Indoor environmental quality and energy performance of buildings are becoming more and more important in the design and construction of low energy, passive and zero energy buildings. At the same time, improved insulation and air tightness have the potential to resulting in a deterioration of the indoor air quality in such buildings. Currently, there are no global guidelines for specifying the indoor thermal environment in such low-energy buildings. The objective of this paper is to analyse the classification of indoor thermal comfort levels and recommended ventilation rates for different low energy buildings, and propose a set of indices that would enable better quantification and comparison among low energy buildings. In this study, the building codes and voluntary guidelines have been reviewed on the basis of experience of Finland, UK, Denmark, USA and Germany. The analysis in this paper will be based on the international standard ISO 7730 and EN 15251 in comparisons with other national building codes or standards.