Thermal Efficiency of Windows

By: Elizabeth Wanga

As discussed in past articles of our windows series, historic windows can be restored and improved to enhance their efficiency. Replacing the existing glazing with thermal glazing is another way to do this. As long as the material of the sash is sound, and has the strength and depth to support the additional glazing, this can be a great option for improving a windowas thermal efficiency. Also, this method is more functional and aesthetically pleasing than using interior or exterior storm windows.

In order to determine if a window can accept thermal glazing, the sash must be evaluated to determine if there is sufficient thickness to route the sash and accept the glass and glazing stop. The size of the sash is also a consideration. Many manufacturers of thermal glazing units limit the dimensions the window sash can be.

The additional weight of the window with the thermal glazing must also be considered. The sash weights will need to be increased to balance a double or single hung window. The additional sash weight for windows that pivot or swing may be more difficult to counterbalance. The armatures or hinges may need to be resized to support the additional weight.

Windows gain and lose heat through three methods: conduction through the glazing and frame, radiation from the sun, and air leakage around the components. This last method can be mitigated through improving weatherstripping, as discussed in the second post in this series.

The properties of a window are measured by energy performance characteristics. A U-factor rating measures the non-solar heat flow through the window. Windows with greater efficiency have lower U-factors. The solar heat gain coefficient relates to the fraction of solar radiation transmitted through the window. A lower coefficient corresponds to less heat being transmitted. This is preferred where the objective is to reduce cooling costs. But in a cooler climate, with long heating seasons, or on windows that face north, it may not be advantageous to have a low solar heat gain coefficient. Depending on the window orientation, climate and building design, different types of glazing may be warranted throughout a building

The space between the glass on a thermal glazed window is often filled with argon. Because the argon has a higher resistance than the air, the heat transfer through the window is reduced. This insulated unit works to mainly lower the U-factor, but also reduces the solar heat gain.

There are several coatings or tints that can be added to a window to reduce the solar heat gain coefficient and glare. Low-E coatings are used to control heat transfer. A thin layer of the coating is applied directly on the glass surface. Adding the coating to the manufactured glazing can increase the cost of the window by 20%; however, the reduction in energy loss can be as great as 50%. The coating lowers the U-factor and can be specified with varying solar gain. It should be noted that the coating can reduce the visual transmittance of the window. Heat absorbing tints can also be used to reduce solar radiation; however, since they change the color of the glass, they are not generally warranted on existing and historic buildings.

In our case study window, we reviewed an existing 40ax68a single hung historic wood window. The single pane glass was replaced with double glazed thermal glass with 1/8a outer layer with low-E coating, a 1/4a Argon filled space, and a 1/4a laminated inner layer. The U-factor for the existing glass was originally 0.82. With the new glazing installed, the U-factor was reduced to 0.49. Similarly, the solar heat gain coefficient was greatly reduced from 0.80 to 0.38. The actual energy savings for any building is dependent on many factors including the climate, wall design, the direction the windows face, and the ratio of windows to wall area.

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