Traditional houses and public buildings in Thailand were constructed from materials of low thermal mass and the walls were not insulated. Brick and concrete began to replace wood as walling material before air-conditioning was introduced into the country. New large buildings have glazed and closed windows so interior spaces require air-conditioning and forced ventilation. Up to the present, no insulation is used on walls. This paper presents results from an experimental and simulation study on comparative energy and economic performance of walls used to enclose air-conditioned spaces. The walls are externally and internally insulated to different thicknesses. The spaces are assumed to serve three commercial functions. Results show that insulation can generally help improve thermal performance of walls. but the function that a space serves dictates where insulation should be placed and how cost effective it is. (C) 2011 Elsevier B.V. All rights reserved.