To obtain an insulating foam having excellent (long-term) heat insulation properties and a low density and being useful as e.g. a building material by mixing a foamable composition with a specified blowing agent to form a film which has pores having an interior surface and the interior surfaces of pores of which are continuously coated and hardening the foam to a specified density.

(A) A foamable composition (for example, one obtained from a free-formaldehyde-free phenol/formaldehyde resol resin having a water content of 4-8 wt.% and a viscosity (40°C) of 5,000-40,000 cP) is mixed with (B) a blowing agent containing (B1) a film-forming additive (for example, a mixture of a surfactant with a mineral oil or a vegetable oil and a polymer such as polyacrylate or polymethyl methacrylate) to form a foam which has pores having an interior surface and the interior surfaces of the pores of which are coated with B1, and hardening this foam to obtain a product having a density of 8.009-48.055 kg/m3 and a thermal conductivity of below 0.029 W/m.K.