The architecture of a building is crucial in determining its thermal energy balance and indoor comfort conditions. Knowledge of solar radiation availability and its transmission through a building envelope to the interior of the building helps an architect to design the building in an energy efficient way. Nowadays, in highly populated urban areas, attics are used as living spaces and the building envelope includes inclined external walls and windows in roofs. This paper presents some aspects of modelling the energy balance of rooms with different orientations and with vertical or inclined surfaces of building envelope, with stress on the impact of solar energy. The dynamics of energy flow through windows is analysed in more detail. One dimensional energy flow through the centre of glass area (based on a thermal resistance model), two-dimensional energy flow through the edge of glass area and two-dimensional heat flow through the opaque frame are analysed. The third dimension is also considered in a simplified way by taking into account the specific perimeter of the edge or frame. Stress is put on modelling the solar energy input. Solar radiation is modelled as short wave radiation that is transmitted directly to the room through glazing and as energy absorbed by the building envelope (glass panes, frame and opaque external walls) that becomes internal heat sources and is transferred indirectly to the room. The model developed has been used for numerical simulation using MATLAB as the programming language. This model predicts (amongst other things) the solar energy impact on the energy balance of a room in a building. It allows many cases of rooms and their envelopes to be run and evaluated and as a result both general and detailed conclusions can be drawn. Some results are presented in both graphical and tabular form. (c) 2008 Elsevier Ltd. All rights reserved.