20% humidity in KFW 40+ new construction

You still owe an answer about what is stated in your ventilation concept regarding nominal ventilation and why you believe you know better than the experts and think twice is sufficient instead ;)
Sorry, but in that respect, you are like my girlfriend. Not engaged with the basics, no desire to do so, but completely convinced of her own view

Regarding nominal ventilation, it says something about 0.5. You are probably right in theory. But apparently theory and practice differ greatly there. As often in life.
Why do I believe twice is sufficient? From my own experience (we have been living very well like this for 1.5 years now) and from others. I dealt with it before our construction, but not theoretically, rather practically.
And since we are on comparisons: You are basically the designer who goes from school to university and then thinks he can design something. Sometimes it doesn’t work out in practice then ;).

I always like to compare the discussion about [Kontrollierte-Wohnraumlüftung] with a washing machine. Of course, you can also do without one. It may even be cheaper, especially if you also have a dryer. But I have one and am glad about it.

Washing machine might be a bit exaggerated. But like microwave (which I don’t have, by the way), water or rice cooker (which I wouldn’t want to miss) maybe. You really don’t need it, but it’s super practical, saves time, and once you’ve had it for a while, you don’t want to be without it anymore.

I'll throw in two things:

- I doubt that an EWT only accounts for 5%. Probably (especially with your extreme values) you're easily at 10% or more. Our EWT works excellently. Another advantage: With EWT, the controlled residential ventilation also works up to around -7° outdoor temperature.

So don't think about whether it's sensible in the long term, but: Controlled residential ventilation always only with EWT. Buy immediately.

- The argument controlled residential ventilation vs. window ventilation always comes down to: "I ventilate manually less than the ventilation concept requires, and therefore I have wonderful humidity and no problems." Okay. You can do that. But then I can simply set the controlled residential ventilation to 1/5 of normal performance and achieve the same. You can do that—probably better than window ventilation, because at least this way no humidity peaks occur. If problems arise: your own fault. If not: great. But that has nothing to do with the ventilation method itself...

Does everyone here really have an EWT who has a controlled residential ventilation system? Or is it only with kfw40+ that the low humidity is so extreme? We now have 9 hygrometers distributed in the house. Humidity 2x% except in the bathroom, where it is 3x%. In the bedroom with a humidifier, we average between 41% and 45%.

We don’t have an EWT because it’s decentralized. Set to about 5-6 air changes per day, plus the exhaust fans in the bathroom operate based on sensors. They are also KfW40, solid with Ytong. Move-in on February 11. Current humidity in the study is 56%. As a rule, I still ventilate 2-3 times a day through the windows to get the construction moisture out. Today it’s raining sideways, so I’d rather keep the windows closed ;). It’s a totally upside-down world compared to you.