ᐅ Construction of the Slab Foundation, Base Wall, and Perimeter Insulation

Hello everyone,

I am currently planning a single-family house and am starting to consider the construction of the ground slab and the implementation of the basement base. The house is designed with a basement that is included within the building’s thermal envelope. Rooms in the basement:

1. Laundry room
2. Utility room
3. Room prepared for a possible future sauna, including toilet, shower, and washbasin
4. Workshop

The basement is not initially intended as a living space, but there should be the option to heat it later with underfloor heating. A soil survey has been commissioned but is not yet complete. The neighbors apparently have no issues with groundwater pressure, so my current assumption is to go with a waterproof concrete tank (also known as a black tank).

My question at this point is: what is the best construction sequence for the ground slab and perimeter insulation? I was thinking of a black tank with insulation below the ground slab (from bottom to top):

1. Soil
2. Blinding layer
3. Vapor-impermeable membrane, e.g. PE
4. 20cm (8 inches) XPS insulation
5. Ground slab

The masonry walls would be sealed with bitumen and have 12cm (5 inches) XPS insulation applied to the sides, extending 30cm (12 inches) above ground level. Is this currently considered best practice? Would you recommend this approach?

Another question I have is: if a black tank is used (which I currently assume), which masonry block is best suited here? The current plan is to use a lightweight concrete block with a relatively good U-value. However, I suspect that calcium silicate blocks might be more common for basements. Is that correct?

I would appreciate any constructive advice.

The construction method you described has been standard practice for a long time. However, I’m not sure if you really need 20cm (8 inches) of XPS insulation under the foundation slab. We used 12cm (5 inches), and that was definitely sufficient.

Are you planning to build the entire basement yourself?
Is there a soil survey available?
Nowadays, if a basement is planned, I would generally recommend building it as a fully waterproofed “white tank” structure. Weather conditions will likely change significantly over the next 80 years. In my opinion, that is money well spent.

Allthewayup schrieb:

The construction method you described has been common practice for a long time. However, I’m not sure if you need 20cm XPS beneath the slab. We used 12cm, and that was definitely sufficient.
Are you planning to build the basement entirely by yourself?
Is there a soil report?
Nowadays, if a basement is planned, I would generally recommend constructing it as a “white tank” (fully waterproofed structure). Weather conditions are expected to change significantly over the next 80 years. In my opinion, that’s money well spent.

Not at all. The soil report is still pending. I just wanted to gather information in advance. Why choose a white tank for the basement? Is it because of high groundwater pressure? The additional costs are quite significant, right? Would the construction then use watertight (WU) precast walls that are poured on site?