Solar Heat Gain Coefficient (SHGC)
The Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight.
The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1.
The lower a window's SHGC, the less solar heat it transmits.
Hot Climates vs Cold Climates
Unlike U-Factor where "lower is always better", the ideal SHGC depends heavily on your climate and the orientation of your house.
Southern / Hot Climates
If you live in a hot climate and use air conditioning extensively, you want a Low SHGC (typically < 0.25) to block the sun's heat and reduce your cooling bills.
Northern / Cold Climates
If you live in a very cold climate, you might actually want a High SHGC (typically > 0.40) on your south-facing windows to take advantage of "passive solar heating" during the winter. This allows the sun's heat to enter your home and reduce your heating bills.
Finding the Balance
In mixed climates, you often need a balanced window that blocks the intense summer sun but allows some winter sun.
Modern Low-E (Low Emissivity) coatings are engineered to selectively block certain spectrums of light (like UV and Infrared heat) while allowing visible light to pass through, giving you the best of both worlds.