Finding the way to predict optimal thermal conditions for an office worker would contribute to sustainable building design: the environmental effects would be reduced, the economics of the organization and whole society would improve and there would be indisputable social benefits for the individual and the global community. These benefits stem from the improved productivity of the office worker in most favorable thermal environment and the possibilities to achieve this with lower energy demand. This study uses a new approach, exergy analysis, to recognise the optimal conditions by looking for the combination of mean radiant temperature and room air temperature giving the lowest human body exergy consumption rate. All of the commonly used thermal comfort prediction methods use energy analysis, and it seems that exergy analysis could give more accurate prediction of the conditions giving optimal thermal comfort. The new method is applied to the case of office worker in typical and extreme weather conditions in Finland. The results agree well with the previous analyses, and moreover, the points giving minimum human body exergy consumption rate coincide with the points usually regarded as most comfortable in summer conditions. According to recent studies, people are also most productive at these conditions. (C) 2014 Elsevier B.V. All rights reserved,