In the highly insulated and airtight houses of Hokkaido, strong thermal boundaries create a comfortable indoor environment in winter, but the separation of inside and outside limits people's behaviour, and it is difficult to say that the houses are flexible enough to respond to diversifying lifestyles. The incorporation of a thermal semi-indoor/outdoor space in a house, after realising a comfortable indoor environment with even temperatures, has the potential to restore regional and season-specific ways of living that are being lost, and to be used as a space that brings out the unique ‘mature way of living’ of high-performance housing.