Ethylene-tetrafluoroethylene (ETFE) foil is most commonly employed in buildings as a replacement for glazing to enclose transitional spaces, for example atria and circulation areas, although, increasingly, it is used as a secondary or primary façade for more intensively occupied architectural spaces where thermal comfort needs to be more precisely controlled. Despite the different thermal/optical properties, thickness and form in use of this material when compared to glass, there has been only limited investigation of the influence of these characteristics on the dynamic thermal responsiveness of the envelope and its impact on the thermal environment of the enclosure. This paper presents results of an ongoing investigation into the thermal environment of an approximately 625m2 atrium enclosed by a two-layer ETFE-foil cushion roof recorded by onsite monitoring conducted from April 2014 to April 2015. In order to examine the thermal environment of the atrium during different seasonal conditions, continuous field measurements of indoor thermal environmental parameters were recorded under varying external climatic conditions at different vertical levels and horizontal positions in the atrium with and without operation of the HVAC system and with variable occupancy. The effects of solar transmission, outdoor air temperatures, surface temperatures of the ETFE cushion on internal air temperatures and mean radiant temperatures at different levels of the atrium are compared and discussed