One of the fundamental measures for avoiding indoor overheating or discomfort is
preventing solar radiation from reaching the building and its interior during
summer and respectively allowing it to enter the building in order to heat it
The objective should be to maximize: winter gain (2 * summergain).
The greatest source of heat gain by solar radiation is the direct heat gain through a window.
This could infact increase the indoor temperature far above the outdoor air
temperature even in moderate climates.
Ordinary window glass is transparent for the shortwave radiation emitted by the sun, but opaque to the longwave radiation emitted by the objects in the room. Hence, the radiation once entered through the glass gets trapped.
If solar overheating is a problem, as in tropical climates, there are four methods available for reduction of solar heat gain through the windows: