ᐅ KfW55 EE House with 42.5 cm Poroton Blocks: Is It Under-Designed?
Created on: 5 Nov 2021 18:35
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Cowperwood
Hello everyone,
the planning for our KfW55 EE house is complete. We want to build with 42.5cm (17 inches) Poroton walls without additional insulation. The design engineer recommends the T9 version with a thermal conductivity (λ) of 0.09.
Now, the energy consultant commissioned by the planning office did the calculations and came up with a very good primary energy demand of 20.8 kWh/m² per year. However, the specific heat transmission loss is only 0.2620 W/m²K.
Note: To qualify as KfW55, a maximum of 0.2625 is allowed, so it’s very close—even at the fourth decimal place. Additionally, thermal bridges are accounted for with a relatively low value of 0.013 W/m²K.
My concern: If another energy consultant runs the calculations, they might conclude that the KfW55 standard is not met.
Question: Would you build it this way? Or is there a too high risk that the finished house might perform slightly worse than the plans indicate, causing the entire financing to fail? Would it perhaps be better to choose the T8 Poroton instead?
Kind regards
Theodor
the planning for our KfW55 EE house is complete. We want to build with 42.5cm (17 inches) Poroton walls without additional insulation. The design engineer recommends the T9 version with a thermal conductivity (λ) of 0.09.
Now, the energy consultant commissioned by the planning office did the calculations and came up with a very good primary energy demand of 20.8 kWh/m² per year. However, the specific heat transmission loss is only 0.2620 W/m²K.
Note: To qualify as KfW55, a maximum of 0.2625 is allowed, so it’s very close—even at the fourth decimal place. Additionally, thermal bridges are accounted for with a relatively low value of 0.013 W/m²K.
My concern: If another energy consultant runs the calculations, they might conclude that the KfW55 standard is not met.
Question: Would you build it this way? Or is there a too high risk that the finished house might perform slightly worse than the plans indicate, causing the entire financing to fail? Would it perhaps be better to choose the T8 Poroton instead?
Kind regards
Theodor
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Hausbau 556 Nov 2021 08:44Tom1978 schrieb:
It can’t be due to Poroton. We are building with 36.5 cm Poroton and KfW 55 certification is not a problem. It’s probably related to the windows. We have passive house windows. Such statements are simply far too superficial. What applies to your project does not necessarily have to fit another construction project. The entire building envelope must be considered in terms of heat transmission losses. So, if comparisons are to be made, significantly more information needs to be provided.
Hausbau 55 schrieb:
Such statements are simply answered far too superficially. What applies to your case does not necessarily have to be true for another construction project. The entire building envelope must be considered regarding transmission heat losses. So if a comparison is to be made here, significantly more information must be provided. It probably says “presumably” and not “certainly.” And if the bricks are the same but only thicker, it is MOST LIKELY not due to the bricks.
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Hausbau 556 Nov 2021 08:48Cowperwood schrieb:
I’m also wondering why it’s so tight in our case. I’ll call the energy consultant to ask which values he used and where the weak point might be. Could it be that the slab on grade is perhaps not insulated?Please avoid making general assumptions about individual elements regarding the thermal insulation calculation. Your slab on grade is only one piece of the puzzle and not the most effective focus. The slab on grade contributes only a small part to the overall calculation since the temperature difference (Delta T) between the house and the ground is smaller than for the walls, windows, or ceiling.C
Cowperwood6 Nov 2021 21:17Hausbau 55 schrieb:
Please avoid making general assumptions about individual components regarding the thermal insulation calculation. Your floor slab is just one piece of the puzzle and not the optimal starting point. The floor slab only contributes a small part to the overall calculation because the temperature difference between the house and the ground is less than that for walls, windows, or the ceiling. Am I correct in thinking that the four main components are: walls, windows, floor (slab), and insulation "above"?
Can these then be compared side by side?
Maybe I have too many windows 🙄
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Benutzer2006 Nov 2021 21:34Cowperwood schrieb:
My concern: if another energy consultant runs the calculations, they might conclude that KfW55 is not being met.
Question: Would you build it like this? Or is there too much risk that the final inspection will show the house performs slightly worse than on paper,Where is the problem? If the house is built as planned, the issue is settled. Anyone can run their calculations.This is completely stress-free. In the end, the energy consultant who did the original calculation will confirm that the house was built according to their specifications. Nothing will be recalculated. Two times two is four, it won’t suddenly be 3.98.
The whole discussion is totally unnecessary.
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