ᐅ Offer for KfW 70 house – KfW 55 with additional controlled residential ventilation – is a KfW 70 too airtight?

Hello,

I received the attached offer from a developer.
It is a KfW 70 house with a gas boiler.

To achieve KfW 55, it is sufficient to install either a controlled residential ventilation system for an additional €12,400 (about USD equivalent) instead of the gas boiler, or even just a decentralized ventilation system with heat recovery for only the kitchen/living room and bathroom for €6,500 (about USD equivalent) — meaning the bedrooms would have no ventilation.

Now I am wondering: wouldn’t the KfW 70 offer be too airtight or sealed to operate properly without any ventilation?

If KfW 70 is enough for me, where can I save costs or where can I reduce insulation?
We want to use brick cladding and not polystyrene panels, but mineral wool instead.

Can the hollow bricks be made thinner? Can simple bricks be used instead of hollow bricks?
Can thinner mineral wool be used?

Thank you very much for your answers.

After several discussions with the general contractor, another construction company, and an advisor from the consumer protection agency, it seems that according to the current Energy Saving Ordinance 16 standards, the insulation requirements are already very close to KfW55 level. Therefore, contractors typically offer sufficient insulation as standard.
So, the insulation itself is not really up for debate, except if you want to add even more.

To achieve KfW55, however, some improvements to the building services (mechanical and electrical installations) are also necessary, which explains the optional prices in the above offer.

Another major point of disagreement is the requirement for a ventilation system.
I have now heard various opinions on this:

- The house is too tightly insulated and will develop mold without ventilation
- Either ventilate manually or use a mechanical ventilation system with heat recovery (controlled residential ventilation)
- Mechanical ventilation is purely for comfort; otherwise, manual ventilation suffices
- If ventilating manually, twice a day is enough
- If ventilating manually, at least 5 times, preferably 7 times per day
- Moisture mainly arises during the first year after construction completion (construction moisture), then it decreases significantly (so especially important to ensure good ventilation during this period)
- Mold can only form where moisture accumulates, and moisture mainly occurs where people spend time (from sweating, cooking, laundry) ⇒ if no one is home during the day, ventilation is not necessary then, so morning and evening ventilation is sufficient
- Ventilation systems create air that is too dry

For me, this means:
- Either use an enthalpy heat exchanger in the hope it does not cause microbial issues
- Have many plants indoors, hoping it does not turn into a jungle and you avoid surface mold on the soil and fungus gnats
- Design or operate the mechanical ventilation system at a smaller capacity or lower speed to avoid overly dry air
- Manual ventilation twice a day equals two air exchanges per day
- Controlled residential ventilation does not necessarily need to provide 12 air exchanges per day

Houses should be built airtight—no matter the energy standard. This results in low infiltration and poor (and possibly too humid) air quality. Mold is the worst-case scenario.
Controlled mechanical ventilation with heat recovery is highly recommended.

What is airtightness?
The energy consultant said it should be around 0.6.
My general contractor says that the blower door test usually results in values around 1 to 1.5.

The consultant mentioned this is mostly due to missing or insufficient plastering around wall joints and (electrical) installations in the exterior walls.
He believes that if someone pays close attention to ensuring everything is properly plastered, the airtightness would significantly improve.
However, I can’t watch the plasterer all day.

BeHaElJa schrieb:

This results in insufficient infiltration and poor (and possibly too humid) air.

How does poor air quality develop? From the people present.
How does moisture develop? From the people present and from construction moisture during the first year.

Poor air quality also arises from stagnant air and emissions. We lived in two apartments without mechanical ventilation systems that were relatively airtight. After work, the air was extremely unpleasant, even when no one was present.
The only source of moisture was actually people.

Kaspatoo schrieb:


To achieve KfW55 standard, it is enough to either install a controlled residential ventilation system instead of the gas boiler for an additional cost of €12,400 (approximately $13,500), or even just install a decentralized ventilation system with heat recovery for only the kitchen/living room and bathroom for €6,500 (about $7,100) — meaning the bedrooms would have no ventilation.

If it is true that someone is trying to sell you a house without a heating system but only with a controlled residential ventilation system, and charging you an extra €12,400, you should question whether you are dealing with the right providers.