Abstract: A rigid foam, especially a largely closed-cell polyurethane or polyisocyanurate rigid foam, includes a cell structure being formed by the expansion of rigid foam raw materials with carbon dioxide as an inflating agent. The cell structure is substantially homogeneous and has a cell content of substantially CO.sub.2 inflating agent and proportions of a physical inflating agent being at least substantially insoluble in at least one of the rigid foam raw materials. A process for producing a rigid foam having a cell structure formed by the expansion of rigid foam raw materials with carbon dioxide includes forming a fine-celled rigid foam material by preparing an inflating agent being at least substantially insoluble in at least one of the rigid foam raw materials. The inflating agent is homogeneously emulsified in at least one of the rigid foam raw materials prior to the beginning of a chemical reaction, in the form of the disperse phase of an emulsion having a liquid droplet size being at most equal to 10 .mu.