[dmitriy_ko]

Average American "cardboard house" is better insulated than average European brick or concrete house. Having heavy walls make your house feel "sturdy" but it has nothing to do with energy efficiency. A lot of houses in Europe are just brick or concrete with no additional insulation. In US it's it's rare to have uninsulated houses.

In US more people live in very hot places like Texas and Arizona and people mostly live in single-family houses. Single-family houses obviously takes more energy to cool than "flats". Plus average American house/flat has twice the living area.