There is an increasing awareness of formal ontologies in knowledge-intense problem domains in the AEC industry. This paper analyzes the general use of ontologies, points to the importance of context-dependent ontology descriptions in AEC, and describes the use and management of multiple (external) ontologies as knowledge management tools within a complex information model of a building performance assessment tool CLIP (Computational support for Lifecycle Integral Performance assessment), previously developed by the authors. The paper provides a discussion of our approach, analysis of the benefits and limitations of external ontologies, suggestions for further development and research areas for the integration of multiple AEC knowledge representations.