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A Window into Energy Efficiency

Originally published in Energy Matters April 2019 issue

Windows are a big investment. But with so many types and technologies on the market, it can be difficult to figure out which ones to pick. So we have some tips to help you get started!

A great first step is to make energy efficiency a top priority. In Canada, Energy Star windows have been tested and certified as the best energy performers on the market. According to Natural Resources Canada, Energy Star windows can save an average of 8% on your energy bills. Even better, models with the Energy Star “Most Efficient” designation are up to 40% more efficient than standard windows. So why replace a window with a standard model when you can also lower your ongoing energy use and improve the comfort of tenants?

Take these steps below to choose the best windows for energy efficiency.

Step 1: Understand what the terms and rating means

  • U-Factor (U-Value): This is the rate of heat flow through a window. The lower the U-factor, the more energy efficient the window. U-factor can refer to the glass or glazing or the entire window unit. Make sure to ask what the overall unit’s U-factor is, because it’s the total performance that’s important.
  • Solar Heat Gain Coefficient (SHGC): Is the amount of solar radiation admitted through a window and released as heat into the indoor space and is shown as a number between 0 and 1. A product with a high SHGC is more effective at collecting solar heat in winter, while one with a low SHGC is better at reducing the need for summer cooling. Consider the climate, orientation, and outdoor shading to determine the best SHGC rating for a particular location.
  • Air Leakage Rating: The rate of air infiltration through a window under specific environmental conditions. A product with a low air leakage rating is more air tight. Installation also impacts air tightness, so hiring a good installer is crucial.
  • Check out this Glossary for more helpful terms.

Step 2: Know what window features help your building’s energy efficiency

Comparison of Single-glazed and Triple-Glazed, Medium-solar-gain Low-E Glass
  • U-Factor (U-Value): This is the rate of heat flow through a window. The lower the U-factor, the more energy efficient the window. U-factor can refer to the glass or glazing or the entire window unit. Make sure to ask what the overall unit’s U-factor is, because it’s the total performance that’s important.
  • Window types: Some windows, such as awning and casement, have lower air leakage rates. For a comparison of operating types, go to the bottom of this link. 
  • Frame types: Vinyl, wood, fiberglass, and some composites resist heat flow better than metal. See this pros and cons comparison of their relative performance and find out how spacers impact heat transfer.
  • Glazing: This is the part of the window you see through, usually glass. An insulating glass unit typically has at least two panes of glass separated by a spacer bar, is filled with an inert gas like argon or krypton to reduce heat transfer, and is sealed around the edges to make them airtight. The more glazing layers, the better. Here is a useful comparison of double and triple glazed performance.
  • Low-Emissivity (Low-e) Coatings: These ultra-thin films improve a window’s insulating properties and help control solar heat gain. Low-e coated windows may cost 10-15% more than standard windows but can reduce energy loss by up to 30-50%. These coatings can reduce a window’s ability to transmit visible light, so consider a spectrally selective low-e coating. Learn more here.
  • Inert Gases Between Panes: This will minimize heat transfer between the inside and outside of the window. Krypton is usually used when the space between panes is ~1/4 inch and while more costly it has better thermal performance. Argon can be used where spacing is larger, e.g. ~1/2 inch. See this detailed breakdown.
  • Each window unit should be labelled with its performance ratings. See these Energy Star examples.
  • Attachments: Awnings, operable shutters, and screens help reduce energy use by providing shade while still allowing light in. Learn about passive solar design here.

Step 3: Know your climate zone

  • Ask for models certified for your climate zone or for zone(s) colder than your region to save even more energy. Energy Star models made after January 1, 2020 will have the same climate criteria for all of Canada and will no longer be divided into three zones (1, 2, and 3).

Step 4: Consider sun and wind conditions

  • Older buildings or ones in in cold climates may need windows with medium SHGC values to let the sun’s heat in during winter months. Those in windy areas may benefit from models with compression seals to reduce air leakage, such as casement and awning units. For eastern and northern walls, windows with higher insulation values can help reduce heat loss.

Step 5: Hire a trained, reputable installer

  • Installation impacts performance! Poorly installed windows may cause condensation, cold drafts, or even allow water leaks.
  • Get at least 3 quotes and ask the installer to help you apply for incentives.

Questions? Contact us