ᐅ KfW 40 (plus) standard for a multi-family house cannot be achieved
Created on: 1 Sep 2019 16:57
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curverbox
Hello everyone,
I am planning to build a five-family house. Since the plot is very narrow but long, the planned house will be 20.415m (67 feet) long and only 7.24m (24 feet) wide, which are the maximum dimensions approved by the building authority / planning permission.
The house will have two full stories and a 45° pitched roof. The gables will face southwest / northeast. The exterior walls will be a cavity wall construction with a thickness of 49cm (19 inches), consisting of 17.5cm (7 inches) Ytong blocks, 12cm (5 inches) PUR insulation with a 0.24 air gap, and 11.5cm (5 inches) facing brickwork. There will be no basement. All technical specifications meet the requirements of a KfW55-standard house, including a ground-source heat pump, underfloor heating, decentralized ventilation systems, photovoltaic panels with battery storage, etc.
Nevertheless, according to calculations by my energy consultant, the house only reaches KfW55 standard because of the unfavorable length-to-width ratio, and she has reached the limits of her expertise.
Is this a calculation error or is it really not possible?
I am really at my wit’s end...
I am planning to build a five-family house. Since the plot is very narrow but long, the planned house will be 20.415m (67 feet) long and only 7.24m (24 feet) wide, which are the maximum dimensions approved by the building authority / planning permission.
The house will have two full stories and a 45° pitched roof. The gables will face southwest / northeast. The exterior walls will be a cavity wall construction with a thickness of 49cm (19 inches), consisting of 17.5cm (7 inches) Ytong blocks, 12cm (5 inches) PUR insulation with a 0.24 air gap, and 11.5cm (5 inches) facing brickwork. There will be no basement. All technical specifications meet the requirements of a KfW55-standard house, including a ground-source heat pump, underfloor heating, decentralized ventilation systems, photovoltaic panels with battery storage, etc.
Nevertheless, according to calculations by my energy consultant, the house only reaches KfW55 standard because of the unfavorable length-to-width ratio, and she has reached the limits of her expertise.
Is this a calculation error or is it really not possible?
I am really at my wit’s end...
curverbox schrieb:
Each building component complies with the U-value.
Annual primary energy demand QP / QP,Anf is: 2.7 Reference Energy Saving Ordinance 41.22) kWh/(m²·a)
Transmission heat loss H'T is: 0.192 Reference Energy Saving Ordinance 0.3523 W/(m²·K) So what exactly is your problem with a deviation of 0.0022? Just perform a detailed thermal bridge calculation, and that’s it.
curverbox schrieb:
Unfortunately, you are mistaken. The property is a double plot and already has a house from the 1950s built on it, which is already in my ownership. Next to it, there is still space available of the specified size. Since there is no development plan, new buildings must conform in size to the existing structures, according to the building authority. I’m glad if my concerns about you potentially being misled by real estate sales hype turn out to be wrong. However, my doubts about the naivety of the concept remain (I hope this is understandable). What purpose does brick cladding serve for a rental property of this unit size? It’s neither sought after nor relevant for speeding up the turnaround of a unit on the market—unless we’re talking about places like Bogenhausen or Sylt.
https://www.instagram.com/11antgmxde/
https://www.linkedin.com/company/bauen-jetzt/
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curverbox2 Sep 2019 20:56ypg schrieb:
May I ask why you insist on achieving KfW 40 or 40+ with such difficulty and stress?
I know several tenants... and all of them, without exception, can hardly make use of noisy ventilation systems and/or super-insulated walls and/or underfloor heating alone. In those cases, the window is tilted open all day and the ventilation is throttled... Because it only costs me slightly more, and therefore I...
11ant schrieb:
I am very glad if I am wrong with my concerns about you being a victim of real estate marketing hype. My doubts about the naivety of the design remain (hopefully understandable). What do you want with brick cladding on a rental property of this size? It is neither required nor relevant for selling a residential unit faster in the market—unless maybe we are talking about Bogenhausen or Sylt. In our region of East Frisia, this is the standard construction method; here, 95% of all houses are built with brick cladding.
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curverbox2 Sep 2019 20:57guckuck2 schrieb:
That's ridiculous. A few centimeters (inches) more of perimeter insulation or a modern window profile and you'll be over the limit if they don’t calculate it smaller. Today, I took the time to download EVEBI as a demo version and actually managed to do it. It’s a razor-thin margin with a 0.03 thermal bridge allowance, but it’s possible.
curverbox schrieb:
Each building component corresponds to a U-value
Annual primary energy demand QP / QP,Anf is: 2.7 Reference Energy Saving Ordinance 41.22) kWh/(m²a)
Transmission heat loss H'T is: 0.192 Reference Energy Saving Ordinance 0.3523 W/(m²K) What U-value does each component meet? Does each component comply with the KfW40 requirements according to the individual component method? Then great! You can skip the Q and H'T calculations and use the alternative method. I’ve already provided the source of information—you can also share this with your energy consultant for your next house.
Otherwise, the solution for a small improvement has already been mentioned. But it’s quite an unusual approach to improve a different building component, where there’s more potential for enhancement, instead of increasing the insulation thickness in the walls.
PUR foam behind the brick cladding is actually unnecessarily expensive. An 8cm (3 inches) air gap is also about 4cm (1.5 inches) too wide. Alternatively, you could use 0.032 mineral wool, which also allows for a thinner build-up.