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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Hausbautraum205 Nov 2021 19:18Cowperwood schrieb:
Were any KfW subsidies used for this? I was recently advised by the KfW hotline to definitely build "with the support" and final inspection by an energy consultant. The energy consultant is also involved in the "confirmation after completion," which is required to receive the generous repayment grant.
Additionally, there are supposed to be random inspections by KfW. If these reveal that KfW55 standards were barely met or missed, you have to repay everything.Yes, KfW55. The energy consultant calculated which insulation needs to go under the slab and which in the roof... The energy consultant visited the construction site six times to verify that everything was built according to the KfW application.
At the end, a blower door test was performed.
I don’t understand how the final inspection could turn out different than before. The whole process is based solely on these invoices.
As someone else mentioned, you can also slightly increase the insulation, which is much cheaper than using a different type of brick.
Cowperwood schrieb:
The windows are specified as "triple glazing Ug 0.6," and the gable roof has "24cm (9.5 inches) insulation WLG035." That actually sounds pretty good.This is what we have. It's strange that the KfW55 standard is so tight for you.
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Cowperwood5 Nov 2021 19:47Tom1978 schrieb:
That is what we have. Strange that your KfW 55 is so tight. I’m also wondering why it is so tight for us. I will call the energy consultant and ask which values he used for the calculations and where the weak point might be. Could it be that the ground slab is possibly not insulated?
A similar thread from six weeks ago can be found here: https://www.hausbau-forum.de/threads/kfw-55-mit-dickerem-stein-42-5-cm-porenbeton.41421/
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Cowperwood schrieb:
I’m also wondering why our space is so limited. I’ll contact the energy consultant to ask which values they used in their calculations and where the weak point might be. Could it be that the ground slab isn’t insulated?Here is the scope of work for our ground slab, so you have an example.
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Cowperwood6 Nov 2021 08:37Tom1978 schrieb:
Here is the scope of work description for our foundation slab, so you have an example.And here is the plan for my foundation slab. A solid 12cm (5 inches) thickness, at least!
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