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Installing low-e windows in your home or office will help to reduce your heating and cooling energy needs, which’ll ultimately reduce your carbon dioxide emissions, too. Read on to find out how low-e windows can also protect your furniture and carpet from fading and discoloration.

How to buy a low-e window

Low emissivity (low-e) windows have a thin, invisible layer of metal or metal oxide deposited on the glass, which reflects warmth into a building during cold months and keeps heat from entering a building during warm months. To see how it works, check out The Efficient Windows Collaborative Low-E Coatings illustrations.

Here are some factors to consider when looking for windows with low-e coatings.

1. Cold climate considerations: If you’re trying to keep heat inside during cold weather, the low-e coat should be applied to the inside pane of glass.
2. Warm climate considerations: Those hoping to reflect heat during summer months should be sure that the low-e coating is applied to the outermost pane of glass.
3. Soft vs. hard: Manufacturers have a choice between soft and hard low-e coatings. Soft coatings are more durable than their hard-coat counterparts, but less durable and more prone to becoming damaged. They are therefore applied inside insulated multi-pane windows. Hard coatings, on the other hand, are resilient and can be applied to the exterior of a window, which makes them ideal for retrofitting old windows.
4. Gas fills: Further boost your new window’s energy efficiency by opting for argon or krypton gas fillings, which can be used on their own or in combination. Argon is less expensive than krypton, but since krypton has better thermal performance, it is preferred in colder regions.
5. One, two, or three?: Double-pane windows are definitely more energy efficient than single-pane windows and are good options for most regions. However, if you happen to live in an area that experiences extreme cold, you may want to consider triple-pane windows, which prevent even more heat transfer.
6. Opt for window film: If you can’t afford entirely new windows, look into less expensive window films which can be applied to existing windows.
7. Look into ENERGY STAR: Windows that are ENERGY STAR rated will often contain many of the features listed above and can save the typical homeowner between $25-450 per year in energy costs.

Before you buy

See your way to ideal U-factors for your area: The most important consideration when purchasing new windows is their U-factor. In general, a 0.35 is best for colder climates whereas a 0.65 is suitable for warmer climes. Check out ENERGY STAR’s Climatic Zones map or Efficient Windows Collaborative Window Selection Tool to find out which windows are best for your specific region. For advice on how to read a window’s label, see Consumer Energy Center’s Window Labels page.

Drawbacks

Although low-e windows cost between 10 and 15 percent more than conventional windows, they can reduce energy loss by 30 to 50 percent, which ultimately reduces heating and cooling costs.[1]

Find it! Windows with low-e coatings

Buying low-e windows helps you go green because…

• They provide increased insulation against heat escape during cold weather.
• They reduce solar heat gain, thereby decreasing cooling energy consumption on hot days.
• They can prolong the life of furniture and carpet by reflecting the ultraviolet radiation that often fades and damages.

Windows contribute up to 30 percent of heat loss and gain in a home:[2] 25 percent of a typical home's heating energy load in cooler climates, and as much as 50 percent of the cooling load for homes in warmer climates.[3] Since windows cover between 10 and 25 percent of the exterior surface area in new homes, selecting windows designed to either retain or repel heat is an important aspect of a whole-house energy-efficiency plan.[4]

Low-e coatings can cut a window’s solar heat gain coefficient (SHGC) by upwards of 30 percent.[5] Compared to coat-less windows, low-e coated windows also provide up to 75 percent more protection from ultraviolet light (UV) rays that would otherwise damage (fade or discolor) artwork, furniture, carpeting, or photographs with repeated exposure.[6]

Tax breaks and subsidies

In the US, upgrading your home's windows may qualify you for tax incentives at the federal, state, or local levels. For detailed information, see these resources:

• American Council for an Energy-Efficiency Economy Updates on potential energy legislation.
• Tax Incentives Assistance Project Explains federal tax credits for energy efficiency.
• Alliance to Save Energy Offers an index of energy efficiency programs by state.
• Database of State Incentives for Renewables & Efficiency Provides information on state and federal incentives.
• Contact your utility provider for information on local offers.

Glossary

• low-emissivity (low-e): A microscopically thin metal or metal oxide layer deposited directly on one or more panes of glass (window, skylight) to reduce heat transfer and subsequently the U-factor. This layer is transparent to the visible solar spectrum (short-wave infrared radiation) but reflects long-wave infrared radiation.
• solar heat gain coefficient (SHGC): Solar heat can be admitted through a door or window either by direct transmission or through absorption. SHGC measures the fraction of heat caused by sunlight on a scale between zero and one, smaller numbers indicating lower levels of heat transfer.
• U-factor: The inverse of the R-factor (a measure of a material's insulating ability; higher numbers indicate better performance), U-factor also measures the rate of heat gain and loss. However, the higher the U-factor number (values range from 0.20 and 1.20), the less the product is able to resist heat flow (i.e., the worse its insulating ability).
• ultraviolet light (UV): The invisible rays of the light spectrum found in sunlight that are not visible to humans. UV can cause fading of carpets, fabric, and paint finishes.

External links

• Consumer Energy Center - Windows
• ENERGY STAR - Anatomy of an Energy Efficient Window
• US Department of Energy - Windows, Doors, & Skylights