Window Performance

Several technological advances have vastly improved the performance of windows over the past 20 years.

Multiple Glass Layers

Condensation on interior window surfaces can be reduced by installing energy-efficient windows, using efficient glazing and low-conductivity frames. Each layer of glass and air space works to increase the inside surface temperature of the innermost layer of glass, thereby reducing the potential for condensation. Windows that separate heated space from unheated space or the exterior must be at least double-glazed.

Some condensation around the edges of windows can be expected during cold weather, particularly if the humidity in a house is high. Double-and triple-glazed windows with glazing that have low-conductivity edge seals and low-conductivity frames create warmer interior surface temperatures and reduce the potential for condensation.

Low-Emissivity Coatings

Much of the heat lost and gained through a window is due to radiation, a process whereby warmer objects radiate heat to cooler objects, as in the case of sunlight passing through a window and warming objects in a room. Low-emissivity (low-E) coating is a thin metallic film deposited on glass that acts as a mirror to radiate heat back into a room during cold weather, and back to the outdoors during hot weather, thereby reducing heating and cooling bills. It provides a double-glazed window with about the same energy performance as a window with uncoated triple-glazing, but at less additional cost. Many window manufacturers offer low-E glazing as a standard feature.

Gas Fills

Another innovation is the use of inert gas between layers of glass in a sealed unit in the place of air. Inert gases have a higher insulating value than air because they are heavier, resulting in lower convective and conductive heat losses between the panes of glass. Argon is the most commonly used gas due to its availability and low cost. Many window manufacturers offer windows made with inert, gas-sealed units.

Solar Heat Gain Coefficient

The solar heat gain coefficient (SHGC) is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. Solar heat gain can provide free heat in the winter but can also lead to overheating in the summer. The appropriate SHGC for a given window depends on the climate, orientation, shading conditions and other factors.

Edge Seals

The thermal efficiency of a sealed glazing unit can be significantly improved by using a low-conductivity edge spacer between panes of sealed glass units. Aluminum was commonly used for window spacers but resulted in cold areas around the edges of glazing units. Plastic, silicone and glass fibre spacers are now used to reduce thermal conductivity at the glass edges.

Thermally-Efficient Frames

Window frames are typically constructed of aluminum, vinyl (polyvinyl chloride, or PVC), wood or fibreglass. Each of these materials has unique properties suited for window frame construction, but differ in terms of thermal performance. Aluminum is very conductive (1,000 times greater than vinyl, wood or fibreglass) and, therefore, performs poorly from a thermal efficiency standpoint. Aluminum frames must be thermally broken using low-conductivity materials such as PVC or nylon, which improve the performance of the frame significantly. Frames constructed of wood, PVC and fibreglass all have similar low-conductivity properties Further improvements can be made in PVC and fibreglass by filling the voids and airspaces in the frames with insulation. While the frames may only make up a small portion of the overall window area, the thermal performance of a window is significantly influenced by the frame selection.

Window Selection

The following list shows window selection factors:

In addition, the Harmonized Standard and Canadian Supplement require manufacturers to make windows that are:

All manufactured windows and entrance doors are required to meet forced entry requirements. All site-built doors and site-built windows located within 2 m (6 ft. 6 in.) of the ground must meet the same requirements.

Source : Canada Mortgage and Housing Corporation (CMHC)