Providing a high quality indoor environment is important to protect occupant’s health and well-being, particularly in the home where we spend a significant amount of time. This paper explores indoor environmental conditions in mechanically and naturally ventilated new-build low-energy housing in the UK. Indoor air temperature, relative humidity and carbon dioxide data were collated and analysed from 53 dwellings across 20 different new-build demonstration projects (consisting of public and private developments). The results raise concerns regarding ventilation performance in new-build homes, particularly homes with natural/mechanical extract ventilation (MEV). Significantly less variation of temperature and relative humidity levels were observed in homes with balanced mechanical ventilation with heat recovery systems (p=<0.001), suggesting these systems may help to provide a more stable indoor hygrothermal environment. Average indoor air relative humidity levels were consistently higher in MVHR dwellings. The findings suggest that the type of ventilation strategy can play a significant role in regulating indoor relative humidity and air temperature in new- build thermally efficient homes.