If you want a well-insulated house in Japan you usually need to build it yourself. If you are looking at an older house to buy or rent, insulation is usually lacking, unless the original owner decided to go the extra mile to create a home with insulation. For the majority of older homes, however, they tend to be built cheaply and can be cold in winter and hot in summer.
The construction and home building industry in Japan has come a long way in recent years with active efforts to create zero-energy homes. There are a lot of options now for double or triple-glazed glass windows, non-aluminum frames, insulation, and thermal breaks.
These insulation options are not just limited to new construction, it is possible to renovate an existing home to make it a zero-energy home.
One of these projects was recently completed in Daizawa near Shimo-kitazawa Station in Tokyo. The 2-story home was built in 1987. It has a concrete basement, while the above-ground structure is wood-frame. The house was fully renovated by Rebita and YKK AP, resulting in a contemporary home with numerous energy-efficient features, insulation, and earthquake-retrofitting.