When deciding about replacing your windows, you need to give a couple things consideration. The first being, what shape are your existing frames in? Are they rotten, square, plumb and level?
If they are in good shape, you can consider a vinyl replacement window, sometimes called a pocket window. This will allow you to leave the interior trim and exterior siding intact. Unlike a new window, you are only removing the old sashes and inserting a replacement with sashes and a new jamb for the sashes to ride in.
Attach your new pocket window to the old jambs with screws through the new jambs and into the old. Use window expansion foam for any large cracks and caulk both the interior and exterior. Be prepared to add new stops and add some fresh paint for a quality looking job.
Often times if you plan ahead, you can get new windows made to fit your old window sizes. You will need to measure carefully so the new replacement fits perfectly. Your new window will need to be slightly undersized to allow room for making small adjustments.
A slight draw-back to replacement windows are, you normally end up with a slightly smaller glass size. Be careful if you are replacing a bedroom window that requires egress. Check to verify you're not getting smaller than the building code requires.
