The short version
An insert window slides into the old frame. A full-frame replacement takes everything out down to the opening and starts fresh.
When an insert makes sense
When the old frame is solid, square and dry, and you want to keep the trim and plaster inside. It is quicker and disturbs less.
When full-frame is better
When the sill is soft, the frame is out of square or there are water stains below the window. Taking it all out lets the opening be repaired properly.
If the siding is coming off too
Doing windows and siding together lets each window be sealed into the new wall the right way.


