© 2020 The Authors Thermal zoning is a commonly adopted building energy efficiency initiative, since thermally segregating conditioned spaces is generally expected to minimise energy losses when conditioning unoccupied spaces. When comparing a partitioned building with widely differing heat gains between zones to an equivalent non-partitioned building, ‘zoning’ might not always beneficial. This paper analyses the fundamental thermal processes involved in these scenarios by firstly undertaking a number of steady state analyses, demonstrating that there are scenarios where the thermal energy required to maintain comfort conditions is less for an open-plan arrangement than for a more highly partitioned building. We then performed dynamical simulations of a simple building, confirming the steady state analyses and showing that, for space heating, connecting the spaces can significantly reduce the energy demand. It was concluded that whenever two zones are both conditioned to the same set-points, thermally connecting zones always leads to an energy demand lower or equal to thermally isolated zones. We then conducted simulations of an archetypal residential building with intermittent conditioning of spaces. The results showed that thermally connecting the spaces can be beneficial in climates from cool to warm temperate, with a decrease in energy demand from 2.2 to 9.9%, while this was not beneficial in a hot and humid climate, with an energy demand increase of 0.2 to 2.3% for the thermally connected scenario.