ᐅ Floor Construction for a New Single-Family House KfW55 Standard
Created on: 7 Apr 2016 08:17
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ElBoCaDiLlO
Hello everyone,
this is my first post in the forum. I hope you can help me a bit.
I have planned a prefabricated house, and with the recent changes in the energy saving regulations and since I will be installing an exhaust air heat pump, my insulation requirements have decreased.
To save some money, I wanted to skip the 16cm (6 inches) perimeter insulation under the concrete foundation.
Now, a new energy saving regulation calculation was done. The floor insulation consists of 20cm (8 inches) C20/25 concrete and 8.5cm (3.3 inches) WLG025 insulation under the screed. This results in a U-value of 0.27 W/m²K, which seems quite high for a KfW55 house, doesn’t it?
I’m not very familiar with this, which is also why I handed everything over, but all other values (roof, exterior walls) are around 0.14 W/m²K.
Do you see a simple way to improve the floor insulation, and above all, does it even make sense, or is this value sufficient?
The floors on both ground and upper levels will be fully equipped with underfloor heating.
I don’t have the exact floor structure on hand right now, but I can provide it if needed.
Many thanks and best regards from the sandwich.
this is my first post in the forum. I hope you can help me a bit.
I have planned a prefabricated house, and with the recent changes in the energy saving regulations and since I will be installing an exhaust air heat pump, my insulation requirements have decreased.
To save some money, I wanted to skip the 16cm (6 inches) perimeter insulation under the concrete foundation.
Now, a new energy saving regulation calculation was done. The floor insulation consists of 20cm (8 inches) C20/25 concrete and 8.5cm (3.3 inches) WLG025 insulation under the screed. This results in a U-value of 0.27 W/m²K, which seems quite high for a KfW55 house, doesn’t it?
I’m not very familiar with this, which is also why I handed everything over, but all other values (roof, exterior walls) are around 0.14 W/m²K.
Do you see a simple way to improve the floor insulation, and above all, does it even make sense, or is this value sufficient?
The floors on both ground and upper levels will be fully equipped with underfloor heating.
I don’t have the exact floor structure on hand right now, but I can provide it if needed.
Many thanks and best regards from the sandwich.
BeHaElJa schrieb:
Electric heating is rated better!? Please explain...The so-called primary energy factor (PEF) for electricity is gradually being reduced. This is partly because the share of renewable energy in electricity generation is steadily increasing and is already well over 30%, which I find remarkable.
Electricity has had a very poor primary energy factor, around 2.7 as far as I remember, mainly due to high transmission losses and low efficiency in centralized large power plants. It is now 2.4 (I hope I recall the value correctly).
KfW calculations—used here as part of an overall assessment—are based on the primary energy factor.
As a result, air-to-water heat pumps continuously score better in these calculations. When the PEF for electricity was nearly 3 and the coefficient of performance (COP) of air-to-water heat pumps was around 3 (or lower, depending on hot water supply), the results were debatable. Now, with a PEF of 2.4, it clearly adds up.
Electricity for air-to-water heat pumps is purchased at about 20 cents/kWh.
Gas (PEF 1.1) costs around 6 cents/kWh.
An air-to-water heat pump with COP 3 is therefore almost cost competitive.
As another user correctly pointed out here, this does not mean that a lower KfW rating necessarily results in lower direct heating costs. You pay for the energy source used, not the primary energy demand.
Thorsten
T21150 schrieb:
As correctly explained by another user here, having a lower KFW rating does not necessarily mean lower direct heating costs. This is because you pay for the consumption of the specific energy source, not the primary energy demand.Additional note:
A neighbor with a comparable house (size, enclosed volume, KFW class, number of occupants) uses an air-to-water heat pump and pays an average of about 70 euros per month for electricity for heating and domestic hot water. We pay an average of 48 euros per month for gas.
To provide an answer to the original question:
I just reviewed our energy calculation for our KFW55 according to the Energy Saving Ordinance 2016.
We have a basement.
The U-value for the slab beneath the basement is 0.25 W/m²K. This applies to the entire basement floor (including the floor structure with impact sound and thermal insulation).
This value is very close to yours.
Under the slab, a 10cm (4 inches) Styrodur insulation with a thermal conductivity of WLG 037 was installed, although according to the energy calculation, 8cm (3 inches) with WLG 040 would have been sufficient. So, I probably have a slightly better value than calculated, or the floor structure uses something different from what was calculated so that the overall value balances out to 0.25. I’m not an expert and can only compare the numerical values.
I just reviewed our energy calculation for our KFW55 according to the Energy Saving Ordinance 2016.
We have a basement.
The U-value for the slab beneath the basement is 0.25 W/m²K. This applies to the entire basement floor (including the floor structure with impact sound and thermal insulation).
This value is very close to yours.
Under the slab, a 10cm (4 inches) Styrodur insulation with a thermal conductivity of WLG 037 was installed, although according to the energy calculation, 8cm (3 inches) with WLG 040 would have been sufficient. So, I probably have a slightly better value than calculated, or the floor structure uses something different from what was calculated so that the overall value balances out to 0.25. I’m not an expert and can only compare the numerical values.
Manuel85 schrieb:
Under the slab, a 10cm (4 inches) Styrodur insulation with a thermal conductivity of 0.037 W/(m·K) was installed, although according to the energy calculation, 8cm (3 inches) with 0.040 W/(m·K) would have been sufficient. So I probably achieved a slightly better value than calculated, or the floor construction uses different materials than assumed in the calculation, balancing out to a U-value of 0.25 W/(m²·K). I’m not an expert, I can only compare the numbers mathematically. You mentioned the bricks you are building with in another thread (I hope I’m not confusing you). The brick you referred to has a U-value of 0.21 W/(m²·K) and would therefore not meet the reference value method requirements for KfW 55 standard. Now you’re stating the U-value of your slab, which, according to that method, is exactly on target—and even better than required, as you say.
I wonder which building components in your house have U-values that shift the overall balance the other way. That leaves the roof and the windows, with the latter being a significant cost factor if overdone. Leaving the front door aside.
I’m really interested because we are currently working more intensively on this topic ourselves. It would be great if you could briefly list your values.
I’d like to join the discussion and share the specifications of our KfW 55 house (construction started early 2017):
Foundation slab: 10cm (4 inches) perimeter insulation and 20cm (8 inches) reinforced concrete → U-value: 0.19 W/(m²K)
Lower two-thirds of exterior wall: 15cm (6 inches) aerated concrete + 18cm (7 inches) cavity insulation boards (0.035) + 1cm (0.4 inches) air gap + 11.5cm (4.5 inches) facing brick → U-value: 0.15 W/(m²K)
Upper one-third of exterior wall: External thermal insulation composite system (rendered facade) with 15cm (6 inches) aerated concrete + 18cm (7 inches) polystyrene rigid foam insulation + 1cm (0.4 inches) silicone resin render → U-value: 0.16 W/(m²K)
Roof (cold roof, insulation on timber ceiling of the upper floor): Gypsum board + 26cm (10 inches) insulation + vapor barrier → U-value: 0.15 W/(m²K)
Windows/doors: 5-chamber PVC windows with triple-pane low-emissivity glazing → U-value: 0.95 W/(m²K)
I hope these values provide you with a helpful comparison.
Foundation slab: 10cm (4 inches) perimeter insulation and 20cm (8 inches) reinforced concrete → U-value: 0.19 W/(m²K)
Lower two-thirds of exterior wall: 15cm (6 inches) aerated concrete + 18cm (7 inches) cavity insulation boards (0.035) + 1cm (0.4 inches) air gap + 11.5cm (4.5 inches) facing brick → U-value: 0.15 W/(m²K)
Upper one-third of exterior wall: External thermal insulation composite system (rendered facade) with 15cm (6 inches) aerated concrete + 18cm (7 inches) polystyrene rigid foam insulation + 1cm (0.4 inches) silicone resin render → U-value: 0.16 W/(m²K)
Roof (cold roof, insulation on timber ceiling of the upper floor): Gypsum board + 26cm (10 inches) insulation + vapor barrier → U-value: 0.15 W/(m²K)
Windows/doors: 5-chamber PVC windows with triple-pane low-emissivity glazing → U-value: 0.95 W/(m²K)
I hope these values provide you with a helpful comparison.
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