There is a general agreement that much of the syntax of modern architecture contributed to design buildings that were often inadequately adapted to climate conditions and, consequently, produced uncomfortable buildings. This would eventually spur critical regionalism and criticism over modern architecture. However, there are some renowned examples that, while consistent with modern tenets, prove that assumption to be wrong. Amongst them the E.1027 house designed by Eileen Gray and Jean Badovici stands out. The building is a genuine example of a modern architectural site-specific design considering its effective adaptation to local climate conditions. This research aims to contextualise, quantify predicted thermal performance, and assess the quality of the E.1027 overall design in relation to thermal comfort. Most significantly, the house's adaptive design to achieve indoor thermal comfort values throughout the year. Thus, the original texts of Gray and Badovici on the E.1027 are contrasted with the results obtained from a series of simulations using thermal performance specialist software. A data rich 3D construction model was built which allowed the export of geometrical data to other simulation and analysis tools. The model was also used to evaluate sun exposure, to analyse the influence of shading devices. Two environmental evaluation software programs were used to analyse the impact of the different types and sizes of windows and solar control devices, and how these affected interior illumination levels. A combination of three methods was used to assess the building's expected thermal comfort performance, using the hourly data obtained from the software analysis process. Firstly, Givoni's Bioclimatic Chart was used to define a comfort zone for the building; secondly, Fanger's statistical analysis method, used to define a thermal comfort zone through the predicted mean vote. Thirdly, the Adaptive Model defined by the ASHRAE 55 allowed the evaluation of the thermal conditions in occupant-controlled naturally conditioned spaces.These simulations and environmental analysis results demonstrated that Gray's and Badovici's deep understanding of the sun's impact and the cross ventilation needed to provide comfort can be modelled and confirmed. All passive design architectural elements evaluated in this research proved to have played a key role in the process of providing predicted thermal comfort values, just as it was foreseen by Gray's and Badovici's design, and anticipated in their writings. This comparison demonstrated that the designers did indeed produce a design as sophisticated as it was comfortable, while distilling the two major architectural trends at the onset of modernity: neoplasticism and Corbusian imprint. Moreover, as a corollary, the paper argues that modernity in architecture did not necessarily contradict the idea of comfortable architecture and adaptation to local climate conditions when embracing modern aesthetic principles.