The efficiency of photovoltaic (PV) devices is approximately inversely proportional to the cell temperature and air flow around PV modules installed over or beside a building envelope can facilitate ventilation cooling and so reduce the problem of module overheating. The effect of the size of air gap between PV modules and the building envelope on the PV performance in terms of cell temperature has been investigated numerically for a range of roof pitches and panel lengths under two different settings of solar radation,. It has been found that the mean and maximum PV temperatures decrease with the increase in pitch angle and air gap. The mean PV temepratutre also decreases with increasing panel length for air gaps greater than or equal to 0.1 m whereas the maximum PV temperature generally increases with panel length but decreases when the panel length increases from two modules to three modules and the air gap is between 0.1 m and 0.125 m. Without adequate air circulation, overheating of PV modules would occur and hot spots could form near the top of modules with potential cell temepratures over 80°C above air temperature. To reduce possible overheating requires a minimum air gap of between 0.11 m and 0.15 m for multiple module installation and 0.16 m for single module installation.