Energy prices are skyrocketing, and decesion have to be made between eating or heating. It is time to make our homes more sustainable and thus save energy. In summer, our homes use less energy than in winter. In summer you are outside more often and the heating does not have to be switched on. In winter, we often keep the windows closed to keep the heat in, but unfortunately that is when the problems arise. There is no fresh air coming into our well insulated houses. The lack of fresh air leads to high CO2 levels, which in turn leads to various health complaints.