Natural convection heat transfer in enclosures is an area that has particular significance to a wide range of applications. However natural convection heat transfer from enclosures with openings to the surroundings, such as open windows or doors in buildings, has received far less attention. In this regard, there is still a lack of generalised relationships that can be used in determining the heat transfer in partially open enclosures. The development of such relationships is particularly pertinent to the development of more accurate modelling tools that describe the thermal characteristics and indoor air quality in low energy buildings. As such, this work presents an investigation of the natural convection heat loss from a partly open room style enclosure, with a view to understanding the mechanism and also to developing relationships that describe it. It shows that heat transfer from the partly open enclosure is strongly influenced by both the Rayleigh number and also the opening size.