ᐅ Insulation of the Concrete Slab and Floor Construction in a 146 m² Bungalow

I would like to raise some questions regarding the insulation of the ground slab in 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 ground slab, and it also appears to depend on the specific project.

First, here are our conditions.

It is a bungalow, i.e., single-story, with approximately 146 m2 (1570 ft²) of living space (about 170 m2 (1830 ft²) of floor area on the masonry). Heating is provided by underfloor heating throughout the house. The heat is generated by a ground source heat pump.

Soil properties according to the report: narrowly graded sands and sand-silt mixtures in a medium dense to dense state. The soil is sufficiently to well load-bearing. No concerns about the construction project.

The planned structural shell from bottom to top is as follows:

• Strip footing as frost protection, concrete C25/20, width = 35 cm (14 inches), depth = 80 cm (31 inches)
• Excavation within the frost protection zone to a depth of 50 cm (20 inches) and installation and compaction of a gravel sub-base
• Blinding layer of lean concrete, thickness = 5 cm (2 inches)
• Cast-in-place ground slab of reinforced concrete with steel mesh, concrete C25/30, thickness 25 cm (10 inches)
• Bituminous membrane G 200 S4 (under masonry G 200 DD)
• External masonry of aerated concrete blocks PP2, thickness = 36.5 cm (14 inches)
• Internal masonry of solid sand-lime bricks, thickness = 17.5 and 11.5 cm (7 and 4.5 inches), bulk density = 2.0
• Perimeter insulation outside from the bottom edge of the ground slab to the top edge of the first masonry course

For the floor construction, the following values were used in the energy demand calculation (building energy act compliance):



[TH]Layer[/TH]

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

Interior surface				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
Bituminous membrane	0.5	0.17	1200	0.029
Normal concrete (2400)	25	2.1	2400	0.119
Exterior surface				0.000
Total				4.131

This results in a U-value of 0.24 W/(m²K). The 5 cm (2 inches) lean concrete blinding layer below the slab was not included in this calculation.

We are not building according to KfW standards but aim to meet the requirements of the Building Energy Act. Ways to meet KfW standards should not be part of this discussion.

We wonder if the above structural shell details and floor construction based on the energy demand calculation are optimal regarding the planned underfloor heating, heating costs with the ground source heat pump (which uses electricity and electricity costs are rising), the prevention of thermal bridges, and the preservation of the building fabric in the foundation and base area.

Here are my questions:

• Questions about possible insulation of the ground slab and the structural shell details
• Should we consider insulating below the ground slab with approximately 10 cm (4 inches) of XPS?
• If yes, why?
• If no, why not?
• Are the other aspects of the structural shell design related to the ground slab and base acceptable, or do you see room for improvement? What recommendations would you give?
• Questions about thermal bridges at the transition from ground slab to masonry
• Do the aerated concrete blocks, with their low thermal conductivity (0.09 W/(mK)), sufficiently counteract the formation of thermal bridges, weakening or even invalidating the argument that “thermal bridges should be prevented by insulating below the ground slab”?
• If not, why?
• Question regarding the floor build-up
• Is the floor build-up shown above (3.5 cm WLG 040 and 10 cm WLG 035) beneath the screed sufficient if no insulation is installed below the ground slab? If not, what would you recommend? Is the 9 cm screed layer somewhat oversized?
• How should the insulation below the screed be designed if the ground slab is insulated? In my opinion, the underfloor heating should not warm the ground slab but rather transfer heat upwards. Should the build-up remain as it is, or can the thickness of the insulation layer and screed be reduced? With 10 cm (4 inches) of XPS, the U-value would be about 0.12 W/(m²K). This would leave some room for adjustment.
• Questions about heating costs with a ground source heat pump
• Would additional insulation below the ground slab have a significant effect on heating costs? Electricity prices are likely to rise faster than we would like. The question is whether the current price increases in insulation materials are lower than the electricity price increases. Does anyone have expertise or a well-founded opinion on this?

Thank you in advance for your answers.