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...
C
curverbox2 Sep 2019 10:29boxandroof schrieb:
That’s what I thought. Still, this attempt clearly didn’t help. With a 49cm (19 inches) wall thickness, this is completely unnecessary, uncommon, and expensive.
I doubt the wall thickness or your given figures – see my post above – please calculate it yourself, there must be a mistake somewhere. But the wall doesn’t seem to be your real issue. Where the consultant can get information has been written by @dertill. Sorry, that was my mistake. The 49cm (19 inches) includes Rockwool with a thermal conductivity of 0.035 W/(m·K) and an 8cm (3 inches) air gap. So 17.5 + 14 + 6 + 11.5 = 49.
The calculation showed that if we use PUR insulation, the wall thickness of 17.5 + 12 + 11.5 = 41cm (16 inches) results in a similar U-value.
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boxandroof2 Sep 2019 10:58Thank you for the clarification. With mineral wool as core insulation, the air gap can be significantly less than 8 or 6cm (3 or 2.5 inches). A wall thickness of 49cm (19 inches) with 14cm (5.5 inches) insulation is unusual and not necessary.
Whether using 024 PUR foam to save 2-3cm (1-1.2 inches) is sensible, or even required for KfW 40 standards, must be evaluated holistically.
The wall must meet specific requirements. Now you have changed the wall, and it still isn’t sufficient? If you clearly don’t know where the exact problems lie, then making adjustments to the wall isn’t productive.
Whether using 024 PUR foam to save 2-3cm (1-1.2 inches) is sensible, or even required for KfW 40 standards, must be evaluated holistically.
The wall must meet specific requirements. Now you have changed the wall, and it still isn’t sufficient? If you clearly don’t know where the exact problems lie, then making adjustments to the wall isn’t productive.
curverbox schrieb:
These are the calculated values.
Annual primary energy demand QP / QP,Anf 2.7 kWh/(m²a) (2.7 kWh/(m²a))
Transmission heat loss H'T 0.192 W/(m²K) (0.192 W/(m²K)) Something seems off with your place (maybe the north side has a lot of windows :-p) or with your energy consultant.
My reference value for a building meeting the 2016 Energy Saving Ordinance is 0.37. Subtracting the 45% improvement requirement for the building envelope at KfW40 level, I would expect around 0.2035.
What is your reference value?
curverbox schrieb:
Sorry, that was my mistake, it was 49cm (19 inches) with Rockwool WLG035, including an 8cm (3 inches) air gap. So, 17.5 + 14 + 6 + 11.5 = 49.
The calculation showed that if we use PUR insulation, a wall thickness of 17.5 + 12 + 11.5 = 41cm (16 inches) results in a similar U-value. May I ask why you want to push so hard and with so much effort to achieve KfW 40 or 40+ standards?
I know several tenants, and all of them, without exception, struggle with ventilations systems with high airflow noise and/or super-insulated walls and/or underfloor heating only. They often keep the windows tilted open all day and reduce the ventilation rate…
ypg schrieb:
May I ask why anyone would stubbornly try to reach KfW 40 or 40+ standards?
I know several tenants... and every single one of them struggles with loud ventilation systems and/or highly insulated walls and/or underfloor heating only. They just keep the window tilted open all day and reduce the ventilation flow... They would need proper instruction first :-p
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curverbox2 Sep 2019 13:45Each building component meets the U-value requirements
The 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)
Zaba12 schrieb:
There seems to be something wrong with your place (maybe the north side has a lot of window area) or with your energy consultant.
My reference value for an Energy Saving Ordinance 2016 house is 0.37. Minus the 45% by which the building envelope must be improved for KfW40, I would be at 0.2035.
What is your reference value?
The 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)
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