To obtain a thermally insulating polyurethane web exhibiting low thermal conductivity and shrinkage and improved in thermal stability by incorporating hollow plastic beads having a specified density and a average bead size into a polyurethane casting compsn. having isocyanurate groups.

A polyurethane casting compsn. having isocyanate groups, having an isocyanate index of 90-2,000. pref. 100-1,400, and obtd. by mixing an org. (pref. arom.) polyisocyanate with an org. polyhydroxyl compd. is compounded with 5-15 wt.% hollow plastic beads having a density of 0.1-0.30 g/cm3 and an average bead size of 5-200 μm, a catalyst (e.g. dimethylbenzylamine), and a filler (e.g. glass fibers or aluminum hydroxide) to give a coating compsn. The compsn. is cast on a composite profile to give a thermally insulating web having a thermal conductivity of 0.2 W/m.k or lower, esp. 0.120-0.180 W/m.k.