It has been recognized by many researchers that Occupant Energy Behaviour (OEB), play a vital role in reducing energy use. The previous research into the field of occupants’ energy behaviour can be classed into two categories regarding their methodological approaches. The majority of the research has been done using a quantitative approach where conclusions were drawn from monitored energy data along with large questionnaire surveys or tightly structured interviews, in which several factors that relate behaviour to energy use have been identified and agreed upon among researchers (e.g. age, housing characteristics, set point temperature etc.). Whereas only a few studies have adopted a qualitative method, or combined quantitative and qualitative methods. However the findings using qualitative and mixed methods showed potential benefits in gaining a better understanding of sustainability in people’s everyday lives and the nature of their energy use, which serves as firmer grounding to march towards energy efficiency. The comparison between these two main research methods in the same field is particularly interesting in terms of discussing the fundamental properties of the subject matter, and discovering specific aspects of energy behaviour which these different approaches could bring to the field. This paper reviews research from last decade regarding occupants’ energy behaviour, focusing particularly on studies of residential low energy buildings and Passivhaus, and compares the above three types of methodologies with a quantitative and qualitative methods, then tries to make a case for mixed methods with an emphasis on Grounded theory in this research field.