Wallboard impregnated with phase change material (PCM) will provide thermal storage that is distributed throughout a building, enabling passive solar design and off-peak cooling with frame construction. This paper examines the thermal dynamics of PCM wallboard that is subjected to the diurnal variation of room temperature, but is not directly illuminated by the sun. The purpose of this work is to provide guidelines useful in selecting an optimal PCM and in estimating the benefits of PCM architectural products. The energy stored during a daily cycle depends upon a) the melt temperature of the PCM; b) the temperature range over which melt occurs; and c) the latent capacity per unit area of wallboard. Situations with the wallboard on an interior partition or on the inside of the building envelope are investigated separately. The following findings are presented. The maximum diurnal energy storage occurs at a value of the PCM melt temperature that is close to the average room temperature in most circumstances. Diurnal energy storage decreases if the phase change transition occurs over a range of temperatures. The diurnal storage achieved in practice may be limited to the 2 range 300-400 kJ / m , even if the wallboard has a greater latent capacity. The implications of these findings for test room experiments are discussed.