A process that freezes cellulose nanofibres, graphene oxide and clay nanorods together, produces super-insulating, fire-retardant foams that are suitable for improving the energy efficiency of buildings, reports a study published online this week in Nature Nanotechnology.

Heat-insulating building materials need to be strong, poor at conducting heat, resistant to fire and moisture, and easily fitted to older buildings without compromising architectural design. Previous studies have shown that nanosized one- and two-dimensional materials such as carbon nanotubes can reduce heat conduction, and that other nanomaterials such as clays offer good resistance to fire.

Lennart Bergstrom and colleagues now report that controlled freezing of cellulose nanofibres, graphene oxide and clay nanorods in a mould placed inside a liquid nitrogen bath produce highly porous foams, which have a thermal conductivity that is low enough to reduce the required thickness of insulation materials by more than 50% based on passive house standards. The pore structure is uniform, having parallel millimetre-long tubes, with cell walls that are thin and smooth. The nanomaterials are distributed homogenously in the cell walls and contribute to the mechanical strength, moisture- and fire-resistance of the foams.