Insert vs. full-frame window replacement compared
Same rough opening, different result: an insert keeps the old jambs (no flashing, but less glass — only if the frame is sound), while full-frame tears out to the studs for a new flanged, fully flashed window with the most glass.
What this diagram shows
Two windows in the same rough opening compared. An insert or pocket replacement nests a new window inside the existing, sound jambs: it keeps the old frame and trim, needs no flashing (just sealant and backer rod), is the fastest and least disruptive option, but shrinks the glass by about half an inch to an inch all around — and only works if the frame and sill are sound. A full-frame replacement tears out to the studs for a new nailing-flange window with full sill-pan, jamb, and head flashing and the biggest glass; it is the path when the frame or sill is rotted or out of square. New-construction windows install the same way as full-frame.
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