In the summer months, in Europe, the concern shifts from heating your home to cooling your home. For generations this has been possible without use of air conditioning. A natural way of cooling has been for centuries to dig into earth to take advantage of the rather stable and cool temperatures underground. In home building this has also been a tradition for centuries to build cellars underneath a home.
Wine growing regions have used the cellar as a natural cooling facility for long and short term storage of wine. Digging into a mountain of a valley offered a natural cool storage solution for years without additional energy consumption. Modern buildings should return to this practice and gain cold from below the surface to cool buildings in the warm or hot seasons. Good isolation combined with the cool temperatures from below the surface in summer months will substantially save energy and thereby CO2 emissions. Circulation of the cool air from below to the upper floors is the issue as well as dealing with differential humidity levels. The wisdom of previous generations in home building with no access to air conditioning (avoiding the air conditioning paradox) is likely to experience a revival in the coming years.
Coolness is the new and old wisdom.