A theoretical study has been performed to determine the effect of variations in convection coefficients on the storage of thermal energy in structural materials in buildings. Detailed analytical and numerical analyses have been conducted to study the fundamental aspects of the problem for simple geometries. The detailed analyses suggest that the study of thermal performance of massive structural materials in buildings can be divided into two distinct classes. These two classes correspond to internal partition walls, characterized by similar air temperature profiles on the two sides, and external envelope walls, characterized by dissimilar air temperature profiles at the two surfaces. For massive interior walls, a thermal energy storage effectiveness parameter has been defined in terms of the changes in the diurnal heat storage capacity of the wall with respect to variations in convection coefficients. The interior wall effectiveness has been calculated for a wide range of convection coefficients and for both simple sinusoidal and more complex and realistic inside air temperature profiles; in all cases upper and lower bounds have been obtained. Results indicate that the effectiveness of interior massive walls is quite sensitive to the variations in convection coefficients commonly occuring in buildings. Since the convection coefficients at interior building surfaces vary quite widely within this range, this work suggests that for some climates and building constructions, improved characterization of convection coefficients is needed to permit reliable calculation of the energy requirements of buildings incorporating large amounts of thermal mass.