Man has tried to improve comfort within buildings by improving the thermal inertia and minimize the equivalent thermal conductivity of the envelope of building from time immemorial. Attempt has been made by engineers by increasing the thickness, changing the geometry of the outer wall and also tried several building materials to reduce temperature fluctuations for indoor environment in both summer and winter. The installation of heating and air conditioning to seek comfort in homes, offices and public places has created high energy consumption and consequently, increased the environmental pollution. The use of passive solar architectural techniques can reduce not only the temperature fluctuations but also can solve the environmental pollution. The use of phase change materials (PCM) in the building along with passive solar techniques is one of the solutions. The integration of a PCM layer into an external building wall diminished the amplitude of the instantaneous heat flux through the wall. In this paper a threedimensional transient heat transfer model has been developed and solved numerically using the commercial Thermal analysis package ANSYS.