Abstract: The present invention relates to a reaction mixture for manufacturing an inorganic-filler based closed-cell rigid polyurethane or polyisocyanurate (PU or PIR) containing foam having a calorific value below 6 MJ/kg, preferably below 4.5 MJ/kg, more preferably below 3 MJ/kg, measured according to EN ISO 1716, the reaction mixture comprising: At least one polyisocyanate-containing compound; At least one isocyanate-reactive compound; An inorganic filler composition; At least one physical blowing agent; characterised in that said inorganic filler composition has bulk density higher than 2 g/cm3, preferably higher than 2.1 g/cm3, more preferably higher than 2.2 g/cm3, even more preferably higher than 2.4 g/cm3.