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Window Retrofits – Analysis of Options

In Canada, there are several factors to consider when getting ready to complete a window retrofit. There are also several different options to consider including: complete window replacement, window shades, exterior or interior storm windows, or window films.

The choice for retrofit will depend on the goals of the building. If the windows are nearing end of life, or there are significant moisture and mold issues, a complete replacement would offer the most benefit. If the main issue is heat loss in the winter, exterior or interior storm windows may be more cost effective, and if the main issue is unwanted solar heat gain in the summer, a window film may be the best solution.

Regardless of the option chosen, replacing or updating windows offers numerous benefits including: utility savings, reduction in carbon emissions, increased occupancy comfort, and reduced maintenance requirements.

Replacement/upgrade options are explored in more detail below:

Window Film

The main advantage of a window film is reducing the solar heat gain coefficient of the window in the summer. A high-end window film can also reduce the U-factor, reducing heat loss in the winter. The most effective films on the market can result in the window absorbing 60% less heat in the summer and losing 20% less heat in the winter! Modern, higher-end, films also do not impact window appearance or quality of light.

Interior/Exterior Storm Windows

Storm windows can be a good alternative to a full window replacement if costs are prohibitive and the existing windows are not damaged. The windows can be installed on the interior or exterior of the existing window. Interior windows are a good option for historic buildings that want to maintain the exterior appearance. Storm windows are approximately half the cost of a window replacement and can reduce the U-factor by the same amount as a full window replacement (decreasing heat loss by up to 40%). The disadvantages of storm windows are that they may damage more easily than a regular window, some types can discolour over time, and they have a shorter lifetime than a regular window.

Window Replacement

The type of window chosen when completing a replacement depends on the goals of the building. If the building’s main goal is to reduce utility consumption and costs, and the building does not have air conditioning, then a window with the lowest U-factor (regardless of solar heat gain coefficient) should be chosen. If the building operates heating and air conditioning, then the U-factor and solar heat gain coefficient need to be balanced, or cellular shades or window films should be used in conjunction with the new windows. The Lawrence Berkeley National Laboratory’s WINDOW computer analysis program can be used to find the best windows for you building type.The following is a summary of some common window parameters:

Glass Type Glass Thickness (in) Visible Transmittance U-Factor Solar Heat Gain Coefficient
Single Pane Glass 0.25 89 1.09 0.81
Double-Pane Glass 0.25 79 0.48 0.7
Double-Pane Bronze Reflective Glass 0.25 21 0.48 0.35
Triple-Pane Glass 0.125 74 0.36 0.67
Pyrolitic Low-E Double-Pane Glass 0.125 75 0.33 0.71
Soft Coat Low-E Double-Pane with Argon 0.25 73 0.26 0.57
High Efficiency Low-E 0.25 70 0.29 0.37

*Window parameters referenced from: “The Whole Building Design Guide, A Program of the National Institute of Building Sciences”, www.wbdg.org/resources/windows-and-glazing.

Windows have a significant impact on building energy consumption and occupancy comfort. Even if the temperature in a suite is relatively high (i.e. greater than 22ºC), the tenant could still feel cold due to drafts from the windows. They may increase the temperature in the suite further, thus the building is both losing energy through the window, as well as paying for additional heat that would not otherwise be required. Efficiency Engineering is a full-service engineering firm that can help with the design and implementation of a window upgrade or retrofit. We use a whole building holistic approach to ensure all systems are operating at peak efficiency prior to and after any upgrade and to ensure the full benefits of the upgrade are realized. With more than 25 years of experience in full systems building design with an emphasis on energy efficiency. We pride ourselves on helping clients run a better building.