So 6500 bucks for a decentralized ventilation system is pretty confident. Those are a few holes in the outer wall plus 300-600 € each for the fan and a cable.
The extra cost for an air-water heat pump together with a controlled residential ventilation system (the LWZ is this combined system?!) doesn’t seem totally overpriced to me. But you will have higher heating costs than with the gas heating.
Is that little subsidy for KFW 55 still worth it?
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 between 1 and 1.5.
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Our Blower-Door test showed 0.3... we also have no ventilation system and ventilate manually. After one year, no mold has been discovered yet... waiting and see. Acquaintances built in 2011 with the same construction company and masonry structure. They achieved Kfw 55... also window ventilation. No mold so far...
Ventilation usually only takes place in the morning after getting up and in the evening shortly before going to bed (but then only in the bedrooms)...