Kfw 70 with a gas heating system without solar thermal is very difficult (not sensible?) to achieve. A standard prefab house builder will probably not manage that and above all will have little desire to do so. The building must be very well insulated and also meet the standard of a good passive house (orientation, planning, etc.). Our house will significantly undershoot the insulation values of a KFW 55 house. However, due to the gas heating, KFW 70 is still not achievable without solar thermal. We do not have a controlled residential ventilation system, but according to our energy consultant, that would not have been sufficient instead of the solar system anyway. The solar system is now installed for environmental protection. Economically it certainly isn’t (the controlled residential ventilation system isn’t either, by the way). By the way, I also find Lexmaul’s consideration good. The insulation of buildings will indeed reach a peak at some point. But considering that over 50% of the total waste production in Germany comes from the construction sector, I think one should definitely think about what materials are used in one’s house! The energy consumption of the construction industry is also immense, and a lot can be saved there as well. I am not sure whether it is the right path to heavily insulate buildings with resource-consuming and waste-intensive building materials, only to face the problem in a few decades of where to dispose of all the hazardous waste. For that reason, I advocate that in the discussion about KFW, one should not only talk about the wallet but also about the environment: because that is what this funding should really be about, isn’t it?!