A polyurethane-polyisocyanurate foam that is a reaction product of a reaction mixture comprising one or more isocyanate-reactive materials, a polyisocyanate component, a physical blowing agent, a foam-stabilizing surfactant, at least one trimerization catalyst and at least one catalyst for the reaction of an isocyanate group towards water, wherein: a) greater than 70% by weight of the isocyanate-reactive materials is at least one polyether polyol having a hydroxyl equivalent functionality of 3 and a hydroxyl equivalent weight of at least 100 but less than 400, wherein the at least one polyether polyol is selected from homopolymers of ethylene oxide, copolymers of ethylene oxide and propylene oxide and homopolymers of propylene oxide and the at least one polyether polyol has an average oxyethylene content of greater than 40% but less than 80% by weight and correspondingly an average oxypropylene content at least 20% but less than 60% by weight based on the combined weight of oxyalkylene groups present in the at least one polyether polyol, provided that when the at least one polyether polyol has an average hydroxyl equivalent weight greater than 200, the isocyanate-reactive materials further include at least one crosslinker having a hydroxyl functionality of at least 3 and a hydroxyl equivalent weight of less than 100 such that the equivalent weight of the at least one polyether polyol and the at least one crosslinker combined is less than 200; b) less than 25% by weight of the isocyanate-reactive materials is a polyester polyol; c) water constitutes no more than 0.5% of the weight of the isocyanate-reactive materials; d) the polyisocyanate component includes one or more aromatic polyisocyanates, the one or more aromatic polyisocyanates having an average isocyanate functionality of at least 2.7 and an average isocyanate equivalent weight of up to 150; e) the isocyanate index is 200 to 400; and f) the reaction mixture contains 3 to 9 parts by weight, per 100 parts by weight of the isocyanate-reactive materials, of the at least one trimerization catalyst. a) greater than 70% by weight of the isocyanate-reactive materials is at least one polyether polyol having a hydroxyl equivalent functionality of 3 and a hydroxyl equivalent weight of at least 100 but less than 400, wherein the at least one polyether polyol is selected from homopolymers of ethylene oxide, copolymers of ethylene oxide and propylene oxide and homopolymers of propylene oxide and the at least one polyether polyol has an average oxyethylene content of greater than 40% but less than 80% by weight and correspondingly an average oxypropylene content at least 20% but less than 60% by weight based on the combined weight of oxyalkylene groups present in the at least one polyether polyol, provided that when the at least one polyether polyol has an average hydroxyl equivalent weight greater than 200, the isocyanate-reactive materials further include at least one crosslinker having a hydroxyl functionality of at least 3 and a hydroxyl equivalent weight of less than 100 such that the equivalent weight of the at least one polyether polyol and the at least one crosslinker combined is less than 200; b) less than 25% by weight of the isocyanate-reactive materials is a polyester polyol; c) water constitutes no more than 0.5% of the weight of the isocyanate-reactive materials; d) the polyisocyanate component includes one or more aromatic polyisocyanates, the one or more aromatic polyisocyanates having an average isocyanate functionality of at least 2.7 and an average isocyanate equivalent weight of up to 150; e) the isocyanate index is 200 to 400; and f) the reaction mixture contains 3 to 9 parts by weight, per 100 parts by weight of the isocyanate-reactive materials, of the at least one trimerization catalyst.