@arildsen@mastodon.green Its called a "Swedish skirt" or "Frost-Protected Shallow Foundation" depending on where you are in the world. Basically you insulate not just the house but a large blob of what it stands on. The limited heat loss from the house keeps the foundation from freezing, moving the frost line somewhere under the skirt instead. Very suitable for retrofitting an old house compared to digging down to the frost limit, which is rather deep here and would risk the house falling down, which just stands on these stones. I'm basically at the bottom of what they stand on or slightly below already. Additional benefit is a huge amount of thermal mass that should help the house retain its temperature longer.