Abstract The buildings included in Sweden's old building stock rarely meet the requirements of authorities regarding the indoor environment or energy consumption. Since they are often designed with self-draught ventilation, the cost of switching to mechanical ventilation is high. The windows in the building receives poorer values in terms of u- values, g-values, etc. than produced windows do today, which negatively affects the thermal comfort of people staying in the indoor environment. This case study examines the thermal comfort of two hotel rooms at Elite Grand Hotel since guests experience the hotel rooms in a southerly direction as warm during the summertime. This is due to the direct solar radiation that affects the thermal comfort in negative term. By simulating the building in IDA ICE and comparing hotel rooms with the same design and location on its windows, a room with existing properties is examined with respect to the window and a room where the characteristics of the window have been modified with the window film 3M Prestige 70. According to the manufacturer, this window foil is to reduce the heat from the solar radiation, which can lead to a better thermal comfort for hotel guests. The results of the simulations showed that the 3M Prestige 70 window foil reduced the heat from the solar radiation while improving the thermal comfort of the hotel room. Even during the warmest simulated days, the room is perceived as neutral/just right for an applied window foil when it without foil is perceived as warm. After comparing the installation of window foil and window change, it separated about 716 000 SEK for one floor to the advantage of the window foil. Key words: Thermal comfort, IDA-ICE, window foil, simulations.