Windows in Chicago sit in masonry and face lake wind with rain in it. Original pulley sash in brick bays whistle at the meeting rail and leak at the weight pocket. Limestone sills crack, and the joint under the stool lets water into plaster. A storm panel that does not latch turns a winter night into a draft you can hear. Newer units fog when the seal dies, which is a different complaint from a rotten brickmold.
East elevations take the lake. West elevations take the long winter wind and the summer storms that cross the city. A greystone bay has more joints than a flat opening, and the stain starts there. Paint alone will not stop water that is already behind the casing. If the plaster below the stool is bubbling, the opening is wet, not just drafty. Say which face of the building you are talking about.
Old brick openings are rarely a clean catalog size, and the wall is thick. Ask what happens to the stool, the apron, the exterior brickmold, and the storm. A replacement that ignores the masonry joint will leak in the first driving rain even if the glass is fine. Leaded or divided lites on a facade may be something you intend to keep. Say so before the scope is only about a white frame.
Condensation on the room side of a single pane is winter physics in a tight, humid apartment. Condensation between panes is a dead seal. Water stains that grow only during lake storms are leaks. Those three get mixed up on calls and they should not. If a sash is painted shut and a bedroom needs the opening, say that plainly. Cords, weights, and locks are still the right repair on a sound wood unit.
Name the building and the wall before you wander. Draft, leak, fog, or a sash that will not move is the point. Lake wind does not respect a stuck lock. Call the number on this page.