Windows are responsible, in large part, for heat loss in any house. So if you neglect this important aspect of your home’s energy efficiency, you’re literally throwing your money out the window! When the time comes to replace your windows, it is essential that you demand precise information on the energy performance of various models. The Energy Rating and Energy Star® qualification, both of which consider the entire product and not just one or other of its components, are invaluable guides to making an informed choice.

Rating energy performance
The ER (“Energy Rating”) measures a window’s overall performance, i.e., how well it:

  • delivers solar heat gain in winter;
  • prevents heat loss through the frame, the spacer between the panes of glass, and the glass itself; and
  • helps control heat loss due to air leakage.

The weighted measure of these various performance factors is used to determine a window’s ER on a scale of 0 to 50. The higher the ER, the better the product’s energy performance. The calculation method is determined by the Canadian Standards Association (CSA).

Generally speaking, if a window has a high ER, it will necessarily be better designed and built. That means it will deliver greater comfort as well as better sound insulation, and be less prone to condensation forming on its surface.

Energy Star: Choose the right zone
Energy Star is an internationally recognized symbol that allows consumers to readily identify products that offer high energy efficiency performance. In the case of windows, the ER enters into the Energy Star qualification process, which is also based on testing done by an independent, accredited laboratory.

Remember, however, that you need to look for more than the label with the star logo to make sure you are buying the right high-performance windows for your home. The windows have to be Energy Star–qualified for the specific climate zone where you live. This is fundamental!

For the purposes of Energy Star® qualification, Canada has been divided into four climate zones, based on average annual temperatures: Zone A is the mildest, and Zone D is the coldest. Only Zones B, C and D can be found in Quebec; the minimum required ER for most types of windows are 25, 29 and 34 respectively.

Source: Hydro Québec

Tip: if you live on or near the border between two climate zones, or you simply want to maximize the energy efficiency of your windows, don’t hesitate to choose a product that meets the minimum ER for a colder zone than yours.

Humidity issues and condensation

A certain amount of humidity in your home is desirable for comfort during the cold months. Windows, doors and skylights that are not energy- efficient will often have condensation or frost on them even when the indoor humidity is at a reasonable level. This condensation, in addition to obscuring the view, can lead to the formation of mould on the frames and sashes or can damage insulation and wood within the wall. An ENERGY STAR qualified product will allow for higher indoor humidity before condensation occurs. You may even be able to turn down your thermostat and still feel comfortable. Because humidity levels are higher in bathrooms and kitchens, consider installing a product qualified for a colder zone in those rooms to reduce, or eliminate, condensation. The table below shows when condensation is likely to form.

Maximum humidity before condensation occurs

Outside temperature Standard window High-performance window
0 °C40 %50 %
–10 °C 30 %40 %
–20 °C20 %30 %
–30 °C 15 % 25 %
–40 °C10 %20 %

If the indoor humidity in your home is too high, try the following.

  1. Turn off the humidifier on your furnace.
  2. Ensure that the clothes dryer and the bathroom and kitchen fans are vented to the outside.
  3. Reduce the number of plants in your home.
  4. Store firewood outside.

If you have a mechanical ventilation system (heat recovery ventilator [HRV]), make sure that it is turned on and working properly. Some airtight houses were built without mechanical ventilation systems. If high humidity is a chronic problem in your home, consider having an HRV installed.

A note about exterior window condensation
Condensation may appear on the outside surface of the exterior pane of glass of energy-efficient windows and doors for brief periods. This condensation generally occurs on cool mornings in the spring or fall, just before sunrise, if there is little or no wind. Because the glass is minimizing heat loss, the outside glass surface becomes cool enough to reach the dew point, which causes condensation to form. This type of condensation will rapidly disappear after sunrise. If the condensation is persistent and appears as a circle in the centre of the glass, or is between the glass panes, there may be a problem with the glazing unit, and it may have to be replaced.

Source about humidity and condensation: NRCan, archives