Insulation under the floor slab - Is it sensible? Experiences

According to our energy consultant, the thermal bridges in the basement, when underfloor heating is installed, are so minimal that they can actually be neglected

Thermal bridges have nothing to do with underfloor heating; on the contrary, losses are greater there due to the higher temperature difference. Insulation under the slab is generally not worthwhile. We did it anyway because you can’t access it afterward and because that was how it was planned. Passive house is a completely different concept with justification and does not necessarily have to be more expensive overall. Economically, however, it is probably difficult to compete with efficient heating using electric heating. With direct electric heating, the heating costs quickly become higher than with conventional construction, but you save a lot on technology. That has its appeal.

There has been no sun here in the last few days

Passive house never with wood stove - except in winter with severe subzero temperatures
The house overheats immediately
You don't need underfloor heating, that's really just a luxury
Electricity only, for example through infrared, will also cost a lot of money in a passive house

how do you want to ventilate?

Passive house costs: Are manageable if you use a passive house builder for whom KFW 55 is an exotic. If you build according to one of the Energy Saving Ordinances, maybe even KFW 55, it gets expensive

I myself intend to insulate below the BP with 30 cm XPS,

Better discuss this thoroughly with the involved planner. 30 cm against the ground seems to me total overkill, even in a passive house. Moreover, XPS is relatively expensive. Economically, more can certainly be gained from other components.

Thermal bridges have nothing to do with underfloor heating; on the contrary, the losses there are greater due to the higher temperature difference.

According to our energy consultant and the calculation he showed me, these losses are so small that they can essentially be neglected.

We have the same issue right now: We have a sloping site, meaning our "garden level" is 1/3 underground and opens directly onto the garden at ground level. At the ground-level spots, a frost skirt is planned. Otherwise, there is no insulation beneath the base slab. On the base slab, WLG 035 or WLG 045 is used along with underfloor heating.

This, of course, saves us quite a bit of money (perimeter insulation under the entire base slab would be 5 to 6 thousand euros).

There is no clear consensus regarding perimeter insulation under the base slab here, is there? We are building "only" according to the Energy Saving Ordinance.

A special insulating brick is not planned for us, so no "first row of bricks" with insulating brick either...

I have no professional knowledge, but 045 would personally be too little for me, I would take the 035.

In our first offer, 10 cm insulation under the floor slab was planned, but the calculation showed that this was not necessary to comply with the energy saving ordinance. Therefore, we left it out and saved 2.5k. Instead, I invested an extra 1k in better insulation on the floor slab – from 14 cm 035 to 10 cm 022 + the 3 cm 045 Rolljet that our heating engineer insisted on. According to ubakus, this still improved the U-value from 0.24 to 0.187 and we saved 1.5k compared to the original plan.