A generally accepted way of building passive houses has been to have small windows facing north and large windows to the south. This is to minimize losses on the north side while gaining as much solar heat as possible on the south. ln spring 2001, 20 terraced houses were built outside Gothenburg partly in this way. The indoor temperature is kept at a comfortable level by passive methods, using solar gains and internal gains from household appliances and occupants. Heat losses are very low, since the building envelope is well insulated and since modem coated triple-glazed windows have been installed. The purpose of this work was to investigate how decreasing the window size facing south and increasing the window size facing north in these low energy houses would influence the energy consumption and maximum power needed to keep the indoor temperature between 23 and 26 degrees C. Different orientations have been investigated as well as the influence of window type. A dynamic building simulation tool, DEROB-LTH, was used and the simulations indicate an extremely low energy demand for the houses. The results show that the size of the energy efficient windows does not have a major influence on the heating demand in the winter, but is relevant for the cooling need in the summer. This indicates that instead of the traditional way of building passive houses it is possible to enlarge the window area facing north and get better lighting conditions. To decrease the risk of excessive temperatures or energy needed for cooling, there is an optimal window size facing south that is smaller than the original size of the investigated buildings. (C) 2005 Elsevier B.V. All rights reserved. (Less)