New, green and cheap rigid foams presenting outstanding performances for thermal insulation are described. Such ultra-lightweight cellular materials are mainly based on renewable chemicals: tannin and furfuryl alcohol, are very easy to produce and have thermal conductivity as low as 38 mW/m/K. Compared to previously reported tannin-based foams, these new materials are much “greener” and present improved resistance to compression and to water. Especially the formaldehyde, formerly used as cross-linking agent of tannins but known as a volatile and harmful chemical, could be successfully removed from the formulation. The as-obtained, 2nd generation, tannin-based foams are totally stable and have an expected interest for thermal insulation of buildings.