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.