The selection of the type of glazing as well as its proportion to the total wall area are very important towards achieving a desirable indoor environment. This paper uses a dynamic thermal-circuit zone model to study the influence of the glazed openings percentage (GOP) and type of glazing on the maximum and minimum indoor temperatures, during and after solar hours. The model comprises of four major structural heat-flow paths, corresponding to the external and internal walls, glazing and horizontal slabs. The simulation is performed for winter and summer seasons and for a zone with and without a power plant, with an external wall having south orientation. In the latter case, the optimum GOP leading to the minimum energy consumption is determined. The zone formation comprises masonry walls with insulation in the center plane, concrete horizontal slabs with and without insulation and three different types of glazing (double, double with low emissivity (low-epsilon) coating and double with reflective film). As GOP varies, computer results show how the type of glazing and the insulation of the slabs affect the indoor zone temperatures and energy efficiency.