Windows filled with see-through wood layer help hold in heat

A see-through aerogel made from wood could replace air in double-glazed windows and make them as insulating as walls.

Windows with air sandwiched in the gap between plates of glass can be made better insulators by either increasing the number of glass panels, which can affect visual quality, or expanding the width of the air layer — but anything beyond around 1.5 centimetres becomes detrimental to the insulation effect because convection currents circulate more easily.

To address this, Ivan Smalyukh at the University of Colorado Boulder and his colleagues used nanofibres of cellulose to create an aerogel, a solid gel containing pockets of gas, that could function better than air in double glazing.

“We have a very unusual combination of properties, which is a very high transparency aerogel that also has very high thermal insulation,” says Smalyukh. “You could think about 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 in a window.”

To make the aerogel, they first suspended cellulose nanofibres from wood in water, then replaced the water with ethanol. Next, they dried the aerogel by raising the temperature and pressure, replacing the ethanol filling pockets in the material with air, then adding silicon compounds to the surface to make it water-repellent, preventing condensation when used in windows.

Tiny pockets of air embedded in the aerogel mean it can be used to fill a wider space without the convection effects you would get with air alone. An aerogel filling around 2.5 centimetres wide could make a window as insulating as a wall.

“This is a really nice development that could be easily employed as a retrofit to existing windows,” says Steve Eichhorn at the University of Bristol in the UK. “The reduction in heat transfer, with the added benefit of maintained transparency and low haze, make this material truly remarkable, and all with a sustainable material, cellulose.”

There will be challenges to scaling it up, but there are already processes for producing cellulose nanofibres at scale which make it feasible, says Eichhorn.