How To Fix a Window that Sticks or Won't Open
Double hung windows are windows having an upper and lower sash that slide up and down within a sash channel. Single hung windows have a similar appearance except that only the bottom sash moves. The window sashes can become difficult to operate for a variety of reasons, but repairs are often fairly straightforward.
First, lets identify the parts of the window. The window sashes run in the sash channels on the side jambs. The inner channel is between the interior stop and the parting strip, and the outer channel is bounded by the parting strip and the outer stop.
If a window will not open at all, the first thing to check is whether it has been painted shut. Using a razor knife, carefully cut the seam between the stop and the sash along the sides of the window and between the sash and the parting strip. Cut between the sash and the sill or top jamb. This may have to be down both from the inside and the outside. If the sash channels were painted, score the paint between the sash and side jamb this may require removal of the stop trim pieces. Now try to move the window. If it still won't budge, go to the next step.
When the sash binds in the sash channel, the stop, parting strip, jamb or the sash may be warped or swollen. Furthermore a build up of paint in the channel can also cause binding. To break the bond between the sash and the rest of the frame, you will need to force a putty knife into the seam between the sash and the stop. Slide the putty knife in as far as it will go, ideally all the way to the side jamb. Repeat this all the way up each side of the sash. You also need to repeat this on the outside of the window between the parting strip and the sash. Now try and move the window. If it still won't budge, it may be necessary to remove the stops and then remove the sash.
After you get the sash to move or you have taken it out completely, remove any built up layers of paint from the sash channel. If the wood is warped or swollen, sand or plane the stop, parting strip or side jamb as needed to allow for proper movement of the sash. Remove a small amount of material at a time. Alternatively, you may be able to remove the stop and reposition it to allow the sash to move smoothly.