[stuaxo]

Depends on the country.

In a crowded Island like the UK, doubling the wall thickness absolutely makes a difference.

[seszett]

Are walls generally 15 cm thick in the UK? Here in Belgium, which is certainly not less crowded than the UK, my exterior walls are 30 cm thick. 40 cm for the back wall that was insulated further recently. That's for a town house in a city center.