Which window type provides only a 50% opening to the total area when opened?

Horizontal sliding windows are designed to operate by sliding one pane horizontally over another, which results in only one half of the window opening at a time. This means that when the window is opened, only 50% of the total area is available for ventilation or passage, making them less effective in providing airflow compared to some other window types that can open fully. This functionality is particularly important in fire situations, as it can impact smoke evacuation and creating a flow path.

In contrast, casement windows can fully open outward, providing 100% of their area for airflow. Bay windows are typically comprised of multiple panes that may not open at all and generally provide more views than ventilation. Fixed windows, by definition, do not open at all, hence do not allow any airflow. Therefore, the unique feature of horizontal sliding windows is their limitation on opening, making them distinctive in how much area they allow to remain accessible when opened.