Wherever there is a window, there is heat transfer through it; windows account for 10% - 30% of heat loss from buildings. The Integrated Learning Centre's west face contains a "curtain wall," composed of eleven different types of window, each with different coatings and performance characteristics.
Each of the fourteen windows mounted on the west face of the ILC has known thermal specifications calculated by the manufacturers; the placement and setup of the windows allows these properties to be tested using solar heat gain measurement technologies developed right here at Queen's.
Windows have differing insulation and efficiency characteristics, because different windows are geared towards different environments and uses. Some are better for warm climates; they might let through less visible light but provide more protection against radiant heat transfer, for example. All windows insulate against air leakage and direct transfer, but some perform better than others.
Heat transfer happens in four different ways:
- Air leakage: Leakage accounts for half of all heating and cooling losses. Leakage occurs around the outsides of a windowpane. Well-made glass-to-frame and frame-to-frame joints are essential to mitigate against leakage.
- Direct conduction: Heat is conducted directly through the frame, or through the pane itself. Placing low-conductivity materials between the panes, such as air or argon gas, cuts down significantly on conduction losses.
- Radiant transfer: Glass absorbs heat energy from its surroundings, and radiates it into cooler air. This causes both higher air-conditioning costs in the summer and higher heating costs in the winter.
- Convection: In the winter, warm interior air gets cooled when it comes into contact with the glass. It becomes denser, and drops towards the floor. The space it leaves is filled by warm air from above, and this creates a cycle of air movement that is self-perpetuating - and chilly.
Convection can be ameliorated by good insulation or by adding a heater under a window to break up the cold-warm cycle. Most conduction losses occur at the frame, which is generally made from metal. In double- or triple-glaze windows, large gaps are left between the panes to insulate against both conduction and convection. These gaps are most often filled with air or argon.
Windows must not only insulate against direct losses but also against radiant heat transfer. In warm climates, windows must reflect heat energy away, and in colder climates they must trap heat inside.
Manufacturers have designed reflective coatings that allow shorter-wave visible light through windows, but which reflect long-wave infrared energy. Windows treated with this low-emissivity coating reduce cooling and heating loads by suppressing solar heat flow.
The measure of how efficiently a window insulates against heat transfer is called its â€œUâ€ value. The lower the â€œUâ€ value, the better the insulation. Windows also include a â€œSolar Heat Gain Coefficientâ€ (SHG) rating, the amount of solar energy that passes through the window.
For the Kingston climate, windows with both a low SHG coefficient and a low U-value are ideal, because of our highly variable temperatures. Windows must both keep heat inside during the winter and air-conditioning costs low in the summer.