In some climates people spend approximately 90% of their lives indoors. Thus, it is very important to maintain healthy and comfortable conditions in buildings. The building sector is responsible for about 40% of the energy consumption and contributes related CO2 emissions worldwide. Knowledge about the heat transfer and storage in concrete components of the building envelope andmaterials that comprise the envelope is vital in evaluating the environmental and energy performance of buildings. Providing sufficient thermal properties of the building envelope is crucial in delivering optimal indoor conditions while reducing the energy consumption in buildings. This paper presents a case study that focuses on evaluating the environmental performance of a reinforced precast concrete slab that incorporates the void forming system. Numerical analysis and fieldmeasurements were performed to investigate thermal properties of the floor slab in the operating demonstration Engineering Building at the National University of Ireland Galway in Ireland. Firstly, the numerical finite volume models of the slab in the demonstration building were developed and validated with the ‘real-time’ physical measurements. Those measurements were obtained from thedemonstration building, including temperature sensors embedded in the slab, outdoor air temperatures, thermal imaging, etc. Those validated models allowed investigation of the phenomena of heat transfer and storage in the slab and their influence on indoor environmental conditions, including occupants’ health and comfort. The numerical models and ‘real-time’ field measurements were also supported by detailed laboratory experiments regarding the material properties of concrete. The proposed analysis will (i) identify possible design/operationdrawbacks; (ii) suggest optimised, cost-effective and sustainable designs; and (iii) allow for the development of new environmentally friendly and energy efficient structural building components.