In response to policy and legislative requirements aimed at reducing carbon dioxide emissions, building designs are increasingly becoming more airtight and energy-efficient. However, previous literature raises concern that buildings designed to meet these requirements present significant challenges relating to overheating and poor ventilation. Concurrently, some designers are opting for low-carbon materials, such as straw bales, not only to reduce embodied carbon dioxide emissions but also for occupant health benefits. While straw bale buildings are recognised for their thermal efficiency, there is limited knowledge regarding their indoor environmental quality (IEQ) performance during occupancy. This paper presents a case study of the environmental performance of an airtight, energy-efficient straw bale dwelling with a decentralised ventilation system. The dwelling is located in southern England, UK and through this case study, the research aims to assess whether the home provides thermal comfort conditions year-round, with attention to summer and winter comfort, as well as a review of the relative humidity and total volatile organic compounds (TVOC). Data were collected for 14 months and assessed against recognised IEQ benchmark guidelines. The findings indicated no overheating occurrences despite a summer heatwave; however, elevated TVOC concentrations were observed during the summer compared to those measured in the winter. Results for the indoor temperature differ from those reported in previous research on UK airtight homes, highlighting the necessity for comprehensive IEQ research in a larger number of dwellings constructed with natural building materials to ascertain whether sustainable building practices enhance occupant comfort.