Overall Thermal Transfer Value (OTTV): How to Improve Its Control in Hong Kong Sam C. M. Hui Post-doctoral Fellow Department of Architecture The University of Hong Kong (cmhui@hku.hk) Abstract The overall thermal transfer value (OTTV) standard has been implemented in Hong Kong for more than two years. It is the right time to re-examine it and to find out ways to improve its control. The concept and brief history of the OTTV method are reviewed. The methodology for developing OTTV standards is explained and the OTTV standards in some developing countries are compared. Limitations of the OTTV standard in Hong Kong are discussed. The ways to enhance the standard are proposed. It is hoped that a more comprehensive building energy standard integrating all aspects of architectural and building services designs can be developed in the coming future. Keywords Overall thermal transfer value, building energy standard and building energy efficiency. Introduction Building energy standards or codes are becoming more and more important in energy efficiency policies [1]. These standards can help raise concern and awareness of building energy conservation, promote energy efficient designs in buildings, encourage the development of energy efficient building products, and form a basis for assessing building energy performance and developing energy efficiency programmes [2]. Many countries in the world are now developing or upgrading their building energy standards in order to achieve the energy efficiency goals. In Hong Kong, the Government has implemented the building energy efficiency regulation in July 1995 [3]. The overall thermal transfer value (OTTV) method is used as a control measure for building envelope design in commercial buildings and hotels [4]. The controls on OTTV aim at reducing external heat gains through the building envelope and hence the electricity required for air-conditioning. After two years, the Government is about to review the OTTV legislation. It is the right time to re-examine the OTTV method, assess its limitations, and to find out ways to improve it for better energy efficiency and effectiveness. OTTV Concept and Brief History OTTV is an index for comparing the thermal performance of buildings. It is a measure of the average heat gain into a building through the building envelope [5] and consists of three major components: (a) conduction through opaque walls, (b) conduction through window glass, and Hui, S. C. M., 1997. Overall theraml transfer value (OTTV): how to improve its control in Hong Kong, In Proc. of the One-day Symposium on Building, Energy and Environment, 16 October 1997, Shangri- la Hotel, Kowloon, Hong Kong, HKIE BS Division/CIBSE/ASHRAE/PolyU, pp. 12-1 to 12-11.