Choosing the Right High-Efficiency Windows
Installing high-efficiency windows is an excellent way to improve energy efficiency and save on heating and cooling costs. But with so many options on the market, how can builders and building managers know whether they’re selecting the right windows?
There are a variety of options that can enhance the appearance and performance of high-efficiency windows and determine the best choice for specific climates or building types.
Almost all new windows have two layers of glazing. Dual-glazed windows have a relatively low U-value, which measures the window’s effectiveness in resisting heat conduction. However, adding a third layer of glazing reduces the U-value by about 35 percent. The important thing to consider is whether the cost of the additional glazing layer justifies the cost. If a building is in a cold climate, where heating costs are significantly higher than air conditioning costs, the answer is usually yes. In other climates, dual-glazed is often the most cost-effective choice.
Air or Gas?
Another glazing consideration is whether to fill the space between the glazing layers with air or gas. Convection causes the air between lites to move and transfer heat from one surface to another. The rate at which convection occurs depends on the temperature difference and distance between the lites. When the rate of convection is higher, energy loss is higher. Replacing air with an inert gas reduces convection, lowering the U-value. But like adding a glazing, using gas costs more. Therefore, this is also more cost-effective in cold climates.
In standard glass, energy absorbed by the glazing is conducted or radiated away, in the form of heat, both into and away from the building. The amount of energy radiated depends on the emissivity rating of the glazing. Reducing the emissivity reduces the amount of heat radiated into a building in warm weather, as well as out in cold weather.
Low-emissivity, or low-e, glass uses a thin coating to limit the amount of ultraviolet and infrared light that can pass through glass. Low-e glass is most cost-effective in moderate to colder climates, and in buildings with large areas of glass.