In healthcare facilities, patient safety and well-being are known to be influenced by the built environment (BE). However, mechanisms linking BE to patient safety and wellbeing are not well-understood, which hinders the prevention and mitigation of undesired outcomes. In this paper, the Functional Resonance Analysis Method (FRAM) is used to model the functions carried out by caregivers and patients in an Intensive Care Unit (ICU), supporting the analysis of how variability in meeting BE requirements propagates across the work system and impacts patient safety and well-being. The FRAM model was based on observations of everyday work in the ICU, interviews with 24 professionals, and analysis of documents. One scenario derived from the FRAM model is discussed, emphasizing impacts on patient safety and well-being. Results show the utility of understanding how healthcare services work under real circumstances, as a basis for BE design. Findings also indicate that the variability of everyday healthcare services may either amplify or dampen the impacts of unfulfilled BE requirements on patient safety and well-being.