Understanding Solar Heat Gain Coefficient in Replacement Windows

In today's world of replacement windows the Solar Heat Gain Coefficient plays a major role in determining the energy efficiency of a replacement window. As energy costs rise it is becoming more and more important to understand how each aspect of the replacement window's performance can help to lower your energy costs and make your home more efficient.
The Solar Heat Gain Coefficient is only one measure of a window's energy efficiency, but it is an important one. The Efficient Windows Collaborative defines Solar Heat Gain Coefficient in the following terms:
Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window's shading ability. It is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater it's shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.
Translating Solar Heat Gain Coefficient
Now that we have the official definition of Solar Heat Gain Coefficient, let's break it into layman's terms and find out how it applies to your specific case.
For a good example of solar heat gain you should look no further than your own skin. Think of your skin as a window to your body. When you are in the sun you feel the sun's warmth on your skin. As you remain in the sun you start to feel your skin warming up as well. If you remain in the sun for a longer period of time, your nerves under your skin start to pick up that it is hot. When it gets warm enough you start to sweat to cool down.
The feeling you get when the sun is warming your skin is solar heat gain. Your skin is admitting the solar radiation and absorbing it inward. As more and more heat is absorbed your skin heats up until it is not able to absorb anymore and it starts transmitting heat to your body. Once enough heat has been transferred, your nerves tell you it is hot and it is time to kick on the A/C and you begin to sweat.
The Solar Heat Gain Coefficient works the same way with windows. As the sun hits the window, the window begins to absorb the heat. As the window heats up it begins to transmit heat across the glass and into the interior of the house. Your thermostat then registers that the temperature in the house is rising and it is time to kick on the air conditioner.
In order to reduce the amount of heat gain into the house, the Solar Heat Gain Coefficient needs to be lowered. Using the skin example, this is is accomplished by wearing clothing that allows you to be in the sun and feel its warmth without getting completely hot. The clothes act as an assistant to your skin to help block the radiant heat. For windows the Solar Heat Gain Coefficent is lowered by providing "clothing" in the form of Low-E Coatings and gas filled insulated air spaces.
Low E Coatings reduce the Solar Heat Gain Coefficient by providing a translucent, reflective coating that is applied to the inside of the insulated glass. Although it causes some light tinting on the window, you can not see the low e on the glass. The Low E provides a reflective quality that helps repel the sun's radiant heat trying to enter the house. Because it reduces the amount of heat moving across the window into the house, it keeps the interior cooler, reducing the need for the air conditioner to run.
Gas filled air spaces also help to lower the Solar Heat Gain Coefficient of the window. Insulated glass is made of two pieces of glass "sandwiching" an air space. Normally this air space is just a vacuum sealed space without any air. To reduce the Solar Heat Gain Coefficient this airspace is filled with an inert gas, usually argon or krypton. The inert gas is heaver than oxygen and its molecules do not move around as much when heated up.
The gas acts as a sponge to help absorb the sun's heat and keep the Solar Heat Gain Coefficient low. As the sun hits the window, its radiant heat hits the inert gas. Because the molecules do not get as excited and move as much, the heat is not easily transferred across the glass and into the house. The result is a much cooler interior when the sun is beating down.
Applying it to the real world
It is obvious that a lower Solar Heat Gain Coefficient can be good for preventing heat gain inside a house. However, the importance of the Solar Heat Gain Coefficient varies depending on where you live. If you live in a warmer, sunny climate, you want to look for the lowest solar heat gain coefficient you can get. If you live in a climate with very cold winters and mild summers, you might not want a low Solar Heat Gain Coefficient.
This is because you want to be able to capture as much of the radiant heat from the sun as you can for warmth inside the house. For northern climates and colder areas you should instead look for the best U value you can find in a window. If you are shopping for replacement windows it is a good idea to understand all replacement window ratings and how they apply to your project.