The naturalness effect: A global open-database analysis of ventilated and mechanically cooled spaces

A key but insufficiently examined phenomenon in thermal comfort is the "naturalness effect", the tendency for occupants to perceive naturally ventilated (NV) environments as more comfortable than air-conditioned (AC) ones under similar thermal conditions. To validate this effect on a global scale, we conducted an open-database analysis of over 72,000 records from the ASHRAE Global Thermal Comfort Database II. We employed Chi-squared tests, logistic regression with cluster-bootstrap for sound inference, and a temperature-controlled, country-level meta-analysis to compare thermal sensation distributions between NV and AC spaces. Results confirm a statistically significant naturalness effect. The comfort advantage for NV is most pronounced in moderate-to-warm temperatures; in the 28-31°C range, 42% of NV occupants reported neutrality compared to only 29% in AC spaces, challenging the assumption that mechanical cooling always ensures superior comfort. A key underlying mechanism was identified: occupants in NV buildings exhibit significantly more responsive adaptive clothing behaviour than their AC counterparts. While a country-level meta-analysis revealed significant heterogeneity, highlighting the importance of regional context, the pooled global effect remained stable. After controlling for temperature, the odds of reporting thermal neutrality are 36% higher in NV environments (Odds Ratio = 1.36, 95% CI [1.15, 1.47]). Overall, this study provides strong, cross-climatic evidence that cooling strategy systematically influences thermal perception, validating the adaptive comfort model and supporting the design of more energy-efficient buildings with wider temperature setpoints and enhanced occupant control.