A New Thin Film Could Make Windows as Insulating as Walls

Despite making up only between 8% and 30% of the exterior surface area of buildings on average, windows leak half of the heat from homes, offices, and other structures in the winter — and in the summer, allow heat indoors. Buildings consume more than 30% of all energy generated worldwide, so their thermal losses add up to a significant amount of resource use and carbon emissions.

Now, a new thin film that can be manufactured at window-sized scales could make it possible to let light in without losing heat. The material, reported in the journal Science, lets through 99% of light in the visible range but barely conducts heat at all. It is made of a lightweight network of polysiloxane nanotubes cushioned with air.

“In this material, we make very tiny pores that are much smaller than the distance between molecules of air at room temperature, so they cannot bump against each other and exchange energy,” says study senior author Ivan Smalyukh, a professor of soft matter physics at the University of Colorado, Boulder. In addition, he adds, the polysiloxane nanotubes are nearly opaque to infrared wavelengths.

Smalyukh and his colleagues conducted the research under a U.S. Dept. of Energy funding program that aims to support non-standard approaches with a high risk of failure. When they began about 10 years ago, Smalyukh recalls, they had developed tiny samples of insulating material that was less transparent than glass. Over the years, the team refined those materials so that the newest window film is “as good or better than most modern walls” at insulation, Smalyukh says.

The researchers call the films mesoporous optically clear heat insulators, or MOCHIs. They make them by first mixing a surfactant (cetylpyridinium chloride) with water, where the surfactant molecules self-assemble according to their hydrophobic and hydrophilic ends into tiny cylindrical tubes. The researchers then add methyl trimethoxy silane and glacial acetic acid to hydrolyze that silane. The surfactant tubes act as a template for the resulting polysiloxane network, which is crosslinked with tetramethyl ethylene diamine.

This mixture is then put in a mold to gelate at 55°C for up to 72 hours, depending on the desired thickness of the final film. With this method, the researchers could fabricate MOCHIs of up to a square meter in size and more than 3 cm thickness.

These slabs have nearly the same refractive index of air, making them extremely transparent even when viewed from a wide angle. Their thermal conductivity was extremely low, at only 10–12 W/m-K. Versions of the film just a few millimeters long could be adhered to plastic or glass, meaning that MOCHIs could be used on existing windows to retrofit buildings for better energy efficiency. Notably, the researchers reported, MOCHIs are also more soundproof than glass, reducing sound transmission by 5–10 more decibels than double-paned windows, depending on the frequency.

Smalyukh says his team is now hoping to engage with industrial partners who might be interested in scaling up the manufacturing process. The researchers are also hoping to improve the flexibility of materials. If they could be incorporated into clothing, Smalyukh claims, they could provide a great deal of insulation without much bulk.

Bhardwaj, A., et al., “Mesoporous Optically Clear Heat Insulators for Sustainable Building Envelopes,” Science, doi: 10.1126/science.adx5568 (Dec. 11, 2025).