This article was published in Solar Energy (http://www.ises.org/shortcut.nsf/to/sej), the official refereed journal for the International Solar Energy Society. It was based on Southall’s experimental work carried out at the Energy Monitoring Company’s test cell at Cranfield University, UK, which was funded by McEvoy's EPSRC project. The paper brought together information from previous modelling and experimental work to describe in detail the solar gain factor and window pre-heat as a function of incident solar radiation, and the U-value and window pre-heat as a function of window size. An innovative research methodology was employed in this instance by installing a supply window air window in an advanced outside test cell, and exposing it to all environmental parameters. Also, the advanced nature of the test cell allowed indirect measurement of the window heat flows so no instrumentation on the window itself could disturb the air flow/heat transfer within the window. Experimentally this work was significant and original in two main ways: • The impact of size and aspect ratio of the window on performance was determined for the first time in an experimental set-up. • It was shown that when the solar gain factor was combined with the ventilation pre-heat the overall solar gain factor was significantly higher, more accurately demonstrating the energy benefits of the ventilated window. Theoretically the article was novel in that it provided for the first time mathematical descriptors for the relationship between U-Value/pre-heat and window area, allowing a simpler calculation route for the performance of the window.