Enhanced human heat exposure in summer in a Central European courtyard subsequently roofed with transparent ETFE foil cushions

Due to their properties and dimensions, courtyards regulate their microclimate, which, above all, mitigates extreme situations and reduces energy consumption. Mainly for increased energy saving, existing courtyards in temperate climates are often subsequently roofed transparently, converting them into atria. Due to various advantages roofs made of ethylene-tetrafluoroethylene (ETFE) foil cushions have been increasingly used over the last four decades as an alternative to traditional glass roofs. Although ETFE foil cushions have a higher transparency to long-wave radiation than glass roofs, they also create human thermal discomfort in the spaces below. To assess its extent, a 6-week field study was carried out in the summer of 2012 in an atrium roofed with ETFE foil cushions and in a comparable neighbouring courtyard located in Freiburg (southwest Germany). The results for air temperature (Ta), mean radiation temperature (Tmrt) and Physiologically Equivalent Temperature (PET) indicated a significantly higher human heat exposure in the atrium than in the courtyard, which averaged 5.6 K for Ta, 6.3 K for Tmrt and 7.1 K for PET. Primary drivers for the enhanced human heat exposure in the atrium were the reduced loss of radiant heat caused by the ETFE foil cushions and the reduced natural ventilation inside.