Positive input ventilation is the usual name for systems in which 'fresh' air is supplied to a dwelling from the roof space by means of a fan. It was originally developed in the 1970s to deal with condensation in dwellings without adversely affecting the operation of open-flued combustion appliances. While there is considerable anecdotal evidence that the installation of these units can improve indoor humidity conditions, measurements of performance have been few. This paper gives a summary of the results from performance measurements made in a BRE test house and in 15 occupied dwellings. It will be of particular interest to dwelling designers, landlords and domestic property managers.