KfW55 versus KfW70

We can choose whether we want KFW55. We just have to install 2000kwh of photovoltaics on the roof. If the insulation is proper, it really only depends on the technology. It probably doesn't work with insulation alone.

Good morning,

Why do you think it is not true in most cases?

I assume you are here on the forum searching for information on the topic of house construction; maybe you are already specialized. Like you, most users here are looking for info on this or that topic. For them to find what they are looking for, it requires that responding users provide complete information to the questioners.

In post #3 you write shortly, "if your house is designed according to Kfw 70, for example a ventilation system (decentralized is more cost-effective) is enough to reach KFW 55" without going into detail about what "designed according to KfW 70" means in your building project. From my experience, it is not sufficient to just equip a monolithic 36.5 cm or ETICS-clad masonry with a thickness of 17.5 cm with a ventilation system. Usually, this also includes insulation below the floor slab as well as a filled or stronger brick and, of course, the technology. The leap from building according to KfW 70 to KfW 55 is a mix of several measures, consisting of insulation and technology.

Because you have omitted substantially important statements about your building project, I have called your answer bold. It could mislead unexperienced laypeople to assume, for example, that a 36.5 cm masonry + ventilation system is sufficient to achieve the efficiency house standard KfW 55.

Rhenish regards

I have to agree with you, I should have provided more information.
Exterior wall 17.5 cm red brick + 10 cm insulation + clinker brick
Interior walls calcium silicate brick
Floor slab is not insulated from below
Solar for hot water support
Decentralized fans with heat recovery

Originally, KFW 70 was planned, our architect advised us to install the fans to reach KFW 55. If I remember correctly, we were also able to reduce the thickness of the insulation because of this.

Regards.

Solar for domestic hot water support

With gas condensing boiler? And then KfW55? They didn't really want to calculate the exact additional costs for us, but it was supposedly significant, so we are building "only" KfW70. (Sand-lime brick + 14 cm ETICS, air-water heat pump, controlled residential ventilation + heat recovery, ..., Without considering the controlled residential ventilation, HT'current = 0.318 W/m²K and QP''current = 51.3 kWh/m²a were calculated. That would be KfW70. Unfortunately, I do not have a calculation considering the controlled residential ventilation.)

Hello,

Thank you for the additional information!

Exterior wall 17.5 cm red brick + 10 cm insulation + clinker
Interior walls made of sand-lime brick
Floor slab is not insulated from below
Solar for hot water support
Decentralized fans with heat recovery

Initially, KFW 70 was planned, but our architect advised us to install the fans to achieve KFW 55. If I remember correctly, we were also able to reduce the thickness of the insulation as a result.

I may be mistaken... but I can't imagine that the aforementioned measures – especially the gas condensing boiler – are sufficient to achieve the KfW 55 energy-efficient house status. Even my colleague from the competitor company with his Nibe® and comparable wall construction had a hard time meeting his promises; ask various experts and they will confirm this to you. I suspect that this is one reason why he now generally sells a small photovoltaic system together with it.

Have your architect confirm to you, in case my gut feeling is right, that the aforementioned measures actually achieve KfW 55. If not, and you are also required to provide proof to the KfW, this can get quite expensive.

Rhenish regards

we are building according to KFW 55, this was also approved by the KFW Bank. I hope they have taken a close look at everything. On Friday, the roof work begins, so we are already fully underway.

Your skepticism makes me thoughtful, but as a layperson, I cannot judge whether the energy certificate was created correctly, I can only hope so.

For the exterior wall, we are using a hollow brick with a lambda value of 0.18.

Best regards.