As sustainable development increases its significance in policy-making, methods for quantifying the sustainability of a project become more important. One such method is life-cycle assessment (LCA). In this study, an LCA assessment of a radiant cooling system was conducted for a retrofit project of a small office building. The studied building is located in Sant Cugat in north-eastern Spain. The radiant cooling system was also compared with a conventional alternative, an all-air variable air volume system. The goal of the study is to provide a generalisable methodology for conducting an LCA-assessment in a retrofit project involving cooling. The methodology of the assessment consists of two parts. Building energy models in IDA-ICE 4.8 were used to determine the energy use of the systems and the resulting thermal comfort conditions in the building, while SimaPro 9.4 was used to carry out the LCA assessment. A major novelty in the study is the use of thermal comfort as the functional unit for the LCA assessment. The results show that the radiant system has a lower environmental impact in all ReCiPe2016 midpoint impact categories during the systems' estimated lifetime of 50 years. However, a sensitivity analysis revealed that while the radiant system's environmental impact is mainly dependent on the manufacturing process, the conventional system's impact is largely determined by its operational energy use. Therefore, the conventional system is significantly more sensitive to decarbonisation of electricity production.