Condensation water window UNILUX new construction

Hello, we have a huge problem.
We have been living in the new Kfw55 house since early summer, and now on all windows there is pronounced condensation forming on the lower part of the glass and frame with wet joints/corners on the windowsill (unfortunately made of wood -> mold), room temperature approx. 23°C, relative humidity approx. 55%, decentralized ventilation system. The blower door test was good, so everything is airtight. The installation looked professional to me, in the timber-frame house everything fits very precisely, sealing with self-expanding foam tape, Styrofoam insulation wedges under the window at the bottom.

On Monday, the architect will come with the window manufacturer. But perhaps someone from the forum can already say something or has had similar experiences. I think in a new build with new windows something like this should not happen.

Unilux plastic windows, "ISO STAR"
Profile depth 88 mm with six profile chambers, frame 76 mm, sash 78 mm
ISO thermal insulation glass Ug 0.5 - glass structure: ISO 4-18-4b-18-4, 26 mm
Warm edge
Thermal transmittance value 0.76 W/m²K

at all windows in the lower part of glass and frame pronounced condensation with wet joints/corners on the windowsill (unfortunately made of wood -> mold), RT about 23°C, rRF about 55%, decentralized ventilation system. Blower door test was good, so everything is tight.

Exactly, that's why. The window is the weakest link. This is where the biggest temperature differences occur and cause the indoor humidity to condense on the cold glass.

I find 55% too high, considering you have a ventilation system. Is it possibly not set correctly? You probably have to ventilate additionally so that your moisture can get out. Do you have a lot of construction moisture inside despite the timber construction from plaster or screed? In any case, your body perspiration as well as showering and cooking habits make the air moist every day, that has to get out of the house.

There is a risk of mold from about 70% relative humidity.

I'm really sorry for you.

Ventilation systems with heat recovery, but without enthalpy, would have to reduce the humidity in a occupied household to about 25% in a well-insulated house and current subzero temperatures.

The cold air contains very little water vapor. The warm air can absorb significantly more water vapor. Example: when 1m³ of air is heated from -5°C and 90% to 20°C, the relative humidity in the same m³ without kitchen, shower, breathing, or plant influences is only 17.6%.

55% in winter, something is not right. Could this possibly be happening in a room with ventilation congestion and the volume flow of the ventilation system is set too low?

The problem exists throughout the entire house. A total of 8 Marley heat exchangers are installed, 4 on the ground floor, 4 on the upper floor. They mostly run on low level (1 of 3), in the utility room on 2, because at level 2 the noise is already annoying. According to the manufacturer, the air exchange at level 1 is 16 m³ per hour.

There should no longer be any construction moisture. The windows were open all summer and apart from screed, there was no wet material here.

55% in winter, something doesn't add up.

Might be unusually high if you assume constant ventilation, but at the same time not too high, rather very comfortable for humans especially in winter. I would agree with ypg there. Yes, unusually high considering there is a controlled residential ventilation system, but not worryingly high. Water then condenses on the window and needs to be removed, otherwise of course the wood will rot.

Here is a comparison: in the evening, we have 55% humidity in our living kitchen (just under 60m²), WITHOUT a ventilation system (2 persons).
Windows 4-12-4-12-4 mm / Ug value: 0.7 W/m², no signs of moisture in the corners at all.
However, only KfW70. I don’t know how much that matters. Move-in 08.2015.

I agree with Alex, 55% is good, but relatively high with controlled residential ventilation.

Even in the bedroom (65%-70% RH in the morning, 13m²), with venetian blinds in the window corners, no moisture. Here the window is 6-12-4-12-8 / Ug 0.7 W/m² K

Good luck tomorrow and not the usual "You ventilate incorrectly"

Best regards
Heiko