not take up water.

For the insulation board it would also be possible to build that in as one layer with an effective µ-value, but in this case you can not access the distribution of the moisture, but only the content over the whole layer. If you do not want to see the distribution, you can try to do it that way. I would guess, that it speeds up the calculation.

The out of memory error is caused by the iter.txt. The variable keeping that can not be written. It is not related to the free disk size. If you change the output of the residues from 10 to 100 (Numerical Settings -> Enhanced), less iteration steps will be stored and that should work. That may have been solved in the latest version of WUFI 2D 4.2, but I can not check at the moment.

For an easier/ faster calculation you also could try to replace the air layers by the ones with the short name, as the mathematical handling is much easier for WUFI 2D. Please check the material information about air layers before, if they also hit your needs.

Christian