The drive toward reduced energy consumption is being enforced through increasingly stringent statutory regulation which is impacting on the way that contemporary housing is designed and constructed is leading to the use of new materials, systems and technologies. The impacts of these, particularly when they become mainstream, are not yet well understood. This is an important problem for social housing providers who retain a contractual obligation to their tenants and are therefore concerned about possible energy and health impacts. The aim of this research was to examine the environmental performance of new-build housing through the monitoring and evaluation of 20 typical new-build contemporary flats in London. Problems with ventilation provision were endemic. In the winter 90% of bedrooms had CO2 levels over 1000ppm overnight, due to a variety of design, construction and operation issues. This paper describes the nature and causes of these.