Types of Energy Efficient Windows

Saving on costs through better efficiency.

You slash your heating and cooling bills by choosing the right sort of glass for your replacement windows. There are now advanced solutions for all window types. Clearly, these types of energy efficient glass will cost more than standard types, but the savings, especially if you frequently experience extreme cold or heat, or your house is large, can be very significant over the life of the windows.

Windows with Low-E coatings

The E in Low-E stands for "emittance" and refers to the reduction of radiative heat flow. This is achieved by almost invisible layers of metal or metallic oxide which are deposited on the glazed surface. This reduces the U-value of the window. Typically, low-E coatings (also known as sputtered or soft-coat) are combined with double or triple-glazed windows. Heat transfer principally occurs with thermal radiation from a warm glass pane to a cooler one. The coating is therefore applied to the surface facing the gap between the panes of glass. This significantly reduces the amount of radiant heat transfer.

Gas-filled windows

Another form of energy efficient windows are gas-filled units. Typically double or triple glazed units are separated by a gap filled with air or dry nitrogen and sealed. The sealed unit has air currents within this gap, just like any other air-filled space, and heat rises to the top of the unit and cold air falls to the bottom through the process of convection. The higher the velocity of convection the more heat conduction takes place through the window. The speed of convection can be reduced by filling the gap with a denser gas such as argon and krypton. These gases are non-toxic, non-reactive, odorless and clear and both achieve significant improvements in thermal performance. Krypton achieves superior results than argon, but is more expensive to produce. In some cases the two gases are mixed to combine an energy efficient result at the right price.

Windows with low conductance spacers.

The panes in most double glazing units are separated by aluminum spacers. While aluminum is strong and easy to manipulate, it is also an excellent conductor of heat. There a now a range of less conductive materials available to perform the same function, such as stainless steel, and changes in window design that increase the insulation. These are called warm edge technologies and typically include a spacer, sealant and a desiccant. Such technologies can produce significant reductions in a window's U-factor.

Combining technologies

High-performance glazing is tailored to suit your climate. Low-E coatings, multilayer glazing and gas fills can be combined to produce an energy efficient window which is perfect for your home. The variation is achieved by modifying the amount of solar gain. High Solar Gain, Low-E glass is suitable for climates where heating dominates and solar gain is desirable. Moderate Solar Gain, Low-E glass is suitable for climates where both heating and cooling are required. This type of glass will typically have an argon gas fill. Low solar gain, Low-E glass, or spectrally selective low-E glass, reduces heat loss in winter and heat gain in summer. This is most appropriate for warm climates where cooling is the important energy expense. This is a more energy efficient window than most tinted or reflective glazings as, not only does it reduce solar heat gain, but permits daylighting, thus reducing the cost of artificial light.

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