Insulation under the floor slab - Is it sensible? Experiences

I completely agree with you. It depends on the overall concept; ultimately, it's about achieving the desired insulation values with as little capital investment as possible. Insulation under the floor slab avoids additional thermal bridges that cannot be avoided with insulation on the floor slab. Of course, these heat losses can be compensated by other measures to reach the desired insulation value... overall concept would be the keyword again.

The question of what is economically sensible... that is philosophical, and each side will present a calculation that says something different. It simply depends too much on all the assumptions about the future price development of the house and energy costs.

Received the thermal protection certificate today... As said, ordered and requested was KFW 55 from our side. With the 80 mm Styrodur recommended by the house builder below the floor slab, we now achieve KFW 40

Option A:
I now have the following possibilities: completely leave out Styrodur, receive a discount of almost €2200 for the omission of the originally offered 100 mm Styrodur at the floor slab, and perhaps achieve KFW 55 or somewhat worse and live with the resulting heating costs.

Option B:
I am happy about KFW 40 and accordingly lower heating costs, as well as the discount for 80 mm Styrodur compared to 100 mm Styrodur (which I don’t know yet but won’t be impressive).

Option C:
I leave everything as originally with the 100 mm Styrodur, will thereby be even better than KFW 40, but how much exactly I will never know (because a new thermal protection certificate would have to be created, which no one does for free) and I probably will never notice savings in heating costs either because the comparison is missing....

Hi

definitely pro insulation
You will never get this opportunity again at this price.

Olli

Hi,
We have insulation under the floor slab. Without underfloor heating. On top of that, parquet.
Often it is still too “cold underfoot” for us.

So I would recommend an additional underfloor heating.

Best regards,
Michael

I would also tend towards KfW40. Do you use KfW for financing? Then you already have €5000 more repayment subsidy compared to KfW55. If a photovoltaic system is planned, it is also worth considering going for KfW40+, then there is another €5000 more and the storage is almost paid for, which otherwise is not really worthwhile. Photovoltaics are worthwhile anyway.

We have insulation under the floor slab. No underfloor heating. Parquet on top. It is often still "cold underfoot" for us.

Without underfloor heating it will always remain "cold underfoot." No matter how much you insulate.