Understanding Solar Heat Gain
or Solar Radiation

Understanding Solar Heat Gain (SHG) is essential before any window purchase.

Another major mode of heat transfer through windows is SHG or solar radiation. Radiation is the transfer of heat from one solid to another through electromagnetic waves. It is the movement of heat through space without being conducted from molecule to molecule (conduction) or carried in a gas or liquid (convection).

For instance, you feel radiant heat as you face a camp fire, although the air around you is still cold. All parts of your body within “sight” of the fire’s heat will be warmed, but your back will remain cool.

Objects are constantly radiating heat to other objects. In fact, people radiate heat to cooler objects around them, including cold windows and doors. This radiant heat loss from our bodies can have a significant effect on our comfort.

The biggest source of radiant energy is the sun. When the sun’s radiation strikes glass, some of it is reflected back outside, some is absorbed, and the rest passes through the glass to the inside of the building.

How much solar radiation passes through a window depends on the time of the year, the direction the window faces, how much external shading there is, whether they're double glazed and the ability of the glass to reflect solar heat.

In winter, solar heat gain is usually welcome, as it adds heat to the building.

However, in the summer, SGC through north, west and east facing windows is usually a major source of heat buildup.

Windows and doors with double glazing, and low-E coatings keep far more solar radiation out of the building than windows and doors with single panes of glass.

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 that is admitted through a window, both directly transmitted and absorbed, then subsequently released inward.

SHGC is expressed as a number between 0 and 1.

Problems associated with solar heat gain:

• Heat gain in summer
• Discomfort for occupants sitting in the path of solar radiation through the glass
• Fading of drapes and upholstery from the sun’s radiation

Reducing Solar Heat Gain through windows should always be an important factor prior to any window purchasing decision.

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