The purpose of this project has been to investigate the durability and lifetime of energy-efficient windows - to see how they age and whether their U-value changes with time, and also analyse problems with this type of windows. The thermal insulating properties of energy-efficient windows are mainly determined by the U-value of the glazed area. Therefore, the project has primarily included what’s happening to the glass part of the windows. In a number of buildings, energy-efficient windows have been inspected concerning function, condition and durability. The age of the studied windows differ between two and thirteen years. The glass part of the windows has been studied concerning number and location of low-emissions coatings. Gas concentration has been measured in those windows that contain a two-glazed sealed unit. The occupants of the buildings have also been interviewed. Calculations have been carried out in order to study how the U-value changes with numbers of low-emission coatings, their locations, gas filling and type of spacer in the sealed glazing unit. Calculations have also been carried out concerning frequency of condensation on the outside of the windows. The inspections show that most of the windows are in good condition. However, there are some failures in gas concentration and low-emission coatings. The measured gas filling is generally in the range of 75-92 %, which is far too low for most of the inspected windows. In two buildings, the low-emission coating was mounted on the wrong side of the glass. Values of gas filling for over 300 newly manufactured sealed glazing units are available at SP. These data shows that the average gas concentration has been 86 %. In 15 % of the units gas concentration was lower than 85 %. More than 2% of the units had no gas at all. If energy-efficient windows have good quality when they are new, the durability also seems to be good. A deterioration of the quality, for example too low gas filling or coatings mounted on the wrong side of the glass, deteriorates the U-value which leads to increased energy use. It is important for the U-value of the window how long the gas stays in the sealed glazing unit. From the work described in the literature, and from measurements made at SP, it is evident that, on the whole, gas-filled units are generally gastight, with little gas leakage. The condensation on the outside of energy-efficient windows is sometimes regarded as a problem. Usually, it can be accepted if its efficient cause is known, i.e. that the window is well insulated. No one of the interviewed persons regarded it as a problem. Key words: Energy-efficient windows, sealed glazing units, gas filling, low emission layers, U-value, durability.