Question about guideline for assessing condensation problems in hydrophobic mineral fiber

[WEI LI]

Hi experts,

Regarding the "Guideline for assessing condensation problems in hydrophobic mineral fiber", is there any explanation about the reason using 1 mm specimen for measured free saturation material, but use 1 cm specimen for no measured free saturation material?
I understand we wanna examine a thin layer, but why 1 mm for the one while 1 cm for another?
Thank you.

[Daniel]

Dear Wei Li,

in vapor permeable fiber insulation the dew water will normally form directly at the cold interface between the permeable and the next tighter material.

If we have measured values, we go up to the limit in the most exterior 1 mm to limit the water content to the measured water retention of the material. If it exceeds that value, we have to assume, that the water can run off. Here we evaluate the water content in the exterior mm in [kg/m³].

In case of not measured values for water retention, we just take a thicker area to be a bit more on the safe side: all water in the exterior cm is assumed to be dew water - and if that exceeds the 200 g/m² value, you should not accept this. Here we evaluate the water in the exterior cm in [g/m²] - so different unit and not depending on the thickness.

So closer to the limit in case of measured values, and more on the safe side in case of unknown water retention capacity.

best regards
Daniel

[WEI LI]

Dear Daniel,

Thanks for the reply. It helps.

[WEI LI]

Dear Daniel,

I am still a bit confused about the necessity of seperating the mineral fiber layer.
In the tutorial, the mineral fiber layer was split, and the outer 1 mm (or 1cm) layer was targeted.
Can we just keep the mineral fiber layer in complete and examine the outer few elements in post-processor.
The reason I raised this question is the 1mm thin layer is leading to numerical errors constantly. Once I removed the thin layer, everything went fine. However, I still wanna investigate the condensation at the surface of the insulation.

Cheers

[Daniel]

Hi Chris,

that is suprising - normally a separate 1 mm layer shouldn't change a lot concerning the numerics, but in single cases the different grid can obviously lead to such problems.

And yes, you also can keep the whole material layer and just evaluate an approx. 1 mm thick area.

Daniel