Air layer exists between glass covers and cells in most building-integrated photovoltaic/thermal (BIPV/T) systems. This layer significantly reduces heat loss and improves the thermal efficiency of BIPV/T systems. However, the frame border that supports the glass cover casts a shadow over cells near the frame according to the air layer thickness, system orientation and location of the sun, thereby affecting photovoltaic performance. This study aims to find out the effect of such air layer on thermal and photovoltaic performance of BIPV/T systems. The distribution of frame shadows and photovoltaic loss caused by such shadows is analyzed. Then thermal and photovoltaic performance of BIPV/T system with different air layer thicknesses is evaluated. The influences caused by the air layer thickness, orientation of the BIPV/T system, solar altitude, and solar azimuth are analyzed. An approach is presented to evaluate the annual performance of the BIPV/T system. The results show that the frame shadow reduces system photovoltaic efficiency to 2.6% (normal efficiency, 13.0%) in the worst case scenario in Hefei (E117 degrees 17', N31 degrees 52'). The maximum annual electrical loss caused by the frame shadow is 70.15 kWh/m(2) at an azimuthal angle of -45 degrees. The quantity of total outputs increases with increasing air thickness. But the maximum comprehensive outputs are 359.95 kWh at 20 mm air thickness. (C) 2016 Elsevier B.V. All rights reserved.