> For a long time I've wanted a variant of this idea in which the exterior walls of the house are separated by a man-sized air gap from the interior walls of the house.

Cue an increase in the number of DIYers and toddlers needing to be rescued after they have somehow managed to fall into the gap e.g. while working / playing in the attic.

Not sure how it would work from an insulation perspective, either.

Houses used to be constructed similarly. It’s actually very good, insulation wise. You have two walls you can insulate & a large air gap: pretty amazing.

But it wastes a lot of space & is expensive

High efficiency houses are actually constructed this way, although the gap is not "man-sized". It eliminates a large amount of thermal bridging.

If it's worth it to the owner, exterior walls will be framed with 2x6 instead of 2x4 to allow more space for insulation.

Mr. Chickadee did an addition like this: https://youtu.be/8fdm9R1Cbm0

Not sure if that’s a Japanese design, or where it came from.