ᐅ Is aerated concrete with a density class of 30 sufficient for a KfW 70 energy-efficiency house?

At the beginning of February, we plan to start building our single-family house (solid masonry). It should meet the KfW 70 standard. Our builder has proposed using 30cm (12 inches) aerated concrete blocks for the exterior walls, built in a "monolithic construction" method, plus a two-layer mineral plaster with a thickness of 15mm (0.6 inches). We have no expertise in this area and are wondering whether this wall structure can meet the KfW 70 standard. The blocks themselves do not have a particularly good U-value.

Ask to see the thermal insulation calculation; it clearly states whether the wall construction meets the requirements for a KfW70 house.
However, it is not only the exterior walls that determine the thermal performance—windows, roof, and basement also play a role. This means a less effective exterior wall can sometimes still be justified for a KfW70 standard.
If you want to be absolutely certain, have the energy saving regulation calculation reviewed by an independent energy consultant. This involves a fee, but it gives you real peace of mind.

When I asked about the thermal insulation calculation, my builder just shrugged and referred to his "many years of experience." He said it would probably be "good enough"... I will spend the extra money and hire an energy consultant to review it.

Hello again,
the builder must provide proof, as required by the Energy Saving Ordinance – which has the force of law. In addition, the KfW increasingly checks compliance with the conditions for individual funding measures. If the calculations are incorrect, the funding will be revoked and – if things go badly – a penalty may be imposed.

That's exactly right; if serious mistakes are made here, it will cost a lot of money. The energy saving regulation must be properly followed. I would not accept anything else.

I also find what some developers get away with quite shocking. They just shrug their shoulders and say, "It will be fine." Then the client has to cover the additional costs. But they don’t care about that at all.