A closed cell, rigid, polymer foam comprising the reaction product of a polyisocyanate and an isocyanate-reactive compound in the presence of (a) a foaming agent, (b) at least about 2 percent by weight of carbon black, based on the weight of the polymer in the foam, and (c) a capped polyoxyalkylene compound which helps bring about a uniform dispersion of the carbon black throughout the foam so that the aged k-factor of the foam is below the aged k-factor of the corresponding unfilled foam.