Windows have a much higher U-value than the opaque envelope and are therefore likely to account for high heat losses/gains. Conversely, its transmittance allows solar gains to enter the building, contributing to the heating loads in the cold periods. High levels of insulation can be achieved with several numbers of panes, low emissive coatings and gas filled cavities, or with new materials such as the aerogel. During the night period, when temperatures drop, the use of movable insulating systems prevents further heat losses. Over the summer, solar gains should be avoided. Passive solutions may include overhangs, reveals, movable shutters or blinds.

The quantity of radiation incident on the facade varies with the time and day of the year. Designing for solar gains strongly depend on the orientation, site layout and sunlight availability.

3.5 Passive cooling

There are a number of different approaches to passive cooling design. They may involve buoyancy and cross ventilation, night ventilation usually associated with strong inertia, ground cooling, evaporative cooling and night sky radiation.

Throughout Europe, a relatively large diurnal swing in air temperature creates the possibility of night time ventilation to cool the building’s heat storage capacity, delaying its release to the space, as well as reducing temperature fluctuations. The ground below 3-4 meters has a relatively stable temperature which may be used to cool or pre-heat the air or a fluid passing through buried pipes. Its strong inertia may also be used for heat storage.

Although night sky radiation can promote cooling of the roof surface, this is rarely used in cities due to the pollution and the greenhouse effect which significantly reduces the radiation exchange to the atmosphere. Conversely, evaporative cooling is commonly found in traditional architecture, particularly in dry and hot climates of southern Europe. Some strategies of indirect evaporation allow its use in more humid climates.