Japanese apartments insulation differs regarding the type of building; concrete or wood but also regarding if it is a rental building or residential building.
In residential buildings, you could find floor heating or double glazing, however in rental building, the only equipment that is supposed to warm the room is your air conditionner.
In my point of vue, japanese insulation is not so efficient, however japanese people use a lot a different way to warm up.
The electric carpet:
You will find it in almost all households.
There is also a version for bed, you but the "bed carpet" under your sheets, and it keeps your bed warm.
The famous kotatsu is a heating table over which an included cover keep the warm.
If your still frozen, you still have the japanese pyjama option...