Nowadays humanity is electric depended. The increasing consumption of electricity negatively affects the environment due to the way it is obtained. Furthermore, the energy usage in residential buildings accounts for 27.1% of the worlds’ total consumption, where governments have started to settle their boundaries. Windows account for a total of energy leakage in a buildings of around 40%, which makes them one of the biggest contributor of energy leakage. Due to this reason, the first objective is to improve the efficiency of the overall building in the designated area. However, there are buildings that cannot be modified, as they are historical and, in consequence, protected by law. In those cases, it is needed to find other ways of upgrading them. Most of the available test methods for calculating the thermal transmittance [U-Value] of a window are performed in specialized facilities, where a sample is needed in order to perform the test. As obtaining a sample for performing a laboratory test is not possible, an onsite methodology based on the hot box test method was designed and constructed in one of the rooms in Gävle Rådhuset, the main building of the city council of Gävle. The proposed solution was built using insulation materials and thermocouples type T, reducing the cost needed for special facilities and not requiring any destructive method. Furthermore, low emissivity films were installed and tested for calculating the best installation position and to quantify the reduction of the U value. Three different scenarios were compared, two of which were made using the same type of film, though installed in a different position. It was found a U-value for the overall 2-pane window (incl. frames) of 2.19W/K·m2 (for a scenario without film, where the outdoor of the window was used as outdoor environment). Additionally, a reduction of 33% of the U-value was obtained, when a low emissivity film is applied between the two glazing surfaces of the window. On the other hand, when installed in the region recommended by the manufacturer – the innermost glass – the reduction of 22% has been achieved, which is slightly lower than the value given by the manufacturer of 26%. The feasibility of the adaptations of the method used for this study seems to be properly made, despite that a more complex calculations for the U-values and testing with known U-value materials is needed to be able to assure the correct and trustable operation of the apparatus. Furthermore, with the results obtained, the option for outdoor environment for calculating the U-value that seems more correctly is choosing the external window surface temperature.

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