Windows and doors contribute up to 30 percent of heat loss and gain in a home, by direct conduction through the glass and via air leakage through the window assembly. The US Department of Energy (DOE) estimates that heat loss and gain through today's windows account for approximately 4 percent of total domestic energy consumption. Efficiency strategies vary widely based on regional climate considerations: windows can account for 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. Windows cover between 10 and 25 percent of the exterior surface area in new homes and selecting windows designed to either retain or repel heat is an important aspect of a whole-house energy efficiency plan. The DOE advocates reducing the environmental emissions from heating and cooling systems—some 150 million tons of CO2 alone annually—from 20 to 50 percent. Current superwindow technology can cut approximately 1,000 lbs of annual CO2 emissions from the average homeowner's energy consumption. Air leakage around windows accounts for 10 percent of an average home's total air leakage, adding approximately 1 percent to the average heating bill.
What kind of windows do I have in my home?
The United States window market has undergone a transformation over the past 20 years. Whereas multi-pane windows accounted for only a few percent of domestic window sales in the 1980s, approximately 50 percent of the windows in the US today are uncoated double pane windows; the other half remain mostly uncoated single pane models. While even basic double pane windows control heat loss and gain with approximately twice the efficiency of their single pane predecessors, emerging advances in window technology offer a still greater potential for energy savings; however, replacing windows outright is still not considered cost-effective in terms of savings to the average home energy bill, due to prohibitive material and installation costs. Caulking, weatherstripping, and shading options may offer better savings relative to the investment required for homeowners seeking to improve the efficiency of their current windows.
Advances in efficiency
The Department of Energy is aggressively pursuing "Zero Energy" window technology, achievable through windows that drastically reduce heat transfer to levels that can be offset by their advanced daylighting and solar heating capacities. While such windows are likely several years away from reaching consumer markets, numerous energy-saving features are available in today's window stock, including:
- Layers of glazing: multiple glazings are additional layers of glass panes or plastic films used to improve a window's resistance to both heat transfer and sound transmission. Double-glazing insulates nearly twice as effectively as single glazing, and adding a third or fourth glazing layer provides even better performance, particularly in colder climates.
- Gas/air fill: Up to one inch of space between layers of glazing greatly improves a window's heat resistance properties; manufacturers fill this space with either air or a denser, low-conductivity gas such as argon or krypton to provide further protection against heat transfer.
- Coatings: Low-e—or low-emittance—coatings allow visible light to pass through a window while reflecting infrared heat radiation. Virtually transparent coatings of silver or tin-oxide can be customized for different climates to either reflect exterior solar heat or retain interior warmth. Traditionally, tinted glass or film have likewise served to limit solar heat gain, and recent advances have improved the transparency of tinted coatings and made them increasingly common in cooling-dominated climates.
- Edge spacers: Used to hold multiple panes of glass apart while maintaining an airtight seal, edge spacers were once constructed almost exclusively from highly-conductive aluminum. Within the past 15 years, manufacturers have introduced edge spacers made from steel, silicone foam, and butyl rubber to parallel the increasingly effective insulating properties of improved glazing technology.
- Framing materials: Wood, vinyl, and fiberglass frames offer improved insulating properties compared to the aluminum and steel frames in older window assemblies. Wood provides effective heat resistance, but requires more maintenance than other materials, while low-maintenance vinyl insulates well but can warp and peel over time. Fiberglass frames are strong insulators and show little tendency to expand or contract, but—like wood and vinyl both—can be two-to-three times as expensive as aluminum and steel.
Window ratings and performance indicators
Four additional factors should be considered when buying new windows; the recommended levels for each vary based on regional climate considerations:
- U-factor: A measurement of a how effectively a window protects against heat loss, U-factors in today's window stock usually range between .20 and 1.20; the lower the U-factor, the better a window insulates.
- Solar heat gain coefficient (SHGC): Indicates how well a window blocks heat from sunlight on a scale between 0 and 1; the lower the rating, the less heat that passes through the glass.
- Air leakage rate: On a scale from 0 to 1, the air leakage rate measures the cubic feet of air that leak through the gaps in a window's assembly per minute for each square foot of frame surface area; today's more efficient windows have air leakage rates between .01 and .06 cfm/ft.
- Visible transmittance: The amount of light that enters through a given window, represented on a scale of increasing magnitude from 0 to 1.
Subsidies and incentives
In the US, upgrading your home's windows may qualify you for tax incentives at the federal, state, and 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.
Related health issues
Older single-pane windows and leaks in window casings increase the likelihood of condensation within a home, which can contribute to interior mold growth. Molds produce allergens and irritants, as well as toxic substances called mycotoxins, all of which may contribute to hay-fever symptoms, rashes, and asthma attacks in individuals who breathe mold-contaminated interior air. The US Centers for Disease Control (CDC) also warns that the paint on older window assemblies may contain lead, which can be easily dispersed in the air during renovations and is toxic when inhaled or ingested.
- frame: The fixed, outer portion of a window that holds the sash.
- pane: A framed sheet of glass within a window.
- sash: The portion of the window that houses the glass (typically the movable parts of a window).
- window parts: head (top); jamb (sides); sill (bottom); meeting rail (center horizontal or vertical strip); stool (trim piece on exterior of jamb).
- superwindow: A window that employs multiple layers, low-e coatings, and/or gas fillings to achieve a U-factor of .15 or lower.
- condensation resistance factor (CRF): The higher a window's CRF, the lower the likelihood that condensation will accumulate on it's interior.
- low-e coating: An ultra-thin metal or metallic oxide coating that is transparent to visible light but reflects long-wave infrared radiation (i.e. heat).
- Efficient Windows Collaborative
- National Fenestration Rating Council
- Rocky Mountain Institute - Energy Library
- American Council for an Energy-Efficient Economy - Consumer Guide to Home Energy Savings
- US Department of Energy - Building Envelopes Program Fact Sheets: Retrofit Best Practices Guide
- Rocky Mountain Institute - Home Energy Briefs: Building Envelope
- US Department of Energy Abstract - Zero Energy Windows: Conclusion, pg 14
- ENERGY STAR - High Performance Windows
- US Department of Energy - Energy Savers booklet, page 10
- Rocky Mountain Institute - Cool Citizens: Everyday Solutions to Climate Change
- US Department of Energy - Energy Savers booklet, page 9
- US Department of Energy Abstract - Zero Energy Windows, pg 4
- Rocky Mountain Institute - Home Energy Briefs: Building Envelope
- ENERGY STAR - Replacing Windows
- Efficient Windows Collaborative - Winter 2007 Newsletter
- American Council for an Energy-Efficient Economy - New Windows
- ServiceMagic.com - Window Treatments
- National Association of Certified Home Inspectors - Mold, Moisture, and Your Home
- US Centers for Disease Control and Prevention - Indoor Air Pollutants and Toxic Materials