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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CowperwoodC
Cowperwood5 Nov 2021 18:35Hello 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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Hausbautraum205 Nov 2021 18:41In our case, there was never a final inspection. We built exactly to meet the KfW 55 standard just barely. The energy consultant calculated this for us accordingly, intentionally as close as possible to avoid unnecessary costs.
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Cowperwood5 Nov 2021 18:45Hausbautraum20 schrieb:
For us, there was never a "final inspection."
We built exactly to just meet the KfW55 standard. The energy consultant calculated it for us accordingly. So it was deliberately tight to avoid unnecessary costs. Were any KfW subsidies used in your case? I was recently advised by the KfW hotline to definitely build "with support" and a final inspection by an energy consultant. The energy consultant is also involved in the "confirmation after completion," which is required to qualify for the generous repayment bonus.
Moreover, there are supposed to be random checks carried out by KfW. If these show that the KfW55 standard is barely missed, the entire subsidy has to be repaid.
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Cowperwood5 Nov 2021 18:59Tom1978 schrieb:
It can’t be the Poroton. We are building with 36.5 cm Poroton and KfW 55 is not an issue. It’s probably related to the windows. We have passive house windows. The windows are specified in the plans as "triple glazing Ug 0.6," and the pitched roof is listed with "24 cm insulation WLG035." That actually sounds quite good.
Windows can practically be improved (0.5 glass, better frames) or calculated more precisely (manually calculating the frame proportion for large windows instead of standard window values). An energy consultant knows these tricks and should achieve better results. In the worst case, you can opt for WLG32 insulation in the roof or improve the base slab. In short, keep calm and move on.
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