Ideally there should be openable high, medium and low elements, though two elements - upper and lower - are usually simpler and more affordable.
The lower sash should be the major daytime component. That is, it would be adjusted by occupants frequently, on demand, mostly for localised fresh air because this is how people normally perceive and use one.
The upper sash should be the major night-time component, with the capability to be either motorised or manually operated, or preferably both. This would be adjusted by occupants on demand during the day, and operate under automatic, semi-automatic or manual control during the night and other unoccupied periods.
Assuming the windows are restrained while open, they will be relatively secure, especially if only the upper part is open.
The system could also be calibrated locally so that some floor areas are more fully-ventilated and/or cooled than others.
Sometimes acoustic screens and acoustic treatment may be required at openings.
Refinements for control of solar gain, glare and noise should not be part of the basic window system but considered as context-dependent add- ons. Design problems are often difficult to solve with one universal technology; problems can be highly localised, for example addressing the effects on people who do not sit directly next to a window but are remotely affected by glare or draughts.
The system should be compatible with standard security and cleaning procedures because those responsible often set the on/off states of the building for subsequent daytime operation.
The window system should allow window-opening under automatic building management system-type control in response to outside conditions. Automatic opening and closing should normally happen when the building is unoccupied. Occupants often object to automated operation when they are present, particularly of elements which are close to them.
All automatic operations should be capable of being over-ridden locally.
The window system should not confuse occupants as to how it should be set. For instance, at night after a hot summer's day, occupants should expect that upper windows will be relatively wide open and in a cooling mode. In the morning, they will probably partially close the windows to obtain the best conditions. On cooler summer nights, the windows may be in a partially open ventilating mode and occupants will expect to open windows progressively during the day.
Window-opening should not conflict with the operation of blinds, which are often needed in office buildings to avoid vdu glare.
The inertial night-time state of the blinds - internal or external - should be up in warm weather, but perhaps down in cold weather when heat may need to be retained (though there is a danger that people entering in the morning will switch on lights rather than raise the blinds).
The system should be designed to encourage a change in habitual behaviour of occupants so that default settings always favour the optimum use of outside conditions. This means that the defaults will need to change from one set of circumstances to another. Default options will differ depending on the complexity of the control technology of the building skin.