On average, heat lost and gained through windows accounts for approximately 25 to 30% of a structure’s total HVAC energy consumption. This leads to higher operational costs and increased carbon emissions. Older, poorly installed, or simply inefficient windows can allow air to leak in or out, or they may lack adequate insulating properties, resulting in greater thermal heat gain or loss over time.
Fortunately, this situation is not inevitable. Advances in window technology have led to the development of highly energy-efficient windows, thanks to a variety of innovative features, contributing factors, and material choices.
Let’s explore the elements that make a window energy-efficient, compare popular options, and examine a few real-world examples where these advanced windows have been successfully installed.
What Makes a Window Energy-Efficient?
Generally speaking, the simplest way to determine if a window is energy-efficient is to check for an official ENERGY STAR label. These windows meet the efficiency requirements and standards set by the EPA, indicating they are proven to perform better than traditional windows.
However, the presence of a label alone doesn’t explain why a window is energy-efficient. In reality, numerous factors contribute to the overall efficiency of any given window.
Window Frame Material
The structural strength of fiberglass windows, along with their high thermal performance, makes them ideal for large glazing areas in residential or commercial buildings. Photo by Paul Grdina.
While not the sole determinant of energy efficiency, the material used in constructing the window frame significantly influences the window’s overall insulating capabilities.
This is because different materials have varying rates of conduction—represented by the U-value—which indicates how much heat is directly transmitted through the frame itself. Materials with low conductivity (and consequently a low U-value) are optimal for use in energy-efficient window framing.
- Wood:
Wood typically exhibits a very low rate of conductivity and has long been favored as a building material for its inherent insulative properties. On average, wood-framed windows have a U-value that falls between 0.30 and 0.50 Btu/hr-sq. ft-°F.
Wood also offers the advantage of being a highly sustainable choice. However, wood-framed windows can be expensive, demand the most maintenance, and may not last as long as other framing materials (most wood frames typically require replacement every 20 years).
- Vinyl:
Provided they feature an insulated core, vinyl window frames possess a U-value similar to that of wood-framed windows and are roughly equivalent in terms of energy efficiency. Hollow-core vinyl frames, conversely, are less efficient than solid wood frames.
Unlike wood framing, however, vinyl frames are relatively inexpensive and are virtually maintenance-free, while still boasting a long lifespan (30 to 40 years).
- Aluminum:
Western Window System’s aluminum frames. Photo courtesy of Western Window Systems.
While aluminum has historically been used for window frames due to its affordability and minimal maintenance needs, it has a much higher rate of conductivity than other framing options. This makes it challenging to achieve true energy efficiency on its own.
For aluminum-framing systems to be efficient, they must incorporate thermal breaks, which are strips or blocks of insulation positioned between the exterior-facing aluminum frame and the interior-facing aluminum frame. They generally do not achieve the same level of efficiency as wood, vinyl, or fiberglass window frames.
In general, aluminum-framed windows are more expensive, require more maintenance, and are less durable than vinyl-framed windows, though they are cheaper than wood-framed options.
- Fiberglass:
Among all four framing options, fiberglass framing—such as that offered by Cascadia Windows and Doors—tends to be the most efficient, boasting a U-value between 0.20 and 0.30 Btu/hr-sq. ft-°F.
“Fiberglass possesses an inherently low thermal conductivity, meaning without any additional thermal breaks or materials, fiberglass is 500 times less conductive than aluminum,” Michael Bousfield, technical director at Cascadia Windows & Doors, previously stated for gb&dPRO. “That means a large-span double-glazed fiberglass window is more than 100% thermally efficient than a comparable aluminum window.”
On average, fiberglass window frames have a life expectancy of 40+ years and require minimal maintenance.
Number of Panes
PGT Innovations’ Thin Triple Insulating Glass Unit is one of the most energy-efficient window options currently available. Photo courtesy of PGT Innovations.
Similarly, the number of panes (or glazes) in a window significantly influences its energy efficiency, as this dictates how insulative the glass truly is. Single-paned windows offer very poor insulation and allow heat to pass through them with great ease, making them very cold in winter and very warm in summer.
By definition, an energy-efficient window must contain at least two panes of glass, although triple-pane varieties are also available on the market. In both cases, an inert gas (such as argon or krypton) is used to fill the spaces between panes. This gas fill, in turn, helps improve insulation against unwanted heat transfer into or out of the building.
Increasing the number of glass panes also lowers the window’s overall U-value, as multiple panes reduce the rate of conduction.
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In terms of actual energy savings, double-pane windows are estimated to reduce energy consumption by 18% during the summer months and approximately 24% during the winter months—and triple-paned windows can reduce energy use even further. PGT Innovations’ Thin Triple Insulating Glass Unit, for instance—which features two panes of traditional glass and one pane of ultra-thin Corning Architectural Technical Glass—can reduce a window’s U-factor by at least 20%.
Double- and triple-paned windows also provide superior outside noise reduction compared to traditional single-pane windows.
Window Coatings
Large floor-to-ceiling windows at 28&7 flood the interiors with natural light, with corners that are fully glazed. For exceptional energy efficiency, the windows are triple-glazed and incorporate a low-e coating. Photo by Dave Burk, courtesy of SOM.
To further enhance their efficiency, windows can also utilize a variety of coatings, such as low-emissivity (low-E) coatings. A low-E coating is a transparent, microscopically thin layer of film that typically contains metallic particles. These particles help reflect long-wave infrared energy without compromising visibility.
Low-E coatings are the most effective means to lower a window’s solar heat gain coefficient (SHGC), which represents the amount of solar radiation that passes through it. SHGC is expressed as a decimal between 0 and 1—with a lower number indicating less solar heat transmitted.
Although they are more expensive, windows with low-E coatings can significantly reduce energy usage, resulting in lower utility bills. On average, low-E coatings help conserve 50% of interior heat during the winter and reflect 70 to 80% of solar heat gain during the summer.
Weather-Stripping & Caulking
As the name implies, weather-stripping is a material used to seal windows and doors to prevent outside weather from entering. Damaged, ill-fitting, or completely absent window weather-stripping is one of the easiest ways for air (and moisture) to leak into and out of a building.
Different windows require varying amounts of weather-stripping, but the fact remains that any operable window will require weather-stripping between the sash and frame to maintain window efficiency. The most effective window weather-stripping includes tension-seal, reinforced foam, rolled or reinforced vinyl, magnetic, reinforced silicone, and fin-seal.
Non-operable windows and stationary components still require caulking to seal any potential gaps around the edges, preventing air from leaking in or out.
Window Orientation
Casa El Pinar’s south facade is made up of floor-to-ceiling windows and black aluminum frames, guaranteeing natural light and capturing maximum warmth to counteract the cold climate. Photo by Rafael Gamo.
Energy efficiency can be further optimized based on the orientation or placement of the windows themselves, as different directions receive varying amounts of direct sunlight throughout the year.
- If your project is located in a cold climate, all east- and west-facing windows should have a low-SHGC. Similarly, north-facing windows should have the lowest U-value possible, while south-facing windows should feature a high-SHGC and the lowest possible U-value.
- Buildings in warm climates will require that east-, west-, and south-facing windows have both a low-SHGC and low U-value. Additionally, those south-facing windows should receive shade (either from trees, louvers, sun-shades, or other shading providers) throughout the day. North-facing windows in warm climates only need to have a low U-value.
Proper Installation
Ultimately, a window—even an ENERGY STAR-rated one—is only as effective as its installation. If a window is not installed properly, it can allow air to leak in or out, essentially negating its energy-efficient properties.
To ensure your project’s windows are installed correctly, hire experienced, verified professionals who possess the licenses and insurance required by your local government. If the window manufacturer recommends specific installers, take their advice seriously.
Examples of Energy-Efficient Windows in the Field
Now that we are more familiar with what genuinely makes a window energy-efficient and the distinctions between various options, let’s explore a few real-world projects that have successfully reduced their carbon footprint and operating costs by incorporating energy-efficient windows.
- Franklin Mountain House:
Large sheets of glass, overhangs, and well placed windows abound here at the Franklin Mountain House. Photo by Casey Dunn.
In El Paso, the Franklin Mountain House, designed by Hazelbaker Rush, highlights the critical importance of windows’ daylighting capabilities, impacting both energy efficiency and mental well-being.
“We are creatures that instinctively respond to the daily cycles of the sun,” Dale Rush, co-owner of Hazelbaker Rush, previously told gb&d. “It’s imperative to allow the natural light into our homes and to let it dictate our schedule and inform our rhythms.”
Situated in the Chihuahuan Desert, the Franklin Mountain Home receives abundant natural sunlight but is also accustomed to sweltering temperatures. As a result, mechanical air conditioning isn’t entirely avoidable, though its necessity is significantly reduced, largely due to the use of windows from Western Window Systems. These windows feature low-E, argon-filled, dual-pane glass.
- 28&7:
The façade at 28&7 was manufactured in a zero-waste, low-energy process, designed by Shildan Group with a custom glossy black glaze that visually highlights the structural grid. Photo by Dave Burk, courtesy of SOM.
Located in Manhattan’s historic Chelsea neighborhood and designed by Skidmore, Owings, and Merrill (SOM), the 28&7 office building is immediately identifiable by its striking black-glazed terra-cotta facade and numerous large windows.
Engineered for maximum performance, 28&7 extensively uses floor-to-ceiling windows to maximize natural sunlight entry and minimize the need for artificial lighting. To reduce solar heat gain, all these windows are triple-glazed and incorporate a low-E coating to help reflect long-wave infrared energy—both features contributing to a reduced need for mechanical heating and cooling.
These windows, alongside other sustainable elements, helped 28&7 achieve its LEED Gold certification and meet New York City’s Zone Green standards.
- Two Doughboy Square:
In the Lawrenceville neighborhood of Pittsburgh, Two Doughboy Square designed by Desmone Architects mixes contemporary and historical design elements. Photo by Ed Massery.
Designed by Desmone Architects as an extension of their main office, Two Doughboy Square prioritizes the health and well-being of its employees. Windows play a crucial role in fulfilling these goals.
Large, ENERGY STAR certified windows—manufactured by YKK AP—are installed throughout the building to allow natural sunlight to enter (boosting mental health and productivity) and to assist with passive interior temperature regulation. To maximize solar energy from the winter sun, windows on the building’s south side were treated with different coatings than those on the north side.
- The Lookout:
Natural light and light wood are key to the design of The Lookout. Photo by Rafael Soldi.
As its name suggests, The Lookout—a modern residential space designed by Hybrid—underscores the importance of windows for both daylighting and providing expansive views of the outside world.
Naturally, the architects at Hybrid also recognize the value of energy efficiency. This is why all of The Lookout’s windows came from Lindsay Windows, a company that prioritizes energy-efficient window design. These windows feature low-E glass, double- or triple-glazing, and triple weather-stripping to help reduce the need for interior heating and cooling.
- M-22 House:
Strategically placed windows give the M-22 House plenty of light and unbeatable views. Photo by Dietrich Floeter.
Designed by Michael Fitzhugh Architect and situated along a ridgeline overlooking Lake Michigan, the M-22 House utilizes large windows to provide unparalleled views of the surrounding environment and reduce the need for artificial lighting—while also assisting in passive interior temperature regulation.
Manufactured by Western Window Systems, all of the M-22 House’s windows are double-glazed and boast triple-coated low-E glass, along with durable, tight-fitting weather-stripping. During Michigan’s frigid winters, these windows effectively keep the cold out and the heat in, significantly reducing energy waste.