Green roofs have been proposed for energy saving purposes in many countries with different climaticconditions. However, their cooling and heating potential strongly depends on the climate and buildingcharacteristics. In particular, the increase of the thermal capacity of green roofs compared to traditionalroofs, if not controlled with insulation, may lead to higher cooling and heating loads. This paper discussesthe energy saving potential of green roofs adopting a variable insulation strategy. A system consisting ofa plenum located between a green roof and the room underneath and a sensor-operated fan that couples(or decouples) the green roof mass with the indoor environment was developed. The fan is activated andstopped using temperature based rules; the plenum is ventilated only when the fan works, creating avariable insulation system. Four cells with an insulated traditional roof, a non-insulated green roof, aninsulated green roof, and a green roof with the variable insulation system have been tested in a hot anddry climate with mild winters over several years. This paper compares and discusses different plenumcontrol algorithms. Results are particularly promising because the variable insulating system proved toadjust the thermal capacity of the roof effectively. In summer, the non-insulated green roof and the greenroof with variable insulation system achieved the lowest indoor temperature; in winter, the insulatedtraditional roof and the variable insulation green roof system achieved the highest indoor temperatures.Measurements are hence compared with simulations. Finally, the energy saving potential of the newgreen roof system is evaluated.