Existing commercial buildings are surveyed and categorized based on the construction characteristics of their envelope. The envelope heat gain and the resulting cooling load demand are analysed with the aid of energy simulation tool DOE-2.1D. The concept of the overall thermal transfer value (OTTV) is applied to study the association of the envelope designs with the cooling requirement, and a modified approach in assessing the effective envelope heat gains under a sub-tropical climate is proposed. The predicted OTTV gives a good indication of the thermal performance of the envelope under a sub-tropical climate. The energy impact and possible range of cooling-load demands, for various envelope designs, under similar internal characteristics and cooling system, are identified.