Windows are an integral part of the building envelope and offer a lot of flexibility in their characteristic, that allowing the selection of the most climate appropriate and energy optimum combination of characteristics - SHGC and U values, size and orientation.
Solar Heat Gain Coefficient (SHGC) value is determined by the fraction of the solar heat (radiation energy) incident, that the windows allows to pass through it. For example a window with 0.35 SHGC, will let 35% of incident solar radiation to pass through. In a cool climate, windows with a high SHGC allow a greater amount of solar radiation to pass through, resulting in free solar heating (passive solar heating) for the home. Similarly in a hot climate, a lower SHGC (less than 0.3) is recommended, to prevent heat gain during the summer days.
WINDOW U VALUES
A window's thermal transmittance is measured by its U Value is the inverse of the insulation R-value. Thus a lower U-value window, will have a higher insulation property. Between the roof, the walls and the windows, the windows have the lowest insulation value and can account for the maximum heat loss (and gain) to the outside. Therefore In cool climates, it is important to preserve the heat inside the home, and a lower U value (less than 0.3) windows are recommended. A low value is recommended even in hot climates to prevent heat gain from the outside through the windows.
Low-E coated glass has a high visual transmittance and a low SHGC
WINDOW VISUAL TRANSMITTANCE & LOW-E
The amount of visible light transmitted by the window is measured by its visual transmittance or VT. A tinted glass window therefore has a low VT. A higher VT is desirable in windows facing a stunning vista or greens, and should be an important consideration when purchasing windows. Low-E coated windows are spectrally selective (let in parts of the light spectrum, blocking the hot infrared component) windows have a low solar gain (SHGC) window and a high VT.