The improvement of thermal insulating performance of polyurethane rigid foams is a crucial task for their use. In this work, the effect of graphene on these properties has been studied by preparing and testing unfilled, 0.3 and 0.5 wt% graphene-filled polyurethane foams. It was found that graphene is able, at very low content (0.3 wt%), to reduce the radiative contribution of the initial thermal conductivity by both decreasing the cell size and increasing the extinction coefficient. Due to the low graphene contents considered, no concerns about the solid-phase contribution of thermal conductivity arise. Polyurethane-graphene nanocomposite foams showed also slower aging rate with respect to unfilled foams.