ᐅ KfW 40 (plus) standard for a multi-family house cannot be achieved
Created on: 1 Sep 2019 16:57
C
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...
C
curverbox1 Sep 2019 19:56Pinky0301 schrieb:
Are thermal bridges estimated globally, or have they been individually verified?Simple calculation 0.03, no individual component calculationYou have mentioned several times that it’s not enough, but what is the exact value? Guessing where something might be missing is not helpful if you don’t know how far off the target you are!
A detailed thermal bridge assessment can make a significant difference, and your profiles are, to be frank, quite poor. It should have been around 0.78.
For my KfW 55 house, I already have 0.85 for the profiles and 0.6 for the glass. Without a detailed thermal bridge assessment, achieving the 55 standard would not have been possible.
A detailed thermal bridge assessment can make a significant difference, and your profiles are, to be frank, quite poor. It should have been around 0.78.
For my KfW 55 house, I already have 0.85 for the profiles and 0.6 for the glass. Without a detailed thermal bridge assessment, achieving the 55 standard would not have been possible.
C
curverbox1 Sep 2019 20:51Which value do you mean that you want to know?
Um, KfW55 and KfW40 set a maximum value for Ht' (transmission losses depending on the average U-value of the building components) and for the primary energy demand per square meter, in addition to the system technology.
Both are referenced to a standard building according to the Energy Saving Ordinance: The KfW55 house must not consume more than 55% of the energy of the standard building, and Ht' must be no more than 70% of the reference building. (For KfW40, primary energy is 40% and Ht' is 55%.)
The key point: The reference building has the same orientation, volume, and window area. So it does not matter whether your house is cube-shaped or looks like the Great Wall of China; for the Energy Saving Ordinance and KfW, all are equal. If your energy consultant does not know this, then they do not understand what their software is doing. Simply increasing walls by one meter and seeing what happens does not sound very competent to me. And claiming it depends on the shape of the house is simply wrong. The Energy Saving Ordinance and KfW are neutral in this regard.
All of this can be found in the publicly available Energy Saving Ordinance 2014/2016, paragraph 3 (building balance calculation), and in the KfW 153 technical minimum requirements and supplements in the KfW technical guidelines FAQ.
Both are referenced to a standard building according to the Energy Saving Ordinance: The KfW55 house must not consume more than 55% of the energy of the standard building, and Ht' must be no more than 70% of the reference building. (For KfW40, primary energy is 40% and Ht' is 55%.)
The key point: The reference building has the same orientation, volume, and window area. So it does not matter whether your house is cube-shaped or looks like the Great Wall of China; for the Energy Saving Ordinance and KfW, all are equal. If your energy consultant does not know this, then they do not understand what their software is doing. Simply increasing walls by one meter and seeing what happens does not sound very competent to me. And claiming it depends on the shape of the house is simply wrong. The Energy Saving Ordinance and KfW are neutral in this regard.
All of this can be found in the publicly available Energy Saving Ordinance 2014/2016, paragraph 3 (building balance calculation), and in the KfW 153 technical minimum requirements and supplements in the KfW technical guidelines FAQ.
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