The paper aims to illustrate the problem of summer overheating of rooms in specific locations with large windows in new well insulated buildings in moderate climates. Such a problem did not exist in older buildings characterized by relatively high heat transfer (losses) through the building envelope.

Southern and western rooms are exposed to especially high solar irradiation. Solar energy gains can influence the thermal comfort of rooms with large windows significantly, causing high cooling energy demand even in moderate climates.

 A mathematical model of the energy balance of a building has been elaborated with the focus being on heat transfer through windows and solar energy passing through them.

The results show that if the glazed surface of the west and south façade of a building is large, then the annual (summer) cooling demand,  can be twice the annual (winter) space heating demand. This is a new challenge for the energy supply systems and the thermal comfort of new buildings.