Offer KfW70 house - KfW55 house already with additional controlled residential ventilation - Is KfW70 too airtight?

Hello,

I have received the attached offer from a developer.
It is a Kfw70 house with a gas boiler.

To achieve Kfw55, it is enough to either install a controlled residential ventilation system instead of the gas boiler for an additional €12,400 or even just a decentralized ventilation system with heat recovery for only the kitchen/living room and bathroom for €6,500 (meaning the bedrooms would have no ventilation).

Now I am wondering: isn’t the KfW70 offer too airtight/tightly sealed to operate without ventilation?

If KfW70 is sufficient for me, where can I save, or where do I need less insulation?
We want to use brick cladding and mineral wool instead of Styrofoam panels.

Can the perforated bricks be thinner? Use simple bricks instead of perforated bricks?
Use thinner mineral wool?

Many thanks for your answers.

After various consultations with the general contractor, another construction company, and an advisor from the consumer center, it seems that according to the current standard Energy Saving Ordinance 16, the insulation requirements already come very close to KfW55, so that sufficient insulation is directly offered by the contractors.
The insulation should therefore not be altered, except to increase it further.

To achieve KfW55, however, something must also be done to the building services technology, hence the optional prices in the above offer.

Furthermore, the necessity of a ventilation system is a major point of disagreement.
Here, I have now heard various statements:

- The house is too tightly insulated; it will develop mold without ventilation
- Either ventilate manually or use controlled residential ventilation
- Controlled residential ventilation is purely a comfort feature, otherwise ventilate manually
- With manual ventilation, 2x per day is sufficient
- With manual ventilation, at least 5x, better 7x per day
- Moisture mainly occurs in the first year after construction completion (construction moisture), then it decreases significantly (especially pay close attention to ventilation here)
- Later, mold can only develop where moisture occurs, and moisture only occurs where people spend time (sweating, cooking, laundry) ==> if no one is home during the day, no ventilation is necessary, so morning and evening are sufficient
- Ventilation systems produce air that is too dry

For me that means:
- either use an enthalpy heat exchanger in the hope that it does not cause germ problems
- many plants in the hope that it does not become a jungle and surface mold on the soil and fungus gnats can be avoided
- dimension the ventilation system smaller or operate it at a lower rate from the start to avoid becoming too dry
- manual ventilation 2x per day = 2x air exchange per day
- the controlled residential ventilation does not necessarily have to achieve 12x air exchange per day

Houses should be built airtight - regardless of the energy standard. This results in insufficient infiltration and poor (and possibly too humid) air. Mold is just the worst case. Controlled residential ventilation with heat recovery is highly recommended.

What is airtight? The energy consultant said it should be around 0.6. My general contractor says that the blower door test usually results in something like 1-1.5. The consultant said it is usually due to missing/insufficient plaster in the area of wall joints and (electrical) installations in the exterior walls. He says that if someone pays meticulous attention to ensuring everything is properly plastered, the airtightness would already be much better. But I can’t watch over the plasterer’s shoulder all day.

Hieraus folgt eine zu geringe Infiltration und schlechte (und möglicherweise zu feuchte) Luft.

How does bad air arise? From the people present. How does moisture develop? From the people present and from the construction moisture in the first year.

Bad air also arises from stagnant air and emissions. We lived in 2 apartments that did not have [Kontrollierte-Wohnraumlüftung] and were relatively airtight. After work, the air was extremely unpleasant.. without anyone being there. Moisture was indeed only from people.

To achieve KfW55, it is sufficient to either install a controlled residential ventilation system for an additional 12,400€ instead of the gas boiler or even just a decentralized ventilation with heat recovery for only the kitchen/living room and bathroom for only 6,500€ (meaning the bedrooms would have no ventilation).

If it is true that someone wants to sell you a house not with heating but only with a controlled residential ventilation system and that even for an additional 12,400 euros, you should ask yourself whether you are talking to the right providers.