Thermal value of window tints

Asked April 28, 2014, 10:50 PM EDT

We are buying a chalet-style house with a glass face to the west-northwest. Would window tint have any thermal value in the winter? Also, will it provide any significant protection from solar heat in the summer?

Michigan

1 Response

Orientation of a home and its openings, particularly windows, is a significant issue in solar performance, primarily contributing to heat gain and heat loss. 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 widow systems can also play a significant role throughout the day… and night.

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 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 a window assembly.


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.

Assuming that the home indicated in this question is located in Michigan (as indicated from the state of its origin), the primary consideration would most likely be keeping heat inside during the winter. Radiant heat loss through large window areas could be significantly improved with a properly designed application of a low emissivity film, although conduction performance, determined by the widows U-value, would not improve as significantly.

Regarding solar heat gain during the summer, the west – northwest orientation of the windows would allow some potential for late day heat gain. Widow film could help this, but 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.

Window film may offer a significant increase to the performance of the window glass, but 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 an understanding of the “return-on-investment” for all applicable energy efficiency strategies.

Excellent, research based resources on the subject include:
“Windows and Glazing” – Whole Building Design Guide – National Institute of Building Science
http://www.wbdg.org/resources/windows.php
and
“About Window Films” - Florida Solar Energy Center – University of Central Florida http://www.fsec.ucf.edu/en/consumer/buildings/homes/windows/films.htm