I think it depends on the builder - my mother in law rebuilt her (~100 year old) home in a mid-sized city in Japan about 15 years ago. It seems comparable to american houses -- good insulation for winter (which is good since the only heat is a split-unit heat pump downstairs, none in the bedrooms), noise isolation between rooms (and between floors) seems similar to american wood frame construction. Of course, the tatami room downstairs has paper doors, so if you end up sleeping there, you hear everything in the living room and kitchen.

Are Japanese houses even worse than American houses ? From a European stand point most American houses are built cheaply and especially their insulation is much worse than in Middle Europe. At least that is what I've been told and what I've experienced some years ago.

How old of American home are you comparing to?

If you are looking at a 40 year US home, it may only have R15 or R20. If you are looking at a new normal home in the midwest, you are looking at an R40 or so. If you custom build you can pay the extra few hundred bucks and get R60 or more..

Well in Austria the current standard is triple pane windows (my parents house form the 70ies had double pane windows).

I don't know the current percentage, but passive houses (https://en.wikipedia.org/wiki/Passive_house) are getting more and more in this area.

Not beeing in the construction business I can't give you any details how the way we insulate our houses compares to your information. Most of the houses here are built with concrete and bricks, so they're insulated differently (styrofoam, as much as I know).

Not sure this is a useful comparison?

Chicago average temperature: http://www.holiday-weather.com/chicago/averages/

Sydney average temperature: http://www.holiday-weather.com/sydney/averages/

In Sydney, your primary concern is heat. In Chicago, your primary concern is cold. They have different strategies to combat. I.e. good blinds could cut your cooling bill 50% in Sydney. Good blinds will not make a dent in your Chicago bill..

Austria in Europe, not Australia on the Southern Hemisphere :)

Innsbruck average temperature: http://www.holiday-weather.com/innsbruck/averages/

That's pretty close to Chicago.

There are contextual concerns though, I'm often told that western-standard insulation would be very expensive in japan: you'd need to dehumidify like mad to keep the entire thing from moulding and rotting.

What about Florida, Georgia, Louisiana? Humidity doesn't seem to be an issue with homes there. How is Japan any different?

We run HVAC systems 100% of the time basically. If you turn a house HVAC off in Florida for a few months the house will generally be full of mold.

I'm not sure that this is any different from the West (at least considering the many places in which I have lived).

Pay more, get more.