ᐅ Insulation (Exterior Walls + Underneath Foundation Slab) for Watertight Basement Makes Sense

Hello everyone,
I initially decided against this insulation due to the costs:
- 14cm (5.5 inches) perimeter insulation on the basement exterior wall (approx. 18m (59 ft) long x 2.5m (8.2 ft) high): €6,600
- 10cm (4 inches) insulation under the floor slab (approx. 10m (33 ft) x 10m (33 ft) basement): €7,300
Now I somewhat regret that and am considering whether to commission this work now.

The basement is partially exposed above ground.
All exposed basement exterior walls are planned with 16cm (6.3 inches) ETICS (external thermal insulation composite system). Now it’s only about the exterior walls that are below ground. The basement is not a living space (no WC, no heating).
The construction is being done by the general contractor. The architect’s advice on the underground areas was basically “you get what you pay for.”
Question:
- Is it sensible to do the perimeter insulation on the exterior basement walls at that price?
- Same question for the insulation under the floor slab. Here I believe it could be somewhat difficult since the building permit / planning permission is already in place. Alternatively, I am thinking about using glass gravel. I don’t know the price of glass gravel.
- A friend suggested "insulation under the basement ceiling." He says that otherwise, thermal bridging could cause cold spots. What do you think about this?
THANK YOU for your opinions.

An exterior wall is a must, in my opinion, otherwise you will very likely get mold and other issues.

Having insulation under the concrete slab is unnecessary luxury; we did it ourselves. It didn’t bring any benefit. It doesn’t save on heating costs, and it doesn’t add any other value either, so definitely skip it.

Bookstar schrieb:

In my opinion, exterior wall insulation is essential; otherwise, you will very likely encounter mold and other issues.

Insulation under the slab is unnecessary luxury—we did it, but it didn’t make any difference. It doesn’t save any heating costs and doesn’t provide any added value, so definitely skip it.

Hello Bookstar,
thank you for your response. I will add the exterior insulation for about €2300 (approximately $2500) extra. There is a choice between 10 cm (4 inches) or 14 cm (5.5 inches).
I will skip the insulation under the slab.
What kind of basement do you have? (waterproof concrete basement, habitable basement, etc.)? When was the house built?

Built 3 years ago, a combination of residential and utility basement made of waterproof concrete. No bitumen coating.