For today's building operation business, intelligent building performance analysis solutions with the strength for high replication potential are on urgent demand. While ICT is available throughout today's daily life, it is however still a challenge to bring the right data at the right time to the right decision makers in order to impact energy efficiency and savings for buildings, while operating in a highly competitive market place. With a multitude of BMS solutions and their proprietary performance calculations approach in the market, improvement of Building Performance needs to address other data sources as well, like geometric data, weather, occupancy, building services, and control/actuation data. These diverse data sources ask for an integrating ICT solution for data acquisition, storage, and analysis contribute to Building Performance calculations with emphasis on Key Performance Indicators (KPIs). Accommodating such information exchange demands the services of an effective and efficient middleware. This paper provides the description of an integration middleware and discusses relevant complexities experienced through achievements concerning the EU FP7 CAMPUS21 project.