Comfort standards and variations in exceedance for mixed-mode buildings

Mixed-mode buildings operate along a spectrum from sealed heating, ventilation and air-conditioning to 100% naturally ventilated, but little is known about their occupants' comfort expectations and experiences. Exceedance metrics, which quantify the percentage of time that a building's environment falls outside an expected thermal comfort zone, can help address the comfort trade-offs in building design and operation. Practitioners were polled on exceedance use in practice and comfort models and exceedance metrics were analysed: several comfort standards using EnergyPlus simulations of a mixed-mode building with radiant cooling in California's 16 climate zones. Results indicate that comfort model choice significantly influences predicted exceedance. Exceedance using PMV-PPD and the adaptive comfort models from ASHRAE Standard 55, EN 15251, and the Dutch NPR-CR 1752 frequently differed by 10 percentage points, often with 2-4 percentage points across the adaptive models. Yet, recommended exceedance limits often fall between 3% and 5% total. Exceedance predictions are also sensitive to uncertainties in predicted neutral comfort temperatures and variations in building envelope performance, solar heat gain, thermal mass, and control precision. Future work is needed to characterize comfort better in support of improved comfort modelling, exceedance targets, building design and building operation, and the development of related codes and standards.