KFN house without ventilation system - Experiences

This is something special for us: We don't have a bedroom, but a "multi-purpose room" that more than lives up to its name.

But precisely for this reason, you should reconsider your answers when someone asks. You cannot generalize from yourself to others and then keep your response short like

In our house, completed in 2019, a ventilation system would be pure waste of money without any added value.

But that's exactly why you should reconsider your answers when someone asks.

The phrasing "you should ... reconsider" sounds like kindergarten pedagogy. But OK: a mechanical ventilation system is a good investment in many houses, but by no means a "must-have feature." The OP's question cannot be answered with a blanket statement due to the limited information. Besides my house, there are plenty of other examples. Better?

Maybe one should simply read through the funding conditions of the kfw. I quote:

A ventilation concept must be created specifying the required outdoor air volume flow and the solution for implementation, for example by applying DIN 1946‑6. The responsibility for initiating the implementation of ventilation engineering measures lies with the builder.

VV - good luck achieving that without controlled residential ventilation. No hints like "I don't need that because I live in a clay bunker with 300 cubic meters of room volume" or "for me, that's money thrown away" will help.

DIN 1946-6 clearly states: user-independent, so no "I'll just open the windows now and then"...

There are enough other examples besides my house.

Which would also have to be explained then

Better?

Kindergarten level is never good for adults. That also includes debating terminology.

The DIN 1946-6 clearly says: user-independent, so no just opening the windows now and then...

A window rebate ventilation meets both the criteria of DIN 1946-6 and can be used in the construction of a KfW 40 house. A mechanical ventilation system is not the only solution. This at least answers the first part of the OP's question: yes, it is possible without mechanical ventilation. Maybe someone who lives in a house with window rebate ventilation can report.

But OK: Mechanical ventilation is a good investment in many houses, but by no means a "must-have feature".

Let's define "many": 98% sensible, 2% exceptions with special building physics conditions etc. For the standard house with standard use, a "must".

A window rebate ventilation meets the criteria of DIN 1946-6 and can be used in the construction of a KfW 40 house. Mechanical ventilation is not the only solution.

A window rebate ventilation IS mechanical ventilation. Only manual ventilation is not mechanical ventilation. You have to build window rebate ventilation and KfW 40 with a lot of effort. You cannot ventilate a standard KfW 40 house like this, otherwise it is no longer a KfW 40 house.