Insulated Concrete Formwork (ICF) is a site-based Modern Method of Construction (MMC). As a MMC, ICF has several advantages; increased speed of construction, cost and defect reduction, safety, among others. Moreover, the ICF wall construction method has similar benefits to any other heavyweight structure (such as strength, durability, noise attenuation). However, its thermal performance is not yet well-researched and understood. Using computational analysis and empirical evaluation, the aim of this research was to analyse the thermal performance of an existing ICF building; and to develop evidence about its transient thermal behaviour and how the latter is affected by the inherent thermal inertia of the concrete core. The results demonstrated that the ICF fabric showed a slow response to changes in boundary conditions, providing a stable internal environment. The concrete core of ICF was found to act as a buffer to the heat flow, reducing the transmission losses by 37%, compared to a lightweight wall with equivalent insulation. The analysis showed that although ICF is mostly considered as an insulated panel, the element's thermal mass is not as decoupled from the internal space, as has been thought the case.