It has been known for centuries that housing and health are inextricably linked. However, most of the evidence so far comprises cross sectional studies, which can only assess the relation between housing and health outcomes rather than provide convincing evidence that better housing improves health. A systematic review of intervention studies (carried out in 2001) found that housing improvement may lead to small improvements in self reported physical and mental health and reductions in some symptoms, but adverse effects on health are also possible.1 However, the evidence is patchy and robust study designs are rare. Of the 18 studies identified in the review, six were prospective controlled studies and only one was a randomised controlled trial.1

In this week's BMJ, Howden-Chapman and colleagues report a large randomised controlled trial from New Zealand assessing whether insulating …