Energy-efficient windows: What they are and how they work

• Energy-efficient windows often feature multiple glass panes, gas fills, low-e coatings, and well-insulated frames.
• Proper installation is crucial to ensure a window's energy efficiency benefits are fully realized.

Heat gain or loss through windows accounts for 25% to 30% of our home’s heating and cooling energy use. Drafty windows are another common issue that leads to high utility bills and inconsistent indoor temperatures. Both factors force your home’s HVAC system to work overtime to maintain a comfortable temperature, leading to higher energy costs.

Switching to energy-efficient windows can address these costly problems when done correctly. 

This guide dives into what makes a window energy-efficient and what to look for when shopping for one that suits your home, climate, and budget. 

What makes a window energy-efficient?

Many factors make a window energy-efficient, but the glass type, coating, gas fills, and frame materials are things to prioritize when shopping for new windows. You’ll also want to consider where you live and your home’s orientation, as certain features benefit different climates and sun exposure. 

The best energy-efficient windows feature multiple panes of glass filled with an inert gas, typically argon or krypton, that further insulates the window. Wood, fiberglass, and composite frame materials are usually the most energy-efficient, depending on your climate and willingness to maintain them. 

Next, consider the coating. Windows with a low-e coating or film cost more, but may reduce energy loss by 30% to 50%. Different low-e coatings are designed for hot or cold climates and can even control the amount of visible daylight that comes through your windows. 

An often overlooked aspect of energy efficiency is installation. 

“You can buy the highest quality windows on the market, but a poorly executed installation can undermine the energy efficiency benefits," says Kiril Ratia, founder of Ratia Construction.

Ratia says that if a window installer or a carpenter uses the wrong-size nails, they can puncture the window's sealing mechanism. A good installer should also place insulation around the window, add flashing tape, put weather stripping around the sashes, and finish the seal with caulk. Anything that compromises the seal of the window and the opening it fills will lead to drafts.

Understanding energy ratings and labels

Window energy ratings and labels guide buyers to make the best choice, but they can feel overwhelming at first. Here’s a quick breakdown of common energy ratings and labels—and what to look for. 

A window U-factor rating provides insight into a window or door's energy efficiency by measuring its heat transfer through the glass and glazing. The lower the number, the better, as this indicates a more energy-efficient product. 

Solar Heat Gain Coefficient (SHGC) is the amount of solar radiation transmitted or absorbed through a window, glass door, or skylight. The lower the SHGC rating, the better the product is at shading and the less solar heat it transmits. A window with a higher SHGC rating keeps heat in the home during the winter. Choose windows with an SHGC rating that best suits the climate and your home’s orientation. 

Visible transmittance (VT) is a performance rating measuring the fraction of visible sunlight entering your home through the window. This rating ranges from 0 to 1, with 0 indicating that no visible light enters the home. Typically, higher VT ratings are ideal because they reduce the amount of electric lighting your home needs during the day. 

Air leakage ratings measure how much air seeps into your home through a closed window. Look for windows with a low air leakage rating, as this indicates greater energy efficiency. 

Lastly, look for windows with ENERGY STAR certifications. The U.S. Environmental Protection Agency manages this rating, and it indicates that a product is energy-efficient. For the best results, use the ENERGY STAR Climate Zone Finder to match the window type to your home's climate zone. 

How energy-efficient windows can lower energy costs

Energy-efficient replacement windows decrease the amount of heat your home loses in winter and help keep it cooler in summer. If you live in a colder climate, a gas-filled, low-e window helps reduce heat loss. In contrast, homeowners in warmer climates should prioritize windows with a solar-control film or coating to keep their homes cool. 

ENERGY STAR-certified windows can save homeowners up to 13% on energy bills when replacing single-pane windows. If your electric bill is around $150 per month, this is a savings of around $234 per year. If your home has inefficient or older double-pane windows, you’ll likely benefit from replacing them with more energy-efficient windows, reducing energy costs and improving curb appeal and resale value. However, these benefits are often a long-term result, rather than immediate savings. 

When energy-efficient window replacement makes sense

Old or drafty windows account for 25% to 30% of our home’s heat gain and loss, which dramatically affects monthly energy bills. If you notice rising energy costs, drafts, or condensation between window panes, consider caulk or weatherstripping to update existing windows, or replace them entirely for the best results. 

Ideally, replace all of your windows at once (especially if they’re the same age), or work room by room to target the worst offenders. 

Costs and considerations

Window costs vary significantly based on the window materials, installation team, and energy-efficiency features. Most homeowners can expect to pay between $400 and $1,500 per window (with the higher end of the range reflecting premium wood, fiberglass, or composite windows). 

When shopping for windows, consider your local climate and which features are best suited to maintaining a comfortable temperature year-round, as this will keep your energy costs lower. 

In terms of return on investment, homeowners can expect long-term results in several ways. 

“Homeowners can typically expect to recoup 60% to 70% of their window investment at resale, but the real payback begins immediately through reduced energy bills," says Phil Adkins, president at Adkins & Sons, a home improvement contractor. In my experience, most families notice a meaningful reduction in their utility expenses within the first year, particularly in regions with harsh winters or hot summers where HVAC systems work overtime.”

Bottom line

Choosing an energy-efficient window that’s a worthwhile investment depends on where you live, your home’s orientation, and your budget.

Prioritize energy-efficient features that suit your climate, and choose products certified by reputable programs like ENERGY STAR to reap the greatest benefit. Before replacing, assess your home’s windows for drafts and lackluster features, such as single-pane glass, to determine your needs. Then work with several installers and collect multiple quotes to compare pricing, warranty coverage, and personalized recommendations. 

FAQs

What is a good U-factor for windows?

A good U-factor for windows is as low as you can afford, with the lowest value typically falling around 0.20. Lower U-factors indicate the window is well-insulated, helping to cut energy costs and keep your home more comfortable. 

Are ENERGY STAR windows worth the cost?

Yes, ENERGY STAR windows can lower your home’s energy bills by an average of 13% when you replace single-pane windows. This can save you several hundred dollars a year in energy bills if you’re upgrading from a single-pane window, making it worthwhile for many homeowners. 

Is it better to replace all windows at once or one room at a time?

It’s typically better to replace all windows at once if your budget permits, since contractors often offer a lower installation price, saving you money on labor. In addition, replacing all of your home’s windows improves your home’s energy efficiency, helping you to save on utility bills.