Insulation of the floor slab and construction of the floor in a 146 m² bungalow

I would like to raise some questions regarding the insulation of the floor slab in relation to our construction project. I have already read several threads on this topic, but opinions seem to differ on whether to insulate below and/or above the floor slab, and it also seems to always depend on the respective construction project.

First, our framework conditions.

It is a bungalow, i.e., single-story, with about 146 m2 of living space (about 170 m2 of floor area on which the masonry is built). Heating is provided by underfloor heating throughout the house. The heat generation is done via a geothermal heat pump.

Soil condition according to the report: fine-graded sands and sand-grit mixtures in a nearly medium-dense to dense formation. Soils sufficiently to well load-bearing. No concerns against the construction project.

The following structural shell constructions are planned from bottom to top:

[*
Strip footing as frost skirt, concrete C25/20, width = 35 cm, depth = 80 cm
[*]Excavation of soil within frost skirt to 50 cm depth and installation and compaction of the sub-base made of gravel layer there
[*]Clean layer of lean concrete, depth = 5 cm
[*]Formed floor slab made of reinforced concrete with mesh reinforcement, concrete C25/30, depth 25 cm
[*]Bitumen welding membrane G 200 S4 (under masonry G 200 DD)
[*]Exterior masonry made of aerated concrete PP2, thickness = 36.5 cm
[*]Interior masonry made of solid lime-sand bricks, thickness = 17.5 and 11.5 cm, bulk density = 2.0
[*]Perimeter insulation outside from the underside of the floor slab up to the top edge of the first masonry layer


For the floor construction, the following were used as a basis in the energy demand calculation (Building Energy Act verification):



[TH]Layer[/TH]

[TH]Thickness [cm][/TH]
[TH]Lambda [W/(mK)][/TH]
[TH]Density [kg/m3][/TH]
[TH]R [m2K/W][/TH]














































































































Inside				0.170
Tile	1.5	1.0	2000	0.015
Cement screed	9	1.4	2000	0.064
Vapor retarder	0.02	0.17	1500	0.001
System board (WLG 040)	3.5	0.040	125	0.875
EPS (WLG 035)	10	0.035	125	2.857
Bitumen welding membrane	0.5	0.17	1200	0.029
Normal concrete (2400)	25	2.1	2400	0.119
Outside				0.000
Total				4.131



This results in a U-value of 0.24 W/(m2K). The fact that a clean layer of lean concrete with a thickness of 5 cm is installed under the floor slab was not included in this calculation.

We are not building according to KfW standards but want to meet the requirements of the Building Energy Act. How one could still build to KfW standards should not be part of this discussion here.

We are wondering whether the above structural shell executions and the floor construction according to the energy demand calculation are optimal regarding the planned underfloor heating, heating costs of the geothermal heat pump (which uses electricity and costs are rising here as well), the formation of thermal bridges, and the preservation of the building fabric in the foundation and base area.

Here are my questions:


[*]Questions regarding possible insulation of the floor slab and the execution of the structural shell
[LIST]
[*]Should we consider an underside insulation of the floor slab of about 10 cm XPS?
[*]If yes, why?
[*]If no, why?
[*]Are the other aspects of the above-mentioned execution of the structural shell in the floor slab/base area okay, or do you see potential for improvement here? What recommendations would you give?


[*]Questions about thermal bridges at the transition from floor slab to masonry


[*]Do the aerated concrete blocks counteract the formation of thermal bridges sufficiently due to their low thermal conductivity (0.09 W/(mK)), thereby weakening or even refuting the argument "thermal bridges should be prevented by insulating under the floor slab"?
[*]If no, why?


[*]Question about the floor build-up


[*]Is the above shown floor build-up (3.5 cm WLG 040 and 10 cm WLG 035) under the screed sufficient if no insulation of the floor slab is installed? If not, what recommendations would you make? Is the screed layer at 9 cm not somewhat overdimensioned?
[*]How should the insulation under the screed look if the floor slab is insulated? In my opinion, the underfloor heating should not heat the floor slab as well but preferably transfer the heat upwards. Should the construction then not remain as it is or can one save some thickness of the WLG insulation and the screed? With 10 cm XPS, the U-value would be 0.12 W/(m2K). That would still leave some room.


[*]Questions about heating costs using the geothermal heat pump


[*]Would additional underside insulation of the floor slab have a strong effect on heating costs? Electricity prices will probably rise faster than we like. The question is whether the current price increases of insulation material are lower than the price increases of electricity. Does anyone have expertise here and a well-founded opinion on this question?

Thank you in advance for your answers.