Foaming a polyurethane foam at above atmospheric pressure conditions using a mixture of polyisocyanate and a specific mixture of low molecular weight (high OH number) polyether and graft polyols produces viscoelastic foams with high firmness, low energy losses and high temperature sensitivity such that firmness is reduced by at least 25% when the foam is heated from room temperature (e.g., about 70° F. (21° C.)) to 100° F. (38° C.). The foam-forming ingredients are mixed together and foamed at controlled pressures in the range 1.05 to 1.5 bar (absolute), preferably 1.1 to 1.3 bar (absolute).