Rising temperatures as a result of climate make the cooling of building envelopes and creating thermal comfort for more and more people a challenge while energy use must become more efficient. In addition to active (electricity-driven) systems, passive cooling methods are being developed for night-time as well as day-time cooling of buildings. A passive cooling skylight under development at Åbo Akademi has demonstrated a night-time passive cooling effect of ~100 W/m2. Its performance depends strongly on the gas used inside the skylight, that will pick up thermal radiation (at long wavelengths, LW) via a lower window and after a natural convection transfer inside the skylight, release the heat to the sky via an upper window. Proof-of-concept work at ÅA involved comparing the use of air with the performance when using carbon dioxide CO2, ammonia NH3 or pentafluoro ethane, i.e. hydrofluorocarbon refrigerant HFC-125. Best results reported were obtained with the latter, but as a result of the 2016 Kigali Amendment to the 1986 Montreal Protocol on HFCs, an alternative for HFC-125 needs to be found, because of its global warming potential (GWP). With GWP = 1 for CO2, = 0 for NH3, for HFC-125 the GWP = 3500. Since a passive cooling skylight can be ranked as a refrigeration installation it cannot contain a gas with a GWP >150 after 2030 under European regulation. The prime material property of a suitable gas is a high emissivity / absorption in the LW range 8-14 µm, also known as the atmospheric window, where, apart from water vapour, nothing prevents radiative heat transfer from Earth directly to space. The hypothesis was that HFC-152a or HFC-41, with GWP values < 150 could replace HFC-125 for this skylight application. Besides GWP, other features that must not prevent the widespread use of the technology are flammability and chemical / physical stability in general, toxicity and, of course, costs. CFD simulations (Ansys Fluent) were used the calculate the passive cooling (LW) heat fluxes, temperatures, the convection flow field, and the transported heat inside the skylight, comparing the mentioned gases. and The results show that the performance of HFC-152a (117.8 W/m2) and slightly less so HFC-41 (115.4 W/m2), both with a GWP < 150, can match the performance achieved so far with HFC-125 (117.3 W/m2), compared to 100.5 W/m2 with air.