ᐅ How to Achieve KfW40 Standard? Energy Systems and Cost Efficiency

With Ytong blocks, you can achieve KfW55 energy efficiency with a 36.5cm (14.4 inches) thickness when calculating creatively ("With detailed computational proof, 36.5 cm Ytong PP 2-0.35 lambda = 0.08 W/(mK) is possible for KfW Efficiency House 55 funding."), and with 42.5cm (16.7 inches) even without calculation tricks. KfW40 also seems possible monolithically if you offset that with photovoltaics and increase insulation elsewhere. Otherwise, a 48cm (18.9 inches) block is used. However, Ytong is not just one product; the website lists the editions ThermStandard, ThermSuper, and ThermUltra...

In terms of cost, there doesn’t seem to be a clear trend whether monolithic construction or external thermal insulation composite systems (ETICS) are cheaper. At least, I haven’t found any definitive information online. Personally, I prefer not to have polystyrene foam attached, and the higher-quality insulation materials also come at a higher price.

user386dx schrieb:

Hello, this is a roof-integrated system, which makes it more expensive, more complex (cooling & under-roof layer), causes higher efficiency losses, and last but not least: warranty issues.

Roof-integrated: yes
More expensive: yes (this is the only real disadvantage)
More complex: no – no separate cooling required, no special under-roof membrane needed
Efficiency losses: moderate (not really a disadvantage if the system is designed according to the demand)
Warranty: unproblematic, as the handover between trades is very well managed.

There are advantages as well:

• simply an attractive roof
• only low voltage on the roof (<120V)
• no added fire load
• reduced electrical fields (if that is considered relevant)
• flexible planning and good space utilization due to smaller module sizes (irrelevant for us, but can contribute to more architectural freedom — positioning dormers, windows, choice of roof shape…)
• hardly sensitive to shading (parallel connection instead of series connection; irrelevant for us)

If you consider the photovoltaic system purely as an investment, it usually doesn’t pay off (unless you assume a higher added value to the property).
If you consider aesthetics, you simply invest – much like with flooring – in the look and enjoyment. Technically, the system is mature, so there is no need for concerns.

For the next house, I would choose this option again and understand anyone who prefers mounting large panels on top of the roof. Either way, photovoltaic systems on roofs are a good contribution.

The original poster should form their own opinion. It’s always a pity when people say, “If I had known this existed, then…”

As a note: Our architect and energy consultant (who mainly designs KfW40/passive houses) once said that most KfW40 houses are timber frame constructions because this building method makes it relatively easy to achieve the required values with appropriate insulation without extremely thick walls.

For comfort: My sister and her husband built a log house (I believe with 14cm (5.5 inches) thick logs) more than 20 years ago. That was sufficient energy-wise at the time. Always a great indoor climate – the best I have ever experienced or felt. Years later, my parents built another log house next to it with logs that were 2 to 4cm (0.8 to 1.6 inches) thicker. Again, a great indoor climate. A few years ago, both houses were retrofitted with 8cm (3 inches) wood fiber insulation boards plus cladding. This was also done to get rid of the old ugly (too dark) paint. Sanding would have been more expensive than adding insulation 😎

nordanney schrieb:

Quite simple: the maximum feed-in is limited to 70% of the nominal output.

Ok, thanks!
Do you also know if, for example, when applying for KfW40Plus, you are not allowed to claim other subsidies, such as storage grants (in our case, the state of Saxony)?

superzapp schrieb:

As a side note: Our architect and energy consultant (who mainly builds KfW40/passive houses) once said that most KfW40 houses are timber frame constructions because this building method makes it relatively easy to achieve the required standards with suitable insulation without extremely thick walls.

This is often said, and at first glance, it’s easy to believe that stuffing slightly thicker mineral wool between timber studs is simpler than installing an external insulated render system (EIFS) or filling masonry with insulation. On the other hand, switching to a 6cm (2.4 inch) thicker aerated concrete block isn’t exactly groundbreaking on site either. And timber stud walls for KfW40 have about 40cm (16 inches) thickness as well. I think this is more of a story by prefabricated builders to differentiate themselves from those using solid construction.

WilderSueden schrieb:

This is often said, and at first glance, it seems easier to just stuff slightly thicker mineral wool between the wooden studs than to install an external thermal insulation composite system (ETICS) or fill bricks with insulation wool. On the other hand, switching to a 6cm (2.5 inches) thicker aerated concrete block (Ytong) isn’t exactly groundbreaking on the construction site. And timber frame walls in KfW40 standards are about 40cm (16 inches) thick anyway. I think this is more a story from prefab builders trying to distinguish themselves from solid construction builders.

We are building with "standard" 36.5cm (14.5 inches) solid brick walls monolithically, and according to the energy consultant’s binding statement, it already complies with KfW40 Plus. There are simply too many parameters and adjustable factors that can influence the outcome. Any generalization can be inaccurate.