Thermal conductances (U-values) and thermal resistances (R-values) are discussed throughout the literature as the appropriate parameters for characterizing heat transfer through walls. Because the quoted numbers are usually determined from handbook values of material properties, they have several drawbacks: (1) they do not take into account degradation effects, (2) they ignore construction irregularities, and (3) they do not take into account multidimensional heat flow. This paper examines the use of field measurements of heat flow and surface temperatures to determine the U-values of walls. The effects of thermal mass on measurements of wall U-values are described in detail, using two data interpretation techniques to estimate the U-values of insulated and uninsulated cavity walls, with and without brick facing. The errors in U-values estimation are determined by comparison with an analytical model of wall thermal performance. For each wall, the error in the U-value determination is plotted as a function of the test length for several typical weather conditions. For walls with low thermal mass, such as fiberglass-insulated cavity wall, it appears that, under favorable test conditions, a 6-h measurement is adequate to measure the U-value within about 10% uncertainty. For masonry walls, the measurement time required is considerably longer than 6 h. It is shown that for masonry walls, and in general, the optimal measurement time is a multiple of 24 h because of the effects of diurnal weather fluctuations.