I'm a little unclear on how to measure windows for replacement. Most reading I've found says to measure the opening for the sash— sill to head, and jamb to jamb.
My questions are:
Why isn't it the rough opening size that matters? It seems like the window could have any size of frame / any amount of material between the window opening and the frame of the house? How can I be sure that by measuring the opening, I'll receive a window that will fit into the rough opening? (The windows I'm replacing are quite old; likely >100 yrs)
If it is the rough opening size that matters, how should this be measured? It is not ideal to tear out walls and/or remove the window to access the framing, and then have to wait in this state for weeks for the replacement window to arrive!
If it's indeed the window opening that should be measured, there are still several places where I could choose to measure— In a double-hung window, the track where the window rides is inset from the trim by 1/4-1/2 inch. I would assume it's this wider dimension I want, but I'm not positive. Similarly, the window rests about 1" below the interior sill. Again I would assume the longer height dimension, but want to be sure.