Utilizing thermal insulation in the building envelope, in harsh climates, will considerably reduce the heat gain through the envelope of the building and consequently its energy consumption when calculating the cooling loads or performing its energy analysis. The performance of thermal insulation material is mainly determined by its thermal conductivity (k), which describes the ability of heat to flow across the material in the presence of a gradient of temperature. In practice, the k-value is normally evaluated at 24°C (i.e., k₂₄) according to relevant ASTM standards. Actually, the thermal insulation materials when used in the building envelope are exposed to significant and continuous temperature and moisture change, due to varying outdoor air temperature, solar radiation, and air moisture content. Accurate prediction of the cooling/heating load and, consequently, the sizing of the heating, ventilating, and air-conditioning equipment requires an accurate prediction of the heat transfer through envelope components. The thermal resistance of most thermal insulation materials depends on operating temperature, and the temperature to which the insulation materials are exposed varies, depending on the thermal resistance of the materials, the location of the insulation layer within the assembly system, and the effective temperature which depends on the amount of solar radiation received on the surface of the assembly. The main objective of this paper is to investigate the effect of the operating temperature and the moisture content on the thermal conductivity of various densities of polystyrene insulation material, which are widely used in Oman under hot-humid climate.