Modern buildings in Hong Kong are characterized by high window-to-wall ratios. This leads to large energy consumption due to the solar radiative heat gain. The problem is now of great concern to the industry not just because of the energy usage, but also due to the environmental problem that arises from overusage of energy. The government is considering implementing some regulations for controlling the cooling load in centrally air-conditioned buildings, The concept of overall thermal-transfer value (OTTV) is proposed to be used as a guideline for designing the building envelopes. However, those equations for calculating the OTTV recommended by the draft design guide are rather unclear. Many parameters that influence the OTTV equation have not been studied thoroughly yet. This paper aims at carrying our a parameterization study on the OTTV equation. The window-to-wall ratio, shading coefficient, wall absorptance and wall heat capacity are found to be important parameters. Modification of the equivalent temperature that appears in the OTTV equation is recommended Finally, the effect of overhang on the solar load value is also investigated.