A Stiff, Tough, and Thermally Insulating Air- and Ice-Templated Plant-Based Foam

[Image: see text] By forming and directionally freezing an aqueous foam containing cellulose nanofibrils, methylcellulose, and tannic acid, we produced a stiff and tough anisotropic solid foam with low radial thermal conductivity. Along the ice-templating direction, the foam was as stiff as nanocellulose–clay composites, despite being primarily methylcellulose by mass. The foam was also stiff perpendicular to the direction of ice growth, while maintaining λ(r) < 25 mW m(–1) K(–1) for a relative humidity (RH) up to 65% and <30 mW m(–1) K(–1) at 80% RH. This work introduces the tandem use of two practical techniques, foam formation and directional freezing, to generate a low-density anisotropic material, and this strategy could be applied to other aqueous systems where foam formation is possible.