Accurate, spatiotemporal, real-time indoor environmental quality (IEQ) monitoring and assessment are significant to safeguard occupants from environmental threats and make decisions on building sustainability. However, it is challenging for existing indoor spaces due to capturing real-time environmental and as-is data, in both space and time scales. Further, visualizing and analyzing data demands a robust data integration platform. Employing the design science methodology, this study develops a holistic method to create digital twins (DTs) of existing indoor spaces for improved IEQ monitoring and assessment. Real-time environment data from Internet of Things (IoT) sensor deployments and as-is data from laser scanning are integrated into a cloud-based data integration platform to create a DT. The study uses the IEQ criteria in Building Environmental Assessment Method (BEAM) Plus Interiors, which is Hong Kong’s leading green building certification system, as a checklist. Facilities managers and green building assessors can use the DT to make accurate IEQ monitoring and assessment decisions.