Consumer's Guide – To buying Energy-Efficient Windows and Doors

Catalogue No.: M-92-156/2001E
ISBN: 0-662-27162-9

Section 4 –
How Windows Perform

Before making a decision about which windows to buy, it is useful to review how windows perform, in terms of how they allow a home to gain energy from the sun, and how they affect energy loss when the sun isn't shining.

4.1 Factors Affecting Gains

There are several factors affecting the ability of windows to capture solar heat. They include: 1) placement and orientation; 2) design of the window unit (and the amount of clear window opening); 3) the type of glazing used; and 4) the amount of interior and exterior shading.

Placement and Orientation
Placement and orientation of the window with respect to the sun will be the primary determining factor affecting solar gains, although some gain is possible in all directions from diffuse sky radiation.

During the winter, the sun's low elevation in the sky at midday enables it to shine through south-facing windows (Fig. 21 a). These solar gains can help reduce your heating costs during the winter.

In the summer, when the sun is much higher at midday, very little sun actually strikes a south-facing window (Fig. 21 b). And what sun does reach the window is at such a low angle that it reflects off the window. Awnings or a modest eave overhang can be used to shade south windows in the summer to minimize these unwanted heat gains even further. Properly placed, these shading devices shouldn't interfere with winter solar gains.

Overheating in summer tends to occur more from unshaded west facing windows and, to a lesser extent, east windows. Well-placed deciduous trees will reduce summer overheating while permitting desirable winter solar gains.

Window Design
The design and heat gain factor of a window will also have a bearing on its ability to capture solar heat. A window with a wide frame and numerous small lights separated by mullions and muntins has less glazing area available to capture solar energy (Fig. 22 a). By contrast, a window in the same rough opening with a thin frame and one large light will have a greater proportion of glass to frame area and so will allow more sunlight into the living space (Fig. 22 b).

Glazing Choice
The number of glazing layers will also affect solar gains. For example, a triple-glazed window with ordinary glass reduces solar gain by 20 percent compared to a single-glazed window with the same glazing area. A double-glazed unit reduces solar gain by about 10 percent. (Fig. 23).

Glazing coatings and tints also make a difference. Clear glass transmits the most solar energy into a building. Tinted glass and glass with special insulating low-E coatings can reduce solar gains by up to one third. For example, a double-glazed window with a low-E coating on one glass surface may transmit up to 20 percent less solar heat to the interior, compared to a double-glazed window of similar area with standard glass (Fig. 24). Different types of low-E coatings vary greatly in terms of their effect on solar gains. Some that are designed for southern climates are not appropriate for use in Canada.

Shading
The shading of windows, either from interior drapes and curtains, or from exterior landscape elements such as trees, will also influence the amount of solar gain. On sunny days during the winter, keep the drapes open to admit as much solar gain as possible.

Remember that the type of trees and shrubs you plant near your windows may affect the winter solar gain potential of the windows. Select deciduous trees with thin branching characteristics for southern exposures. They will provide shade in the summer but will lose their leaves in the fall and allow more sunlight through.

4.2 Factors Affecting Heat Losses

There are several processes at work which influence rates of heat loss through window components. These processes follow a basic law of nature: heat energy tends to move from warmer areas to colder areas. There is no way to get around this fundamental principle; all we can do is slow the processes down.

The principal heat transfer processes in windows are radiation, conduction and convection. In addition, air leakage is responsible for a significant portion of heat loss.

Radiation, Conduction and Convection
Absorbed by the inside pane of a double-glazed window, heat moves to the cooler outside pane and is released to the outdoors. This heat loss through windows takes place through the glazing (by radiation); across the spacer material which separates the two glazing layers at their edges and through the frame of the window (by conduction); through the movement of air in the space between the two glazings (by convection); and between the moveable or operable frame components (by air leakage) (Fig. 25).

Radiation losses through the window glass represent about two thirds of the total heat loss in a standard window. Because ordinary glass readily emits heat to colder surfaces (ie., has a high emissivity), radiation losses can be reduced by lowering the emissivity of the glass (hence the term low emissivity or low-E glass).

Conduction losses in windows occur primarily through the edges and frames of the units. Advances in materials and designs that more effectively use insulating materials have dramatically reduced these losses.

Convection losses occur due to air movement between the spaces of multi-glazed windows. If the space is too small, conduction through the air is significant. If the air space is too large, the still air will begin to rise as it is heated on the warm interior side, and fall as it is cooled on the cold exterior side of the window. This convection movement of the air passes heat to the exterior. The best spacing to minimize convection losses is 12 to 16 mm (one half to two thirds of an inch) between the glazings. Other gases (argon, krypton) are often used to reduce convection heat loss. Optimum spacing for these gases can be different.

Air Leakage

Air leakage is a significant contributor to energy costs during both heating and cooling seasons. Most of the air leakage of operable (i.e., openable) windows occurs between the window's sash and frame, or the meeting rails of a sliding sash (Fig. 26). Bigger windows tend to leak less air per unit area. Air leakage can also occur in poorly constructed fixed windows between the insulated glass unit and the frame. (Remember: even in these types of windows, holes are required to effectively drain rainwater.)

Windows with the lowest leakage rates, regardless of type, tend to be fixed windows, that is, windows you can't open. Operable or openable windows come in many types, as described in Section 3. The operable windows with the least rates of air leakage are awning, casement and similar types with a closure mechanism which pulls the sash against a compression gasket, as shown in Fig. 20 (a).

Air leakage can also be a big problem if the windows are poorly or carelessly installed in the rough opening. If the space between the outside perimeter of the window frame and the rough opening isn't sealed with either caulking or foam insulation, air will leak through it. This space should be insulated and sealed before the window trim is attached.

4.3 Balancing Gains and Losses

As we have seen, there is a great deal of two-way "traffic" passing in both directions through windows. South windows often gain more solar energy during the day than they lose at night through convection, radiation and conduction losses.

North windows are usually net losers of energy, while east and west windows tend to be neutral during the heating season. However, during the summer, west windows may be net gainers of energy, posing an overheating problem.

High-performance window technology is pointing the way to significant improvements in this balancing act between gains and losses–maximizing gains when needed, while at the same time minimizing heat transmission as never before.

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