Photovoltaic (PV) modules generate high amounts of heat during electricity production process. The excess heat could lower the overall module efficiency. Therefore, seeking methods of reducing PV cell's heat gain could potentially enhance the overall module performance. One such method, is to incorporate Phase Change Materials (PCM), which could also be integrated with other heat rejection methods. In this paper, a building-integrated photovoltaic-thermal (BIPVT) system comprising a PCM layer as well a gap for airflow beneath the PCM layer is numerically investigated. For comparison purposes, four commercial PCM types were selected, namely RT18HC, RT21, RT21HC, and RT25HC. For each PCM, the cell temperature, overall module's efficiency, and the unit output airflow temperature was investigated. Furthermore, the effect of PCM layer thickness on the aforementioned system performance criteria was also analyzed. The effect of solar intensity apart from the ambient temperature on the module performance was assessed in advance as a novel approach. It was revealed that for a specific layer thickness, incorporating RT18HC, which has the highest latent heat of fusion among the PCMs, yielded the lowest PV temperature, lowest output airflow temperature, and highest PV efficiency. At 1:00 p.m. and for 50 mm of PCM layer thickness, the PV efficiency of the system with RT18HC was improved by 1.71% as compared to the module with RT21. In addition, it was shown that the PCM layer thickness has an optimum value in improving the system performance. Increasing the thickness beyond the optimum value did not yield significant enhancement. For RT18HC, the optimum layer thickness acquired to be 120 mm. Finally, it was shown that the cell efficiency was highly affected by solar radiation intensity, while the output airflow temperature was more affected by the ambient temperature.