How Do Energy-Efficient Windows Work?
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If window replacement is on your home improvement to-do list, consider windows that help you save energy. Energy-efficient windows can improve home comfort, lower energy bills, and reduce your carbon footprint.
Our guide shares the qualities that make a window energy efficient, energy performance ratings, and tips for choosing energy-efficient windows that meet your needs.
What Makes a Window Energy-Efficient?
Energy-efficient windows are made of materials that have insulative properties or block UV rays. In comparison to traditional windows, energy-efficient windows typically have a combination of several of the following features:
- Double or triple-pane glass: Windows with two or three panes of glass fused together provide a more substantial barrier than single-pane windows, keeping temperature-controlled air in and outdoor air out.
- Insulated frames: Fiberglass window frames have air cavities that can be filled with insulation to give them superior thermal performance compared to other options, such as wood windows.
- Insulating gas fills like argon gas or krypton gas: Double and triple-pane windows have insulating gas between the panels to help keep them closer to room temperature.
- Low-E glass coatings: Low-emissivity glass is treated with a coating that filters the amount of UV light entering your home. It keeps heat out in the summer and in during the winter.
- Warm edge spacers: Spacer systems between double and triple panes of glass are designed with materials that reduce condensation buildup and seal in escaping heat.
Understanding Energy Performance Ratings
Energy performance ratings from the National Fenestration Rating Council (NFRC) and Energy Star can help you understand which windows conserve energy. The NFRC tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. Energy Star bases its qualification only on U-factor and solar heat gain coefficient ratings.
The following metrics measure the rate of heat transfer and insulation properties that make windows energy-efficient.
U-Factor
U-factor is the rate of non-solar heat transfer through your window. While some U-factor ratings refer to just the glass or glazing, NFRC U-factor ratings represent the entire window performance. U-factor ratings are measured on a scale from 0.15 to 1.20, with lower U-factor ratings indicating better insulating performance. The optimal U-factor rating for your replacement windows depends on your climate.
- Colder climates: Less than or equal to 0.30
- Mixed climates in the North and Midwest: Less than or equal to 0.32
- Mixed climates in the South and Central regions: Less than or equal to 0.35
- Hot climates in the South that are cooling dominated: Less than or equal to 60
Solar Heat Gain Coefficient
The solar heat gain coefficient (SHGC) measures how much solar radiation passes through the window. The lower the SHGC, the less solar heat it transmits. Lower numbers are ideal for hot climates to minimize heat gain, while higher numbers are better for cold climates to allow more passive solar heating.
Air Leakage
Air leakage (AL) refers to the rate of air movement around a window when there’s a pressure difference across it. Windows with a low AL rating are tighter than those with a high air leakage rating. Proper installation also plays a crucial role in determining AL.
Visible Transmittance
Visible transmittance (VT) is a fraction of the visible spectrum of sunlight transmitted through the glazing of a window. The VT you need for your window is determined by your home’s daylighting requirements and whether you need to minimize interior glare.
Condensation Resistance
Condensation resistance (CR) measures the performance of parts of the window assembly through different temperatures and humidity levels on a scale from 1 to 100.
Types of Energy-Efficient Windows
Energy-efficient windows are available in several window types and styles. A window’s functionality can impact your home’s energy use. Here are some popular window types:
- Awning and hopper windows: Awning windows are hinged at the top and open outward, while hopper windows are hinged at the bottom and open inward. These types of windows have lower air leakage rates because the sash closes by pressing against the frame.
- Bay and bow windows: Created from joining a combination of windows to create a curve, bay and bow windows introduce more light and airflow into your home. The energy efficiency of these windows depends on the size and how the side windows operate.
- Casement windows: Hinged at the sides, casement windows are similar to awning windows in that the sash closes by pressing against the frame to reduce air leakage.
- Double-hung windows: In a double-hung window, both sashes slide vertically, resulting in higher air leakage rates than hinged windows.
- Picture windows: Stationary windows that don’t open, picture windows are sealed directly in the frame, making them energy efficient by design when installed correctly.
- Sliding windows: In a double-sliding window, both sashes slide horizontally, resulting in higher air leakage rates than hinged windows.
The cost of energy-efficient windows ranges widely and depends on window size and materials.
| Window Type | Benefits | Cost Per Window |
| Awning | Lower air leakage | $400–$850 |
| Bay | Increased light and airflow | $900–$7,100 |
| Bow | Increased light and airflow | $1,500–$6,500 |
| Casement | Lower air leakage | $150– $1,000 |
| Double-hung | Affordable and ideal for window air conditioners | $150- $650 |
| Hopper | Lower air leakage | $150–$650 |
| Picture | Sealed in the frame, promoting natural energy efficiency | $65–$700 |
| Sliding | Easy operation and cleaning | $150–$800 |
Choosing Energy-Efficient Windows
Choosing the best energy-efficient windows for your home requires a careful examination of your climate and energy efficiency goals. You can compare various window materials and certifications to determine which windows align with your goals.
- Compare the energy performance ratings of different window options. Ask dealers and window installation professionals how various window types impact energy efficiency. Compare ratings of different styles to determine which one most likely meets your needs.
- Consider the window style and material that best suits your home. Various styles of windows have energy-efficient properties that help you reduce energy consumption. If you dream of sliding windows but are concerned about air leakage, ask about options like low E-coatings and insulated frames.
- Evaluate your climate zone and energy efficiency goals. Your climate determines your heating and cooling needs and optimal energy efficiency ratings. In colder climates, consider selecting gas-filled windows with low e-coatings and a low U-factor. In warmer climates, select windows with coatings to reduce heat gain.
- Get multiple quotes from reputable window installers. Shop around for the best prices and all the features you want before making a purchase. Get multiple quotes and consider the cost of installation to ensure the optimal performance of your new windows.
- Look for Energy Star and NFRC certifications. No matter what style you seek, look for Energy Star and NFRC labels to ensure you find windows with the highest energy efficiency ratings. Select windows with ratings that align with your climate based on your location.
- Review warranty coverage and maintenance requirements. Before making a purchase, learn all the details about different window warranties. Consider what’s covered, how long the warranty lasts, and whether coverage changes over time.
Benefits of Energy-Efficient Windows
Energy-efficient windows provide a wide variety of benefits. Consider how advantages like lower energy costs, reduced noise, and improved curb appeal will enhance your home.
Lower Heating and Cooling Costs
Energy-efficient windows use various components to help reduce air conditioning and heating needs. Depending on the windows you’re replacing and your local climate, energy-efficient windows can reduce your energy bills by up to 13%.
Improved Comfort and Reduced Drafts
Energy-efficient windows typically feature double or triple panes insulated with argon or krypton gas to reduce draftiness. Different window types can also reduce draftiness. For example, the closure type of hinged windows, such as awnings and casement windows, helps prevent air leakage.
Better Moisture Control and Less Condensation
Energy-efficient windows cut condensation on the interior surface of windows with properties that help keep window temperature aligned with room temperature. For improved condensation and moisture control in colder climates, look for windows with a high condensation resistance rating.
Enhanced Soundproofing and Noise Reduction
Double- or triple-pane windows designed to improve temperature control have the added benefit of soundproofing. Triple-pane glass is the best option for reducing noise. Since the same technologies to minimize energy consumption also improve soundproofing, the air leakage rating can help you find the highest levels of soundproofing.
Increased Home Value and Curb Appeal
Replacing your home’s windows can add instant curb appeal and even increase its value. Homes with high energy efficiency ratings sell for 2.7% more than homes with lower ratings, according to Freddie Mac.
Reduced Fading of Furniture and Carpets
Certain types of window coatings and films designed to reduce UV rays can help you reduce the fading of furniture and carpets without affecting your view. Windows with high visible transmittance ratings are most effective at reducing fading.
Environmentally Friendly With Lower Energy Consumption
Using less energy can help you achieve your environmental goals. Energy-efficient windows reduce the amount of power needed to heat and cool your home. These windows allow homeowners to use large, energy-efficient windows to increase natural light.
Our Conclusion
Energy-efficient windows provide substantial benefits, helping you save money on energy bills and enhancing your home’s comfort, potentially increasing your property value. These windows are available in many types and styles to suit various home types.
Learning about energy efficiency ratings for windows can help you choose the windows suited for your climate. Compare Energy Star and NFRC certifications of different windows and consult with professionals to find the best windows for your specific needs and budget.
FAQ About How Energy-Efficient Windows Work
What is low-E glass, and how does it work?
Low-E glass is glass coated with a non-toxic coating that minimizes the amount of UV light that passes through it. By diminishing UV light, low-E glass can improve natural light while reducing cooling costs.
Are triple-pane windows worth the extra cost?
Whether triple-pane windows are worth the cost depends on whether you’re spending more on energy bills due to air escaping from your windows. Triple-pane windows consist of three layers of glass instead of the two that make up double-pane windows. While they help save energy, it can take a long time to recoup the costs. If you have many windows and are planning to stay in your house for a long time, triple-pane windows may be worth the cost.
How much energy can you save with energy-efficient windows?
You can save up to 13% on energy costs by replacing poor-performing windows with Energy Star-certified windows. How much you save depends on the type of windows you’re upgrading from and your local climate.
What is the best frame material for energy-efficient windows?
The best frame material for energy-efficient windows maximizes energy savings by offering insulative properties and a strong structure to support heavy windows. Wood, vinyl, and fiberglass are good frame materials for energy-efficient windows.
How long do energy-efficient windows last?
Energy-efficient windows can last for decades. Quality windows last 15–30 years, on average, but some last more than 50. However, the low-E coating on energy-efficient windows can lose effectiveness over time, diminishing energy savings.
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