Window film may improve window glass performance

Posted June 19, 2014 at 10:03 am

The site orientation of a home and its windows is a significant issue in solar performance and contributes to both heat gain and loss according to Jeff Barber, an architect and housing specialist with University of Missouri Extension.

Typically, buildings that have considered seasonal solar exposure tend to place windows to allow direct sunlight in during colder months and shade openings from sun during warmer months.

“The composition, configuration and overall performance of the glass in these window systems can also play a significant role throughout the day and night,” said Barber.

In some cases, adding a window film to windows can add thermal value in the winter and provide significant protection from solar heat in the summer.

Overall window energy performance is determined by the window’s thermal resistance (U-value), the Solar Heat Gain Coefficient (SHGC) of the glass, the Glass Visible Transmittance (VT) and the window system’s resistance to air infiltration.

“Window tints and films can dramatically affect performance when careful consideration is given to the film used and its function in the window assembly,” said Barber.

When selecting the appropriate window film for a window, look for the Visible Transmittance (VT) value of the film when applied to a single pane of clear glass and the Solar Heat Gain Coefficient (SHGC) for that application. If the VT value is greater than the SHGC value, then this is an appropriate film for a hot climate window.

The Light-to-Solar-Gain (LSG) ratio is the VT value divided by the SHGC value. The higher this ratio, the more spectrally selective the film and the better it is for hot climates.

“Many manufacturers may void the warranty for their windows if film is applied because of potential thermal stresses that can affect the glass, particularly in thermal laminated units. Be sure to check the window manufacturer’s warranty before applying the film,” said Barber.

Assuming a person’s home is located in a temperate climate, the primary considerations for window film would balance keeping heat inside during the winter and heat out in the summer.

“Heat loss through large window areas could be significantly improved with a properly designed application of a low emissivity film,” said Barber. “Conduction performance, which is determined by the windows U-value, would not improve as significantly.”

Regarding solar heat gain during the summer, the west to northwest orientation of windows would allow some potential for late day heat gain. Widow film could help with this, but Barber says a better solution might be exterior vertical shading devices or strategic planting of trees.

“Careful consideration of the exact angles of solar exposure could provide the best and least expensive solution,” said Barber.

However, window film may offer a significant increase to the performance of the window glass.

“Before buying this remedy, a ‘whole-house’ energy audit might reveal other areas for greater saving. The investment in the services of a certified Home Energy Auditor could help to determine priorities and understanding of the “return-on-investment” for all applicable energy efficiency strategies,” said Barber.

More information on this topic is available online at http://extension.missouri.edu.