The design requirements for a shading device depend entirely on a building’s use and local climatic conditions. In a multi storey open plan office building, the occupancy and equipment gains are such that heating is rarely required. In this situation, to avoid unnecessary loads, shading may be designed to completely protect the windows all year-round.
In a domestic building or one that is occupied 24 hours, the release of stored heat during cold nights in winter can be important. In this case, the shading would be designed to fully protect the windows during the summer months, but to expose them as much as possible to direct sun in winter so that they have a chance to absorb heat during the day. In climates where summers are also relatively cold, the requirement may be to allow full solar access all year round.
If you look at outdoor air temperature and the intensity of solar radiation at different times of the year for Perth, Western Australia, it is clear that the transition to colder weather really begins in mid to late March. Thus, in order to take advantage of solar heating, the transition from shaded to exposed should really begin at the same time. This means that the window should remain completely shaded up until mid to late March, with maximum exposure occurring at the winter solstice in mid June.
A convenient date, by happenstance, is the 21st of March. This has the advantage of being the autumn equinox. One characteristic of the equinox is that, for a north-facing wall, the VSA is exactly the same throughout the day. This is an important piece of information as, in summer, the lowest daily VSA occurs at noon, whereas in winter noon sees the highest VSA.
Thus, if the cut-off date for a north-facing shade occurs on or before the autumn equinox, its depth will be defined by the noon VSA. If the cut off date occurs after the 21st of March, the VSA at either the start or end times will determine its depth.