In 2023, researchers modeled fluid-filled microchannel windows; computer simulations estimated the liquid system could cut building energy use by 43 percent

Windows might be the leakiest part of your house. They can drive your heater and air conditioner to run nonstop throughout the year. However, according to a 2023 study, "Multilayered optofluidics for sustainable buildings," published in the journal Proceedings of the National Academy of Sciences by University of Toronto researchers Raphael Kay, J. Alstan Jakubiec, Charlie Katrycz, and Benjamin D. Hatton, there is a solution: windows made up of a layer of fluid that moves around. Computer simulations modeled the liquid window system and showed an estimated 43 percent reduction in a building's energy use compared with existing window technologies.

How does a liquid window actually work

Think of a window pane with a thin layer of glass attached to it by an adhesive. The thin layer contains channels that carry water or another fluid. Based on the type of fluid flowing within the channel, the system controls the amount of light entering the glass, heat blocking, and scattering of light within the room. The 2023 study modeled this system computationally rather than testing a full-scale building prototype. Raphael Kay led the research, with co-authors J. Alstan Jakubiec, Charlie Katrycz and Benjamin D. Hatton, and most of the work was done at the University of Toronto.


Why every window matters this much

Buildings consume a significant amount of the world's energy, primarily due to heating and cooling. Often the weak link is glass windows, which allow heat to escape in winter and pour in during summer. Most of the existing solutions, like tinted coatings or automatic blinds, only address one of those problems at a time. The liquid window is different because it is tunable. One clear liquid can block heat. Another blend scatters light, diffusing harsh afternoon sun without darkening a room.

From a lab model to a real Harvard prototype

Since the 2023 study was published, Kay has moved to Harvard University's John A. Paulson School of Engineering and Applied Sciences, where he is now a PhD candidate working in the Aizenberg Lab under Professor Joanna Aizenberg on a project called Liquafilm, a continuation of his original University of Toronto research, now aimed at building a working, large-scale version rather than a computer model. In a Harvard SEAS feature titled "A Window into Heating and Cooling," Kay compares buildings to bodies, with concrete and glass acting as skin and heating and cooling systems helping regulate indoor temperature. As mentioned in the same article, Kay has been conducting tests for Liquafilm in actual homes, in collaboration with the Harvard Graduate School of Design. The same Harvard SEAS feature reports that heating and cooling account for a large share of the roughly 140 billion dollars U.S. commercial buildings spend on energy each year. A separate Harvard SEAS profile of a 2025 senior capstone project by Brice Austin, "Development of a Platform for Large-Scale Liquid Windows," explains that the project was directly based on Kay's original Liquid Window concept from the Aizenberg Lab, working out how to make these windows at a larger, more practical scale.

What this could mean for your energy bill

The 43 percent figure was computed using models and simulations; it is not based on any measurement taken in the real world of buildings with real people occupying them for an entire year. Liquafilm and similar large-scale prototypes remain experimental research projects, not products available for home installation. Commercial buildings in America are estimated to spend around $140 billion a year on energy, according to the Harvard SEAS feature cited above, a large portion of which goes to heating and cooling.

For now, the technology exists mainly in Harvard labs and a handful of test homes. But the next time you touch a hot window pane on a summer afternoon, it is worth knowing that researchers are quietly working on a version of that glass that could feel almost cool to the touch, while trimming your energy bill at the same time.