The significant share of buildings from total energy consumption across the world has been mentioned and emphasized very well by several scholars. In this regard, there have been major developments and improvements in the expertise of developing and designing buildings to be adequately energy efficient. However, the recent studies show that there is still a considerable deviation between the intended and actual energy consumption of the completed buildings. Hence, this exploratory study aims to discover the origin of success and failure in achieving energy efficiency in building construction projects with a life cycle perspective and based on viewpoints of key participants in the project and constructed building's operation. To do so, 21 semi-structured interviews were conducted with Finnish project professionals representing client, design/planning experts, contractors and building operation/maintenance experts. Both thematic analysis and content analysis methods were employed for analysing the obtained research data. The findings reveal a set of challenges/barriers and solutions/enablers which account for failure and success of achieving energy-efficient buildings. The obtained results contribute to the existing body of knowledge and practices on achieving energy efficiency in building construction projects.