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What Makes Windows Energy Efficient

what-makes-windows-energy-efficientWhen consideringĀ replacing your windows, one major factor to consider is the energy efficiency of the various materials. In order to really make the best choice, you will need to understand what it means for a window to be energy efficient. Although it may initially seem confusing, this is really not a daunting task. By understanding a few key terms, you will have a good grasp on this important characteristic of windows.


Simply put, the U-factor of a window material is the heat lost and gained by the material. There are several different ways this transfer of heat can take place. The main ways are through conduction, convection, radiation, and through the leaking air. Conduction is the transfer of heat directly through the window to the outdoors. The principle of conduction is at work when you touch a hot sauce pan. When your skin touches the hot metal, the metal conducts heat from the pan to your body.

Some types of window materials lose more heat through conduction than others. In order to determine how much heat the material you are investigating will lose or gain, take a look at the U-factor. The U-factor number often comes in the form a fraction or decimal. Windows which are more resistant to gaining and losing heat will have a lower U-factor number. A lower number means the window will have a higher insulating value.

Solar Heat Gain Coefficient (SHGC)

This number refers to the amount of solar radiation, or heat from the sun, which comes through the window. This heat from the sun will contribute to heating the building. A smaller SHGC indicates that less heat from the sun will affect the temperature of the house. If you are interested in windows which will prevent your house from becoming hotter, this is ideal. Particularly in summer time, these windows will decrease cooling costs for the building. A high SHGC number indicates that the window will collect more heat from the sun. During the winter, the extra collection of heat may reduce heating costs.

Other elements which may affect the SHGC is the specific location of the window. A window which is directly facing the sun most of the day will have more opportunity to absorb more heat. However, a window with a high SHGC, which means it is best at absorbing heat from the sun, that stays in the shade the majority of the day may not greatly increase the temperature of the building.

Air Leakage

This factor refers to the speed at which air moves around the window. It is measured in respect to specific pressure differences. In general this number is articulated in terms of cubic feet per minute per square foot of frame area.

If you are trying to keep the temperature of your building at a particular level, a window with a high rate of air movement may make this task more difficult. During winter, you may be trying to keep the cool air out while maintaining plenty of warm air inside. If a lot of air escapes through your windows, your heated air will leave while allowing the cold air to come inside. This may decrease your ability to efficiently maintain a specific temperature in the building.

Window Materials

There are lots of different window materials which can affect the efficiency of insulating your home. One of the main things to consider is the glass used in your windows.

One glass glaze to consider is heat-absorbing tints. This will actually change the color of the glass. The result of this glazing material is to reduce the solar heat gain coefficient. In other words, the window will not absorb as much heat from the sun when a heat-absorbing tint is applied. However, it is important to note that the U-factor of windows with this glaze will not change.

Another option isĀ insulated windows. These windows include two or more panes of glass. The result of insulated windows is to reduce the U-factor. It will also, to a lesser extent, lower the solar heat gain coefficient as well.

You may also consider reflective coatings. This type of glaze will reduce the SHGC of a window by reducing the transfer of solar radiation. This material is a thin metallic layer set on the glass. This is typically used in extremely hot climates. However, this coating reduces the brightness of the room, so it is only used in for special circumstances.