A clear airgel made from wood could replace air in double-paned windows and make them as insulating as walls.

Windows with sandwiched air in the space between the glass panes can be made better insulators by increasing the number of panes of glass, which can affect visual quality, or by expanding the width of the air gap, but anything that exceeds the 1.5 centimeters becomes detrimental to the insulation effect because convection currents circulate more easily.

To address this, ivan smalyukh at the University of Colorado Boulder and colleagues used cellulose nanofibers to create an aerogel, a solid gel containing pockets of gas, that might work better than air in double glazing.

“We have a very unusual combination of properties, which is a very high transparency airgel that also has very high thermal insulation,” says Smalyukh. “You could think of it as a pillow that keeps heat where you need it, and at the same time, you can see through it, so you can use it on a window.”

To make the airgel, they first suspended wood cellulose nanofibers in water, then replaced the water with ethanol. Next, they dried the airgel by raising the temperature and pressure, replacing the ethanol filler pockets in the material with air, and then added silicone compounds to the surface to make it water repellent, preventing condensation when used on windows.

The tiny pockets of air embedded in the airgel mean it can be used to fill a larger space without the convective effects you’d get with air alone. An airgel filler about an inch wide could make a window as insulating as a wall.

“This is a really cool development that could easily be used as an upgrade to existing windows,” he says. steve eichhorn at the University of Bristol in the UK. “The reduction in heat transfer, with the added benefit of sustained transparency and low haze, make this material truly extraordinary, and all with a sustainable material, cellulose.”

There will be challenges in scaling it up, but processes for producing cellulose nanofibers at scale already exist to make it feasible, Eichhorn says.