The present invention relates to an expandable polymeric composition comprising: a) a polymeric matrix containing vinyl aromatic polymers and/or copolymers; b) from 0.1% to 20% by weight, calculated on the polymeric matrix (a), of an athermanous agent; c) from 0.01% to 10% by weight, calculated on the polymeric matrix (a), of at least one non-polymeric brominated flame retardant additive; d) from 0.01% to 10% by weight, calculated on the polymeric matrix (a), of at least one brominated polymer containing at least 50% bromine; e) from 0.01% to 5% by weight, calculated on the polymeric matrix (a), of a synergistic flame retardant additive; f) from 1% by weight to 10% by weight of at least one blowing agent.