Porous self-extinguishing materials based on a polyurethane/polyisocyanurate mixture with density ≤ 350 kg/m 3 , where the polyurethane/polyisocyanurate mixture is obtainable from a reaction mixture consisting of the following components: i) at least one component from the group consisting of: a. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight, b. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, c. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid, and d. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, and ii) at least one incipiently trimerized polyisocyanate with from 15 to 95% isocyanurate content, based on the sum of the number of isocyanate groups and of isocyanurate groups, from the group consisting of diphenylmethane diisocyanate (MDI), its isomers and higher homologs (pMDI) and mixtures thereof, toluene diisocyanate (TDI) and its isomers and mixtures thereof, hexamethylene 1,6-diisocyanate (HDI), isophorone diisocyanate (IPDI), hydrogenated diphenylmethane diisocyanate (HMDI), its isomers and higher homologs and mixtures thereof, pentane 1,5-diisocyanate (PDI) and xylene 1,3-diisocyanate (XDI) and mixtures of the components of the group in the presence of iii) at least one urethanization catalyst and iv) at least one trimerization catalyst, where the NCO index (the ratio of the number of NCO groups to the number of OH groups multiplied by 100) is from 200 to 600. i) at least one component from the group consisting of: a. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight, b. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, c. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid, and d. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, and a. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight, b. a polyalkylene oxide polyol >200 mg KOH/g, started with phosphoric acid of strength from 80 to 100% by weight and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, c. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid, and d. a polyalkylene oxide polyol with OH number >200 mg KOH/g, started with boric acid and with a polyol of functionality from ≥3 to ≤8 and of molar mass from 92 to 350 g/mol, and ii) at least one incipiently trimerized polyisocyanate with from 15 to 95% isocyanurate content, based on the sum of the number of isocyanate groups and of isocyanurate groups, from the group consisting of diphenylmethane diisocyanate (MDI), its isomers and higher homologs (pMDI) and mixtures thereof, toluene diisocyanate (TDI) and its isomers and mixtures thereof, hexamethylene 1,6-diisocyanate (HDI), isophorone diisocyanate (IPDI), hydrogenated diphenylmethane diisocyanate (HMDI), its isomers and higher homologs and mixtures thereof, pentane 1,5-diisocyanate (PDI) and xylene 1,3-diisocyanate (XDI) and mixtures of the components of the group in the presence of iii) at least one urethanization catalyst and iv) at least one trimerization catalyst, <EOS>