As is known double skin facade applications have an important role on passive solar strategies. In this paper, heat losses of a single skin and a double skin facade office buildings were compared to show the effect of double skin on building energy demand for winter period in Istanbul. Calculations have been carried out for the January, which is assumed as the representative month of winter period in Istanbul considering Istanbul's climatic conditions. However there is no any double skin facade application in Istanbul, the thermal effect of the double skin has been theoretically analysed to show the effect of double skin facade on heat losses if this technology would be applied by the construction industry in Turkey. In this study a new method is used in double skin facade's heat losses calculations. The realization of the proposed method is composed of two main steps. The first step is formed from the modification of a previous method, which is proposed by Todorovic, to calculate inter-space temperature. For the second step inter-space temperatures calculated in the first step have been accepted as outdoor air temperature and time dependent heat transfer through the inner skin of the envelope has been calculated by using finite difference numerical approach. It has been assumed that the space between two skins is closed since the calculations have been made for the winter period. It has been concluded that heating energy consumption is significantly reduced in double skin facade building for winter conditions in Istanbul. (c) 2004 Published by Elsevier B.V.