The present invention relates to a polymeric composition which comprises: a) vinyl aromatic polymers and/or copolymers in an amount ranging from 50% to 99% by weight, calculated with respect to the overall composition, b) a polyolefin or a relative copolymer with an ester in an amount ranging from 1% to 50% by weight, calculated with respect to the overall composition, c) olefinic elastomer grafted with a.-vinyl aromatic polymer in an amount ranging from 0.01% to 40% by weight, calculated with respect to the sum of (a)+(b), d) an expanding agent in an amount ranging from 1% to 10% by weight, calculated with respect to the sum of (a)+(b)+(c).