The conviction has become established, windows are the weakest point in the housing in terms of thermal insulation. The heat transfer coefficient k for windows is much higher than for solid walls.
The ko coefficient characterizes the window only in terms of heat loss. The energy balance of windows consists of both losses, as well as the profits resulting from the solar energy penetrating through the windows. In order to characterize the actual share of windows in the energy balance, the introduction of the so-called. effective kef (equivalent), which would also include gains from solar radiation.
Triple glazing covered with a selective coating and filled with gas (C02, argon, krypton, xenon) they have such favorable thermal insulation, that with high solar radiation transmittance they have a negative kef value in the south orientation, which means energy savings without any additional running costs. The above statement is true under the condition, that the degree of use of solar energy penetrating through the window is not less than 60%. This is related to the heat capacity of the entire enclosure of the room. The smaller it is, the lower the efficiency will be, because with a large amount of solar heat, rooms overheat. For this reason, the efficiency of direct solar heating of rooms with a light casing is low in the long term.
In fact the windows are covered at night, and in passive systems, even more accurate and efficient reduction of heat energy losses through windows with blinds is postulated, ROLES, reflective foils, shutters etc. All these efforts are aimed at reducing the value of the heat transfer coefficient through the window to the outside ko, and thus making the window an even more efficient solar energy collector.