An energy-efficient window is a device capable of providing good lighting during the day and good thermal comfort both during the day and night at minimum demand of paid energy. Thermal comfort implies that overheating as well as excessive cooling should be avoided, that draught should be small, etc. Thus energy efficiency involves control of radiative inflow, and of heat losses due to radiation, conduction, and convection. The radiative component to energy efficiency is conveniently discussed with reference to the ambient radiation introduced in Section 3.1. It is suitable to make a separation into the requirements imposed by a warm, a cold, and a temperate climate.
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