This Thesis is focused on the study of the problem of condensation on the inner surface of Fullrigaren building’s large single pane window. This has serious consequences as water on the floor, corrosion or mould growth.

As the climate in Nordic countries is cold for several months a year, windows are a crucial part in building envelopes. Condensation on a window may be suitably discussed only with respect to the climate considered as cold, moderate and warm climates pose different requirements on the windows, and this is why a characterization of Gävle by its climate is necessary.

This Thesis will include the energy analysis of the staircase considered which is required to understand the source of the actual problem. Both heat and moisture transfer will be studied. In this purpose, an IDA model will be built to simulate the conditions throughout the year and hand-made calculations will be done for the average and most critical situations. The results show that condensation will already occur for the monthlyaverage conditions having as an additional problem that if temperature drops below zero it will freeze.

Results will also be compared to an initial installation of a 2 pane window reaching as a conclusion that its original installation would had avoided the problems for most of the time.

The Thesis will end with several proposals posed to solve the problem by either increasing the temperature or reducing the moisture content of the ambient air, and the selection of the best one. The final aim of the Thesis is to achieve an energy efficient window which should provide good lighting during the day and good thermal comfort both during day and night at minimum demand of paid energy. And for this, the selection of the electrically heated window is proposed.