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When it comes to saving money on your home’s heating and cooling, windows play and important role. An average house can lose up to 10% of heat through windows (check out our blog on energy efficient retrofits for details). When doing upgrades, it’s common for homeowners to replace the windows.

When looking for windows, here is some important information to keep in mind.  

Window Types

There are 5 different types of windows typically used today. They include Casement Windows, Awnings, Hoppers, Sliders, and Hung type windows. All of these window types are not created equal and the pros and cons:

Casement windows are hinged at the sides. These windows have a much lower AL rate than a slider or hung windows because it closes against the frame, making a good seal. Casement windows also provide much better ventilation because the window opens outwards, and is capable of catching the passing breeze and directing it into the house.  Casement windows can also open entirely, where as sliders and hung windows are limited to less than half of the window’s total surface area. 

Awnings are windows that are hinged at the top, and open outwards. The performance of this window is identical to a casement window, with two exceptions being that it does not catch the breeze as well, but is much better at preventing rain from getting in.

Hoppers are windows that are hinged at the bottom and open outwards. The performance of this window is identical to a casement or awning window. These windows don’t catch the breeze or prevent rain from getting in, however because of their angle, they are much less likely to give off any harsh glare from the sun.

Slider windows are made up of two or more windowpanes inside individual frames called sashes that slide freely to the left or right. These windows have higher AL because they do not seal against the window frame, they seal by overlapping the sashes, which does not work very well. The only advantage of these windows is that their design is simple, requiring no mechanical parts, and is typically cheaper to make.

Hung windows are identical to slider windows but instead of sliding to the left and right, they slide up and down. These windows are slightly more dangerous than any other windows. Poorly designed hung windows, if not carefully closed or properly locked in place when opened, may fall down which could break the glass or cause injury, most commonly to fingers.

Window Terminology

Here are some common terms you should know when when window shopping:

• U-factor -  Thermal conductivity.  (lower is less/better)
• SHGC - Solar heat gain coefficient.
• VT - Visible light transmitted. (in or out of the house)
• AL - Air Leakage. (less is better)

Glazing  

A window glaze is a film added to the glass pane in your window on the side facing outwards. This is done to improve the performance of your windows in terms of excess heat gain or loss, (lower U-factor) reduction of glare given off by the window, to modify the window’s VT, or simply to change its appearance in terms of colour.

Gas Fills

A gas fill is a heavy gas with a low U-factor that is placed into the gaps between a window’s panes before the window is sealed. The purpose of a gas fill is to eliminate or reduce any convection currents that would otherwise occur in a regular air filled window, which leads to a lower U-factor. Argon and Krypton are most commonly used for gas fills. These gasses are inert and completely non-toxic. Argon is much cheaper than krypton, however, krypton has better thermal performance.

Low-E Coatings

A low-emittance (low-E) coating is made up of a microscopically thin layer of metal or metallic oxide. This coating is applied on the surface of the glass facing into the gap between the window’s panes, and so, can be used in conjunction with regular glazing. This coating reduces heat loss between the panes of glass in the window itself via thermal radiation, ultimately lowering the window’s U-factor. There are low-E coatings designed for high solar gain, moderate solar gain and low solar gain. Higher solar gain is better for cold climates, and low solar gain is better for hot climates. The purpose of this coating is, when used in conjunction with regular glazing, to allow for a better U-factor for the window yet still have a high SHGC, or vice versa. This coating is invisible to the naked eye.

Frames

Window frames represent anywhere from 10% to 30% of the total surface area of a window, so how they are made is an important thing to consider. The most common frame types are metal, metal with a thermal break, non-metal frames (wood) and thermally improved non-metal frames (insulated vinyl, fiberglass, or engineered thermoplastics). Generally speaking, metal frames have the highest U-factor, wood is better, but the best by far is an insulated non-metal and non-wood design.

Spacers

Spacers are the small bars that hold the windowpanes in a window at the correct distance from each other. These bars are usually around the perimeter of the windowpane. In some, though very few, windows today aluminum is still used as the actual material for a spacer because of its structural properties.  However, aluminum has a very high U-factor so it’s something to watch out for when shopping for, or designing windows. Aluminum’s high U-factor creates rapidly changing surface temperatures on the spacer itself. It can also cause condensation to occur around the edge of the windowpane, which lowers the window’s overall U-factor. More recently window manufacturers are experimenting with much better options such as extruded vinyl, fiberglass and thermoplastics.

For more information on purchasing energy efficient windows, visit the Natural Resources Canada website.