Thermal performance characteristics supplied in specifications for materials being used for designing cryogenic and elevated temperature applications are obtained by standard testing techniques under laboratory conditions. It is assumed that these values adequately represent the actual field performance. Recent field performance data indicate that design performances are not being met. A number of general and specific factors that may influence actual thermal performance are outlined and discussed. These include product variability, flaws, anisotropy, direction of heat flow, orientation of the insulation, air and moisture movement, temperature and environmental factors, and installation. Quantitative results are given to illustrate the magnitude of possible effects, and these demonstrate that thermal properties alone should not be the sole criterion for design performance.