Internal insulation with XPS in the basement

Hello everyone,

a basement room is to be insulated from the inside.
With the u-value calculation I arrive at:

Under these conditions, no mold growth should be expected.

Feuchteschutz
During the winter condensation period of 90 days, a total of 0.175 kg of condensate water per square meter occurs in this component. This amount dries out within 81 days in the summer (evaporation period according to DIN 4108-3:2014-11).


# Material sd-value Condensate water Drying period Weight
m kg/m² % days kg/m²
1 1.5 cm Knauf Rotband bonding plaster gypsum 0.08 - 0.0 14.3
2 5 cm rigid foam, XPS 4.00 0.15 8.6 1.8


... on outside 0.15
3 1.5 cm lime cement plaster 0.52 0.17 0.6 27.0


... on inside 0.15 70


... on outside 0.023
4 25 cm reinforced concrete (1%) 32.50 0.026 0.0 575.0


... on inside 0.023 81


... on outside 0.0024
5 0.3 cm bitumen thick coating 150.00 - 0.0 3.2


... on inside 0.0024 42
33.3 cm entire component 187.10 0.18 81 621.2



The calculation on the internet says: "No mold is to be expected."
However, condensation water does occur after all, and that makes me uncertain now.
Is it okay as it is done as described above?
I don’t want to have to tear everything down again...

Interior insulation is always rubbish and in my opinion XPS is the worst thing you can do. It looks good in the calculator because it is extremely tight (so the calculation works out), but in practice it is not feasible. In addition, you have to heat significantly more (warmer) to feel warm. It is better to use something diffusion-open and ventilate regularly.

Hm... that sounds like it would rather be inadvisable...?

Interior insulation is not always nonsense, and sometimes unavoidable. It is important that the insulation is not bypassed, meaning no air gets behind it. This would be ensured, for example, by full-surface adhesion (instead of patch technique). But what are 2.5 cm of insulation supposed to achieve? The costs and effort are clearly greater than any benefit. We have 6 cm of interior insulation above, in the timber construction (flexible rock wool between battens, above that a climate membrane and Rigidur boards). Exterior insulation was not possible in the required thickness, as we have no roof overhang. Yes, the rooms become narrower with interior insulation, but 2.5 cm really brings nothing.

Well.. at least it's 5 cm of XPS, and in a room that is 3.5 x 4.5 meters in area, the "reduction" hardly matters. Excavating outside is just far more labor-intensive.

The only thing... the mold worries me with the XPS

On the lower floor, we have composite panels on the interior side of the exterior walls. So Styrofoam and drywall. There was no mold anywhere. Mold can only develop if the insulation is bypassed. Otherwise, the wall is warmer than without insulation. The 5 cm is correct, I overlooked that item 2 is in front of it and thought there was still a comma. Yes, 5 cm is definitely better.