Differential scanning calorimetry and large scale thermal testing were conducted to determine the latent heat storage in a thermal storage wallboard. Thermal storage wallboard was developed at School for Building, Concordia University, by incorporating about 20% by weight of a phase change material in gypsum wallboard. It was determined that differential scanning calorimetry can adequately predict the performance of a full-scale installation of thermal storage wallboard. Consequently, the expensive large scale tests need only be performed when the development of thermal storage wallboard is well advanced.