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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.

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  • The rough opening is the maximum size. A window should about ~1/2 inch smaller than the rough opening, so there is no compression/tightest on the window/glass. A replacement window should measure the same as outside dimensions of the window frame or smaller(use filler pieces). Trim and maybe sills are use to hide/make neat the space between the outside window frame and the wall. They should be remove temporary to measure.
    – crip659
    Commented Jul 29, 2023 at 23:11

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"Replacement windows" typically go inside the frame of the window being replaced. That makes the window clear area smaller, but avoids needing to rip off the siding to redo the flashing. Not a good idea if the original window frames are themselves leaking, rotted, etc.

If you are ripping out the old windows entirely and doing the work needed to flash the new windows and replace siding, then you get "new windows" to suit the rough opening.

You should be able to measure the rough opening by removing (only) the interior trim (temporarily if you expect a long delay between measuring and new windows arriving.) You may well be able to find "stock size" new windows that suit your rough openings by shopping around, rather than assuming you have to go custom. You can shim the rough opening to fit pretty much any window smaller than it, with adequate attention to detail (and trim to suit.)

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    If your windows are double hung and old, then they've got a massive weight pocket on each side. Whether you opt for "replacement" inside the existing frame or maximize your "new" size is up to you (trim inside and outside are real concerns), but if you go "replacement", be sure to insulate the weight pocket. (I've seen hack work where they just cut the cords and run.) Commented Jul 30, 2023 at 0:26

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