TECHNOLOGY FOCUS - POLYMERS - Preferred Composition: The amount of (I) is 3-50 wt.% with respect to the sum of components (b), (c), (e) and (f). Components (a), (b) and (c) contains not less than 32 wt.% aromatic groups with respect to the rigid PUR foam.Preferred Components: Preferred examples of the organic polyisocyanates (a) are aromatic polyisocyanates selected from 4,4'-, 2,4'- and 2,2'-diphenylmethane diisocyanate, polyphenyl-polymethylene polyisocyanates (crude MDI), 2,4- and 2,6-toluylene diisocyanate and mixtures, especially mixtures of crude MDI with 30-80 wt.% diphenylmethane diisocyanate isomers. Preferred examples of compounds (b) are polyhydroxyl compounds with a functionality of 2-8 and molecular weight of 400-6000, especially polyether-polyols prepared by anionic polyaddition of alkylene oxide(s) to aromatic compound(s) with not less than 2 reactive H atoms and OH, amino and/or carboxyl group(s) as starter molecule. Preferred examples of starter molecules are aromatic polycarboxylic, hydroxycarboxylic and aminocarboxylic acids, aromatic mono- and polyamines, polyphenols and Mannich condensates of phenols, formaldehyde and dialkanolamines, especially 1,2-, 1,3- and 1,4-phenylenediamine, 2,3-, 2,4-, 3,4- and 2,6-toluylenediamine, 4,4'-, 2,4'- and 2,2'-diamino-diphenylmethane, polyphenyl-polymethylene polyamines and their mixtures. Preferred examples of the alkylene oxide are 1,2-propylene oxide (PO) and/or ethylene oxide (EO). Preferred examples of the blowing agent are (d1) water with cyclopentane; or (d2) water with homogeneous mixtures of (d2i) cyclopentane and/or cyclohexane and (d2ii) low-boiling compounds, especially n- or iso-butane, n- or iso-pentane, methyl formate, 1,1,1,3,3-pentafluoro-butane or -propane and/or 1,1,1,2-tetrafluoroethane. SPECIFIC COMPOUNDS - Specific examples of the substituted alkanols (I) are trimethylolpropane, trimethylolethane and their reaction products with alkylene oxides, especially reaction products with a hydroxyl (OH) number of 700-2000 mg KOH/g.    EXAMPLE - Component (A) was based on a mixture of 65.5 parts weight polyether-polyol with OH number 390 and 11.5 wt.% aromatic groups (prepared by block copolyaddition of 50 wt.% PO and 50 wt.% EO to toluylenediamine isomer mixture), 20.0 parts weight polyether-polyol with OH number 950 (prepared by anionic polyaddition of PO to 1,1,1-trimethylolpropane) and 1.1 parts weight N,N,N',N,N-pentamethyl-diethylenetriamine. A control (A') was based on a mixture of 63.5 parts weight polyether-polyol with OH number 440 (prepared by anionic polyaddition of PO on sucrose), 15.0 parts weight polyether-polyol with OH number 400 (prepared by anionic polyaddition of PO on sorbitol), 10.0 parts weight polyether-polyol with OH number 120 (prepared by anionic polyaddition of PO on N-(3-dimethylaminopropyl)ethylenediamine and then anionic polyaddition of EO on the adduct). The other constituents were (A) 8.0, (A') 5.0 parts weight polyoxypropylene glycol (OH number 250), (A) 2.0, (A') 1.5 parts weight foam stabilizer based on silicone (Tegostab B 8467 RTM), (A) 1.3, (A') 2.2 parts weight N,N-dimethylcyclohexylamine, (A, A') 0.5 part weight 47% solution of potassium acetate in ethylene glycol, (A) 1.6, (A') 2.0 parts weight water and (A) 17, (A') 11.0 parts weight cyclopentane. Component (B) was a mixture of diphenylmethane diisocyanates and crude MDI, which contained 31.5 wt.% isocyanate and 56 wt.% aromatic groups. Mixtures of (1) 100 parts weight (A) and 134 parts weight (B) or (2) 100 parts weight (A') and 125 parts weight (B) were made into uniform rigid foam. The foam had a bulk density of (1) 36, (2) 38 g/l and thermal conductivity of (1) 17.8, (2) 20.5 mW/mK at 23 degreesC. The content of aromatic groups in components (a) to (c) was (1) 35.3, (2) 31 wt.%.