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Chesapeake Roofers: Article About Heat Transfer Through Windows

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When discussing different window options, Chesapeake roofers and window experts may mention a number called the U value. This value is a measure of heat loss through a window. The U value encompasses the four ways that heat travels through a window: conduction, convection, radiation and air leakage. By becoming familiar with these four heat transfer mechanisms, homeowners can better understand how energy efficient windows work.

Conduction is the movement of heat through direct physical contact. Conduction occurs when there is a temperature gradient; heat flows from the warmer object to the cooler one. An example of conduction is a porcelain bowl filled with hot soup. Heat from the soup travels through the bowl, making the exterior of the bowl feel warm to the touch.

In a window, conduction occurs when heat passes directly through the glass and frame to the outside. Insulated windows have two or three glass panes that trap layers of air in between. More advanced insulated windows fill the space between panes with a low conductance gas such as argon or krypton. These types of windows block heat from passing through the panes. Heat transfer is also reduced by choosing a less conductive material for the window frame. Wood and vinyl are poorer conductors than metals such as aluminum, so vinyl and wooden frames are more energy efficient than aluminum frames.

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Heat transfer also takes place through the movement of liquids or gases. This process is called convection. When warm indoor air meets a cold window, the air cools, becomes denser and falls to the floor. Near the window, warm air moves to fill in the space left behind by the cooled air. The window cools the newer warm air, repeating the process in a cyclical way. A homeowner will feel this air movement as a chilly draft and turn up the heat, increasing energy usage. Drafts resulting from convection can be reduced by using insulated windows, which have warmer surfaces that don't cool interior air as much.

Another way that heat can travel is by infrared radiation, which is long wavelength, invisible light that is felt as heat. The sun emits infrared radiation that passes through a window, heating a house. Warm objects inside the house also emit infrared radiation. Window glass can be treated with a low emissivity, or low E, coating that reflects infrared radiation. On a hot day, the low E coating blocks the sun's infrared energy from entering the house. During winter, the same coating keeps the house warm by preventing indoor infrared energy from leaving the building.

Air leakage occurs when air seeps through small openings around a window. These cracks allow heated indoor air to escape during the winter and cooled air to be lost during summer. Weatherstripping and tight window closing mechanisms save energy by preventing air from leaking through.

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