Considering the determinant role of energy efficiency opportunities in the building sector, proposing guidelines and technical solutions in order to improve the thermal performance of glazing systems is in priority in Iran and other countries. In glazing systems, a substantial amount of radiative heat transfer results from absorption and emission. Applying low-E (low emissivity) coatings on the glazing system is a solution for reducing the radiative heat transfer by radiation in glazing systems, without a noticeable decrease of visible light transmittance. By selecting the proper glazing type, in a hot climate, the amount of solar heat gain can be reduced significantly. Vice versa, in cold climate, the heat loss by long wave radiation of inner surfaces can be reduced. In this paper, first, the thermal and optical characteristics of local clear and Low-E glasses have been measured in BHRC (Road, Housing & Urban Development (National) Research Center) laboratory, by the spectrophotometer and then the thermal performance of single and double glazing units, with and without low-E coating, in different orientations (North, South, East and West) are computed, compared and analyzed for cold (Ardebil) and hot (Bandar-Abbas) climates in Iran. The selection of these two cities is based on maximum heating and cooling degree day values obtained respectively for Ardebil and Bandar-Abbas. The simulation results indicate that using double clear glazing unit with low-E coating reduces the energy consumption significantly in very hot climates, using mostly electrical energy for cooling. In cold climates like Ardebil, double glazing with low-E coating has a minor impact on annual heating load, because of the great amount of thermal radiation, even in the cold season, the high cost of low-e coating and the considerable payback time, compared to the building life cycle.
