The thermal energy storage (TES) capacity of building materials can be enhanced by using phase change materials (PCM). In this work, the building composite PCM was prepared by impregnation of n-heneicosane (HE) as an organic PCM into bentonite (BNT) clay. The chemical, morphological, and thermal characterizations of the developed BNT/HE composite PCM was performed by scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), differential scanning calorimetry (DSC), and thermogravimetry (TG) analysis techniques. The SEM and FT-IR results showed that the HE was well incorporated into the pores of the BNT and had good compatibility with the components of the composites. The DSC analysis results showed that the composite including 36% HE by mass had a melting temperature of 38.32 degrees C and solidification temperature of 38.34 degrees C; in addition, the composite also can store latent heat of 96.23 J/g and release latent heat of 94.16 J/g. The TG analysis demonstrated that the developed composite PCM had good thermal durability. The thermal cycling test confirmed that the composite had good long-term thermal reliability and chemical stability. The laboratory-scale thermoregulation performance test revealed that the BNT/HE composite PCM wallboard had the effect of decreasing the indoor temperature of the building cell for a considerable period. Use of the developed BNT/HE composite PCM as a wallboard in the construction of energy-efficient buildings will open new practice opportunities in the building industry for passive solar heating, ventilating, and air conditioning (HVAC) purposes. (C) 2017 American Society of Civil Engineers.