When I was an architectural technician, I used the spelling cill (e.g. window cill). I knew of one architect who used sill and stated that this was the older and more correct form. My Concise Oxford Dictionary gives both but is weighted more towards sill .
Also asked, what is a failed window seal?
Moisture Within the IGU: Fogging, hazing or moisture between the two panes of glass mean that a window seal has failed. While a window can certainly fail and not have any moisture between the two panes, it is more common to see moisture between the two panes.
What do you call the inside of a window?
For example, on a single-hung window, the piece you slide to open the window is the sash. The frame has three main parts: the sill, or horizontal strip along the bottom of the frame; the jamb, the vertical sides of the frame; and the head, the top horizontal strip on the frame. The sash also has several components.