This study analyzed the correlations between the meteorological/substrate-moisture variables and the evapotranspiration (ET) to improve the understanding of the factors influencing the thermal performance of green roofs, including cooling the surface and interior temperature of the building, in a subtropical climate. Daily ET of the green roof had been estimated using the FAO Penman-Monteith method to increase the number of ET data and the Nash-Sutcliffe coefficient is 0.51–0.62 compared to the observed daily ET from data of green roof experiments in Taipei during December 2012 and September 2013. Based on correlation analysis, this study then suggests that ET may have a higher influence on Tdiff, i.e. the difference in surface temperatures of green and original roofs, than the meteorological variables or substrate water content (SWC), although the radiative heating is sometimes more influential to Tdiff than the cooling caused by ET of the green roof in thin substrate condition. This study also found that under wet and dry substrates, daily ET is highly correlated with weather-related variables and with SWC, respectively. Furthermore, because of the low insulation of wet substrate, more downward fluxes are sometimes found under high than low SWC conditions. On the contrary, under the dry substrate, increasing SWC helps reduce the downward fluxes. This study, therefore, indicates that an optimal range of substrate moisture appears to maximize the function of a green roof in reducing the surface temperature and the downward substrate-bottom flux. Considering that a counterbalance exists between SWC and ET as well as irrigation can be applied to control SWC, future studies could be aimed at the optimal range of SWC to execute informed irrigation concerning the thermal performance, water-saving irrigation, and storm-water-retention function of the green roofs either in dry period or before storm events.