Aside from providing us with sunlight and views outside, windows have a big influence on the temperature inside our homes. The Department of Energy estimates that in the winter 30% of a home’s heat is lost through windows, while in the summer 76% of sunlight that directly hits a double-pane will enter to become heat. Alternatives to your standard glass panes are starting to hit the market. These smart glass windows, as they’re often called, promise to help keep buildings comfortable in ever-increasing heat without ballooning electricity bills.
Smart glass, however, is still an exceptionally small minority in the marketplace. Its presence in residential buildings is close to none. Luís Fernandes, the head of building technologies at Lawrence Berkeley National Laboratory, estimates that the most common forms of smart glass make up less than 2% of the current commercial market for windows.
But the presence of this energy-saving technology will only grow in the future. Here’s what to know about smart glass windows.
What is smart glass?
Most windows found in both residential and commercial buildings are “static,” which essentially means that the pane can’t change in color or tint. “Smart glass” technology or “smart glass windows” are panels made of glass that can change how much solar energy is allowed to pass through—or how much gets reflected away.
Solar energy has two main components that affect our comfort inside a building: visible light, and infrared light (a.k.a., heat). Smart windows can control either. They might modulate how opaque they are to let in more or less visible light, also reducing the glare you might see from inside a building. Or, these panes might control how much infrared light they reflect to allow more or less heat to enter a building.
How does smart glass enhance energy efficiency in buildings?
Every window—especially poorly insulated ones—is an avenue for energy to transfer both in and out of a building. Heat gain and loss via windows, after all, are responsible for up to 30% of residential heating and cooling energy use. The laws of thermodynamics dictate that heat always wants to escape to cooler areas. So in cool weather, warmth in a building will want to get out, which has an energy cost. In the summer, the opposite happens: Heat penetrates cooler buildings and warms them up via sunlight, which can increase the need for air conditioning.
The biggest boon smart glass can bring to a building in terms of energy efficiency is the ability to control the amount of light and heat that enters a building to reduce cooling needs, in some cases by as much as 20%.1 During a hot summer, smart glass windows can be controlled via electric currents to change their tint in a way that only lets in a certain amount of heat. This keeps the inside environment cool and comfortable, minimizing the need for air conditioning.
However, smart glass is hardly the only facet of a window that can improve energy efficiency. Marc LaFrance, advanced technology and energy policy manager at the Department of Energy, emphasizes that high quality insulation is “the most important window attribute with the most energy savings potential.” In windows—with two or more panes of glass that are hermetically sealed—good insulation will help better control the temperature inside a building both in cold and hot weather, and when used in homes can reduce the total energy load of a house by about 15%.2
What are the environmental advantages of using smart glass?
Beyond increasing energy efficiency and reducing energy-related costs, having dynamic solar control with smart glass can have other environmental benefits. It can help reduce a building’s energy consumption when demand is at its peak—particularly during summer afternoons. This is important because during times when energy demands are high, utilities often need to purchase more expensive energy from different sources, such as from plants or backup systems called “peakers” that burn fossil fuels. One report found that these so-called “peaker” plants emit 1.6 times more sulfur dioxide and almost three times more nitrogen oxides than non-peaker plants per megawatt hour of electricity generated.3 Their carbon footprint is no joke either: U.S. peaker plants emit around 60 million tons of carbon dioxide every year.4
How does smart glass technology work?
There are several different technologies that people might refer to when talking about smart glass. Electrochromics use electricity to change in tint to reflect both light and heat. Thermochromics react to temperature to change tint and reflect only light. And suspended particle displays contain tiny, electricity-controlled microshutters.
Electrochromic glass
Electrochromic glass is the most common and promising technology for smart windows. These panels change in tint when a small electric current runs through them, altering the opacity or color of the glass so that it lets in just the right amount of light or heat. This type of glass can run on a schedule of opacities and colors for different times of day. In some instances, electrochromic windows can also pair with daylight sensors that trigger adjustments to the window settings in response, says Robert Hart, a technology researcher at the Lawrence Berkeley National Laboratory. Some electrochromics also connect to an app, or through a wall-mounted dial reminiscent of a thermostat.
The advantage of electrochromics is that it can reflect both visible light and heat, which some other smart glasses cannot. Electrochromic glass is readily available for use in commercial buildings, though there still are not very many companies producing it.
Thermochromic glass
Like electrochromic glass, thermochromic glass can change its tint. But instead of using electricity, these panes react to temperature and automatically darken when the mercury rises. This principle is similar to transition lenses—eyeglasses that darken into sunglasses once you go outside—which are photochromic and react to sunlight. The downside to thermochromic glass is that you have less control to fine tune your settings throughout the day, which could also decrease energy savings.
Suspended particle displays
In a suspended particle display, millions of black particles are, yes, suspended between two panes of glass. In an untouched state, these particles swirl around and block light from passing through. But when an electrical current runs through the display, the particles line up to let light through. It’s somewhat similar to the way blinds work—just think of the floating bits as tiny light valves.
The advantage of suspended particle displays is that they can go from open to shut very quickly, whereas other smart glass technologies are a little slower to change, says the DOE’s LaFrance. The key disadvantage is that suspended particle displays don’t modulate or reflect infrared solar energy, only visible light. So while they can help with visual comfort and light levels, they won’t really do anything for your energy bill.
The ultimate, ideal window for energy efficiency and comfort would be able to reflect and modulate both visible light and infrared independently of each other, says LaFrance, which is something that no window on the market can really do right now.
How much does it cost to install smart glass windows?
Electrochromic windows are about 5 to 10 times more expensive than normal windows. But, the Biden administration recently enacted tax credits to bring down the cost, and now these smart glass windows can be bought and installed at prices much closer to parity with high quality static (“normal”) windows.
Out of the several barriers to smart glass window adoption, including availability by region and general awareness of smart windows as an option, cost is definitely a large factor. Fernandes of Lawrence Berkeley National Laboratory says that he expects the market to expand once these tax credits are more widely known. It’s hard to say how quickly cities might adopt smart glass, but if you ever see an office window darken in the light, just know that its function is not just aesthetic—it’s a path to energy efficiency.
Other efficiency upgrades you can make right now
While the market for smart glass windows gets its footing—especially in the residential sphere—there’s still plenty homeowners (and even renters) can do to up the efficiency of their homes. Here are three places to start.
Energy-efficient landscaping
Even when you’ve exhausted your options for making efficient upgrades inside your home, strategic tweaks to your landscaping can help keep your home a little cooler in the summer and warmer in the winter. Tree cover in the summer, for instance, can slash your home’s cooling costs by 10%—or more. Here’s what you need to know about the basics of energy-conscious landscaping.
Cool roofing
In the heat of the sun, a home’s roof can easily hit 150 degrees Fahrenheit. A roof covered in materials that reflect those rays, however, can be as much as 50 degrees lower. While there’s no official standard for what’s considered a “cool roof,” there are a host of options for minimizing how much the sun heats up your home from the top-down. Here’s our primer on cool roofs.
Air sealing
Heat naturally wants to escape to where temperatures are lower, which is why warmth tries to creep out of a home in the winter and into it in the summer. A well-sealed home—that is, one where any potential gaps between indoors and outdoors are plugged up tight—can cost as much as 15% less to heat and cool than one that isn’t. Here’s what you need to know about undertaking an air sealing project in your home.
- Pathway to Zero Energy Windows, U.S. Department of Energy, Apr. 2022 ↩︎
- Guide to Energy-Efficient Windows, U.S. Department of Energy, Oct. 2010 ↩︎
- Electricity: Information on Peak Demand Power Plants, U.S. Government Accountability Office, May 2024 ↩︎
- The Peaker Problem, Clean Energy Group and Strategen, Jul. 2022 ↩︎