ᐅ KfW55 compared to KfW70

We can choose whether we want KFW55 standard. We just need to install a photovoltaic system on the roof that generates 2000 kWh. If the insulation is done properly, it really comes down to the technology. Insulation alone probably won't be enough.

Good morning,

vst85 schrieb:

Why do you think it usually does not apply in most cases?

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

In post #3 you briefly stated, “if your house is designed to KfW 70, for example, a ventilation system (decentralized and more affordable) is enough to reach KfW 55” without explaining what “designed for KfW 70” means in the context of your building project. From my experience, it is not enough to simply add a ventilation system to a monolithic 36.5cm (14.4 inches) or external insulation composite system (ETICS or EIFS) wall with a thickness of 17.5cm (6.9 inches). It usually also requires insulation below the slab as well as a filled or thicker masonry unit and, of course, the technology. The step from building to KfW 70 to KfW 55 is a combination of various measures, including both insulation and technology.

Because you omitted substantial important details about your building project, I considered your statement bold. It could mislead inexperienced laypeople into assuming that, for example, a 36.5cm (14.4 inches) masonry wall plus a ventilation system would be sufficient to achieve the KfW 55 efficiency house standard.

Regards, Bauexperte

I have to agree with you, I should have provided more information.
Exterior wall: 17.5 cm (7 inches) red brick + 10 cm (4 inches) insulation + facing brick
Interior walls: sand-lime brick
The foundation slab is not insulated from below
Solar system to support hot water
Decentralized ventilation units with heat recovery

Originally, the plan was for KfW 70 standard, but our architect advised us to install the ventilation units to achieve KfW 55. If I remember correctly, this also allowed us to reduce the thickness of the insulation.

Regards.

vst85 schrieb:
Solar for domestic hot water support

With a gas condensing boiler? And then KfW55?

They were not very willing to calculate the exact additional costs for us, but it was supposedly significant, so we are only building to KfW70 standard. (Calcareous sandstone + 14 cm (5.5 inches) external thermal insulation composite system, air-to-water heat pump, controlled mechanical ventilation with heat recovery, ..., Without considering the controlled mechanical ventilation, a thermal transmittance (HT' existing) of 0.318 W/m²K (0.055 Btu/(h·ft²·°F)) and a specific primary energy demand (QP'' existing) of 51.3 kWh/m²a (16,100 Btu/ft²·yr) were calculated. This corresponds to KfW70. Unfortunately, I do not have a calculation that includes controlled mechanical ventilation.)

Hello,

Thank you for the additional information!

vst85 schrieb:

Exterior wall: 17.5 cm (7 inches) red brick + 10 cm (4 inches) insulation + facing brick
Interior walls: calcium silicate brick
The floor slab is not insulated from below
Solar system for hot water support
Decentralized ventilation units with heat recovery

Originally, we planned for KfW 70, but our architect advised installing the ventilation units 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 wrong, but I find it hard to believe that the measures mentioned—especially the gas condensing boiler—are sufficient to achieve the KfW 55 energy efficiency standard. Even the colleague with the red competitor and Nibe® system, and a comparable wall structure, struggled to meet his promises; ask various experts and they will confirm this to you. I suspect this is one reason why he now generally bundles in a small photovoltaic system.

If my gut feeling is correct, have the architect confirm that the measures listed will indeed achieve KfW 55. If not, and you are required to document compliance to KfW, this could become quite costly.

Best regards, Bauexperte

We are building to KfW 55 standards, which has also been approved by the KfW Bank. I hope they have thoroughly reviewed everything. Roofing work will start on Friday, so we are already fully underway.

Your skepticism makes me think. As a layperson, I cannot judge whether the energy performance certificate is correctly prepared; I can only hope it is.

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

Best regards.