Air conditioning != dehumidifier. One controls air temperature, the other moisture levels.

While air conditioning can reduce humidity, if you have good insulation then your your HVAC won't run that often, but things like cooking and bathing will effect humidity levels, and so a separate apparatus is really needed for that. They're not unreasonably sized:

* https://www.youtube.com/watch?v=OKJnP_6uxcg * https://www.youtube.com/watch?v=D3wd2yVwzvE

My point is just that Japanese homes are not built with any ductwork at all, aside from a vent above the stove and in the shower/bathroom. A "whole-home" device like those you linked would require installing air ducts to every room, and the homes just aren't designed for it.

Actually you wouldn't have to duct the whole house: humidity does not concentrate like heat does, but rather diffuses. So even if you only have a dehumidify one part of the house, the moisture level would level average out (over a given period of time).

Though this does depend slightly on air circulation, but a couple of tubes (e.g.) connecting floors would help balance things out.

Japanese companies produce many quality products that deal with indoor air quality:

* https://news.panasonic.com/global/topics/2017/53539.html

There are stand alone units as well:

* http://www.survivingnjapan.com/2012/08/a-guide-to-dehumidifi...

All that said, a proper building envelope is the first thing that needs to be done. Don't know enough JP codes and construction techniques to comment on that though.