A fiberglass composition is described which is characterized in that it comprises, in percentages by weight: SiO2 50.00-70.00 Al2O3 0.00-3.50 Na2O + K2O 16.00-21-00 CaO + MgO + BaO < 12.50 B2O3 4.00-13.00. Heavy metal oxides 0.05-7.00, preferably between 0,10 and 7.00 %.
