Moulding composition comprising the following components: a) one or more polyisocyanates, b) one or more polyols, c) one or more catalysts catalysing the reaction between polyisocyanate and polyol and d) one or more pyrogenically produced oxides of a metal or of a metalloid, characterized in that the moulding composition comprises, as further component, e) at least one or more siloxanes of the formula R-S1(CH 3) 2 -O-[-S1(CH 3) 2 -O-] n -[-Si(CH 3)R 1 -O-] m -Si(CH 3) 2 -R 2 where R, R 1 and R 2 are identical or different, R and/or R 2 are methyl or R 1, R 1, being identical or different, is - (CH 2) x - z - (CH 2 -CHR' -0) y -R'', R', being identical or different, is -H, -CH 3, - CH 2 CH 3, or phenyl moiety, R", being identical or different, is -H, - -R''', - -NH-R''' or -alkyl, preferably C 1 to C 40 -alkyl, with preference C 1 - or C 6 to C 30 -alkyl, R''', being identical or different, is C 1 to C 40 -alkyl, -aryl or -alkylaryl; optionally also substituted with halogens, n+m+2 = from 10 to 150, preferably from 12 to 85, with particular preference from 15 to 47, m = from 0 to 20, preferably from 1 to 4, x = from 3 to 10, y = from 1 to 40, preferably from 2 to 19, z = 1, where the units (CH 2 -CHR'-O) can be identical or different, with the proviso that for m = 0 at least one moiety R or R 2 is identical with R 1, with the proviso that the moulding composition comprises less than 2% by weight, preferably less than 0.2% by weight, based on the entire constitution of the moulding composition, of physical blowing agent and also less than 2% by weight of water, wherein the composition comprises 100 parts by weight of polyol, from 1 to 30 parts by weight of pyrogenic oxide, from 0.05 to 1.5 parts by weight of catalyst, from 0.25 to 7.5 parts by weight of siloxane of the formula, from 0 to 50 parts by weight of flame retardant and an isocyanate index from 13 to 80. a) one or more polyisocyanates, b) one or more polyols, c) one or more catalysts catalysing the reaction between polyisocyanate and polyol and d) one or more pyrogenically produced oxides of a metal or of a metalloid, e) at least one or more siloxanes of the formula R-S1(CH 3) 2 -O-[-S1(CH 3) 2 -O-] n -[-Si(CH 3)R 1 -O-] m -Si(CH 3) 2 -R 2 where R, R 1 and R 2 are identical or different, R and/or R 2 are methyl or R 1, R 1, being identical or different, is - (CH 2) x - z - (CH 2 -CHR' -0) y -R'', R', being identical or different, is -H, -CH 3, - CH 2 CH 3, or phenyl moiety, R", being identical or different, is -H, - -R''', - -NH-R''' or -alkyl, preferably C 1 to C 40 -alkyl, with preference C 1 - or C 6 to C 30 -alkyl, R''', being identical or different, is C 1 to C 40 -alkyl, -aryl or -alkylaryl; optionally also substituted with halogens, n+m+2 = from 10 to 150, preferably from 12 to 85, with particular preference from 15 to 47, m = from 0 to 20, preferably from 1 to 4, x = from 3 to 10, y = from 1 to 40, preferably from 2 to 19, z = 1, where the units (CH 2 -CHR'-O) can be identical or different, with the proviso that for m = 0 at least one moiety R or R 2 is identical with R 1, with the proviso that the moulding composition comprises less than 2% by weight, preferably less than 0.2% by weight, based on the entire constitution of the moulding composition, of physical blowing agent and also less than 2% by weight of water, wherein the composition comprises 100 parts by weight of polyol, from 1 to 30 parts by weight of pyrogenic oxide, from 0.05 to 1.5 parts by weight of catalyst, from 0.25 to 7.5 parts by weight of siloxane of the formula, from 0 to 50 parts by weight of flame retardant and an isocyanate index from 13 to 80. R, R 1 and R 2 are identical or different, R and/or R 2 are methyl or R 1, R 1, being identical or different, is - (CH 2) x - z - (CH 2 -CHR' -0) y -R'', R', being identical or different, is -H, -CH 3, - CH 2 CH 3, or phenyl moiety, R", being identical or different, is -H, - -R''', - -NH-R''' or -alkyl, preferably C 1 to C 40 -alkyl, with preference C 1 - or C 6 to C 30 -alkyl, R''', being identical or different, is C 1 to C 40 -alkyl, -aryl or -alkylaryl; optionally also substituted with halogens, n+m+2 = from 10 to 150, preferably from 12 to 85, with particular preference from 15 to 47, m = from 0 to 20, preferably from 1 to 4, x = from 3 to 10, y = from 1 to 40, preferably from 2 to 19, z = 1,