Insulation under the floor slab - Is it sensible? Experiences

There is no need to worry that the XPS will compress. It is absolutely standard to insulate under the slab, and I am not aware of a single case of a problem.

We do not have insulation under the basement slab but 10 cm PIR insulation WLG22 on the slab. In the single-story extension, however, we have 12 cm insulation under the slab. According to our energy consultant, thermal bridges in the basement, when underfloor heating is installed, are so minimal that they can basically be neglected. Furthermore, there are now initial experiences with inverted roofs with XPS insulation, where the insulation boards, despite the corresponding specifications, were saturated with water, which in my opinion essentially eliminates the insulating effect of the boards. Therefore, I would rather recommend insulation on the slab if there is pressurized groundwater.

In my opinion, a basic stove is not necessary in a passive house with timber frame construction. On the contrary, the single-story house can even be overheated quite quickly with it. Therefore, I also do not need any storage mass in the wood stove. And besides: How many basic stoves could I probably buy for the saved costs of a heat pump?

And what surcharge do you pay for the passive house standard? We built minimally according to the Energy Saving Ordinance 2016 (insulation ON the slab). Ground source heat pump with surface collectors completely as own work (costs about €12,000 for heat pump, collectors, underfloor heating on 160m² including thermostats/distributors etc.). Heating costs about €40 per month = €480 per year.

Ground source heat pump with surface collectors completely done by own work (costs about €12,000 for heat pump, collectors, underfloor heating on 160m² including thermostats/distributors etc).
Heating costs about €40 per month = €480 per year.

Great price and respect for the work

Oh, from October to March the sun doesn’t shine where you are? Where do you live?

I live in cold and usually cloudy Germany in autumn and winter. And you?

Great price and respect for the work

Of course, it only works if you have a professional who works for you for free (family). Otherwise, to be fair, you would have to add labor costs. And by the way: I don't think I've seen the sun in the past 7 days.

We built a single-family house a few years ago - as of today somewhat below [Energieeinsparverordnung] standard.
Heating costs with geothermal energy max. €40 per month including hot water for 5 people and 300 sqm area.
And that without insulation under the floor slab.

Heating including underfloor heating cost around €25,000 (about 5-6 years ago).

Therefore, I am skeptical about what additional insulation brings.