By Jenny Griffin
Energy efficient windows play a vital role in creating a more attractive and affordable home.
Eco-friendly glazing not only reduces heating and cooling bills, but also provides a healthier and more comfortable living space.
So, what are energy efficient windows?
In simple terms these are windows treated and constructed to maintain a comfortable temperature in a room or building.
For colder locations, energy efficient windows keep out drafts and prevent cold air from penetrating indoors, while keeping warm air inside.
Tropical climates and warmer regions will benefit from windows that are able to open up to allow adequate ventilation, which can reduce the need for cooling.
How Are They Rated?
Energy efficient windows are rated according to their insulating properties – before selecting windows, it is best to understand how these ratings work:
- The U-value (or U-factor) indicates the rate of non-solar heat flow through a window or skylight, with lower U-values indicating a higher insulating value. The U.S. Department of Energy recommends selecting a window with a U-value of between 0.25 – 1.25, but for maximum energy efficiency, choose a window with a U-value of less than 0.5 .
- The R-value is the opposite of the U-value. It measures the ability of a window or skylight to resist heat flow, with a higher R-value indicating higher insulating properties.
- Air leakage rating measures air-tightness of a window.
In some countries energy efficient windows carry the Energy Star label, indicating that they have been approved as a genuinely energy-efficient product meeting standards set by the U.S. Department of Energy and the U.S. Environmental Protection Agency.
Energy Star approved windows can reduce household electricity consumption by as much as 15 percent, and are an environmentally friendly alternative, as they reduce the need to burn fossil fuels that add to global warming .
Technology – Minimizing leaks, Improved Double Glazing, and Low-E Windows
Energy efficient windows have adequate seals to prevent air leaks, and may include a number of other technologies to enhance their insulating capabilities.
Double glazing provides extra insulation to a window by trapping an insulating layer of air between the glass panes.
In cold climates installing double glazed windows is the first step one can take to prevent heat from escaping from a home.
To further increase the insulation properties of a window, double-glazed units can have the air space between the glazing filled with a gas such as argon, krypton, sulphur hexafluoride or carbon dioxide. These all have better insulation properties than air.
The gas is sealed into the pocket between the glass to prevent it from readily escaping .
Efficiency measures are most easily implemented when building a new home, however installing energy efficient replacement windows is generally straight-forward if you’re renovating.
When purchasing double glazed windows look for windows that have thermally improved edge spacers separating the glass panes. While aluminum is typically used as a spacer, it is a good conductor of heat, and therefore heat can be lost through the edges more readily than if a less conductive material, such as stainless steel, is used.
New technologies utilize a single-tape element consisting of a spacer, sealer, and a desiccant, or butyl tape, silicone foam and aluminum spacers with thermal breaks .
Low Emittance Windows (Low-E Windows) have a very thin metal or metallic oxide coating that is applied to one or more glazing surfaces that face an air space in a multi-glazed window, or to a thin plastic sheet that is inserted between the panes.
The low-E coating increases the insulating properties of a window, as it reflects heat back into the room during cold spells, and back outdoors during warmer spells. Low-E windows offer good insulation, and have a low U-factor .
Before purchasing windows with low U-factors make sure to ask the manufacturer if the U-factor specification listed applies to the glazing only, or to the whole unit.
If it only applies to the glazing, the U-factor is likely to be higher, since the frame and spacer effects are not taken into account.
It is also advisable to avoid aluminum-framed windows without thermal breaks, as these can present condensation problems during hot weather.
The best materials for window frames are fiberglass, vinyl, and wood, which have great insulating capabilities .
Shading devices, such as awnings, drapes, roller blinds and venetian blinds, can be used in combination with energy efficient windows for controlling heat and providing shade.
Interior shading devices, such as drapes, can be used to both keep heat in, and to keep heat out. By opening or closing drapes and blinds in the daytime, one can control whether the warmth of the sun penetrates the room or not.
Drapes also provide a layer of insulation, and closing drapes when the sun goes down can provide an effective measure to retain heat in a room at night.
Exterior shading devices, such as awnings and pergolas, are efficient at blocking radiation, and are effective for keeping a room cool.
Cost of Efficient Windows
The number one detractor that puts many homeowners off installing energy efficient windows is the cost involved.
While energy efficient windows are more expensive than most standard types of windows, and the initial financial outlay may be high, money will be saved in the long run. The household’s annual electricity consumption will be reduced, allowing the homeowner to recoup this initial capital outlay.
 U.S. Department of Energy. Selecting Windows for Energy Efficiency: What’s New in Building Energy Efficiency.
 Energy Star. Residential Windows, Doors, and Skylights.
 U.S. Department of Energy. Energy-Efficient Windows.