ᐅ Perimeter insulation 12 or 16 cm extruded polystyrene (XPS)

Hello,

For a single-family house with a concrete wall basement in Baden-Württemberg (average outside temperature around -14°C (7°F) according to the WP map), would you choose 12 cm or 16 cm XPS for perimeter insulation?

What I think about the basement is that it stays pleasantly cool in summer, cooler than the ground and upper floors. If I insulate more, maybe it won’t stay as cool? Or does the 4 cm difference not really matter there? The price difference between 12 and 16 cm is only about 1,500 EUR.

Or is it better to invest the extra 1,500 EUR and then save on heating costs in winter? But the money would have to pay off first through saved heating costs (feels like it would take over 100 years, probably)... hmm

For our KfW 55 house, both 12 and 16 cm XPS perimeter insulation would easily meet the requirements. The thermal transmittance value (U-value) is 0.215.

Regards

Thickness alone doesn’t determine the effectiveness; the thermal conductivity class (WLG) of the insulation material is also important. If 12cm (5 inches) is sufficient at the same WLG, I would go with that. The basement definitely won’t be warmer with 16cm (6.3 inches), considering it will still be surrounded by several tons of soil, right?

Using only 16cm (6.3 inches) XPS insulation, I will probably have lower heating costs in winter, right!?

With the same thermal insulation value (WLG): Yes, probably in the low single-digit euro range.

I doubt that these 4 cm (1.5 inches) are worthwhile, as the effectiveness of additional insulation decreases with increasing thickness.

12 is very little