Insulation under the floor slab - Is it sensible? Experiences

Hello everyone, at the moment I am considering leaving out the insulation under the floor slab. The house itself will be a bungalow with 102 m² floor area, with an air heat pump, underfloor heating, and controlled residential ventilation, and regarding sanitation with a circulation line, built as a timber frame house.

In principle, we ordered a KFW 55 house, but since we have not applied for any KFW 55 subsidies from the banks or the government, it would theoretically be irrelevant to me whether it becomes an Energy Saving Ordinance 2016 house or a KFW 55 house. Due to the lack of subsidies, we would not need the Blow Door Test either.

From initial discussions, I have now found out that for KFW 55, apparently 80 mm of Styrodur/Jakodur would have to be placed under the floor slab.

Actually, I don't care whether the floor slab now has a U-value of 0.20 W/m² x K due to the insulation (values are guessed) or without insulation a U-value of 0.22 W/m² x K (values are also guessed).

What would currently be more important for me is whether I should throw away the estimated €2000 for the insulation and installation in the soil, in order to break even after 20 years or save €20 heating costs every year.

Or whether I should rather invest the €2000 in something on the house that I can effectively use or benefit from? For example, upgrading the equipment or investing in a photovoltaic system (and yes, I know I won't get a photovoltaic system for €2000).

Or is every additional millimeter of insulation crucial with the air heat pump and underfloor heating? (I know the electricity won't get cheaper, hence the point about the photovoltaic system....

We already talked about this topic here recently.....I would do the insulation anytime again

But even more important, I would strongly recommend the Blower Door to you....you simply find many spots where you can still do something and where heat might possibly be lost.....

I would skip the insulation. Better to spend a few euros on improved insulation on the floor slab and actually use the rest as a down payment for, for example, a photovoltaic system (smaller systems aren't that expensive anymore).

... use as a down payment for, e.g., a photovoltaic system (smaller systems are not that expensive now either).

What does that mean in euros or kWp, I would be interested to know?

I would omit the insulation. Better to spend a few euros on improved insulation on the base plate and actually use the rest as a down payment for, for example, a photovoltaic system (smaller systems are not that expensive anymore).

That's exactly what we did. Omitted insulation under the slab and went from WLG035 to WLG023 on the slab.

What does that mean in euros or kWp, I would be interested to know?

You should get prices per kWp from €1,000 (always net, as a business you get the VAT refunded) - depending on the manufacturer/components. I can warmly recommend the photovoltaics forum for this.