Ok, thx. I have now inquired with the company Dämmstoffkönig. The following feedback from the company:
the boards for €14.85 were left outside for a while, see description
The boards for €15.78, for example, have no tongue and groove and have minor defects.
Since both boards are coated with aluminum foil on both sides, you do not need an additional vapor barrier foil.
Then I would rather take the PUR rigid foam boards coated with aluminum foil on both sides WLG 023 for €15.78. Then I would only have to ask the company if they can issue the invoice with reference to the insulation of the top floor ceiling or something similar? And is it a problem if the boards have no tongue and groove?
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I have already received approval for the window replacement in the upper floor apartment under the KFW430 Single Measures program, also the KFW431 program. That means I could now try to have the insulation of the top floor ceiling subsidized retroactively under the 430 program? Do I have to change the application again because unfortunately I did not check that box :-(
However, in the application regarding thermal insulation of floor ceilings under
Point 3.1 Top floor ceiling to unconverted attic spaces: U-value <= 0.14 W/(m2K)*
Point 3.2 Basement ceilings, ceilings to unheated rooms: U-value <= 0.25 W/(m2K)*
Which point applies? If point 3.1, then I would need a U-value of 0.14 for funding eligibility. Can I achieve this value with the aforementioned boards?
Thanks you all for the great help.
Best regards
Chippy