The most common problem with double-hung windows is that they become stuck in their tracks.
This can happen because of high humidity that swells the wood, accumulated grime that blocks the tracks, or repeated painting that seals together the surfaces.
» Double-Hung Window Is Temporarily Stuck
If a sash is temporarily stuck because of high humidity, simply waiting for the weather to change may solve the problem. If a sash moves reluctantly, clean the sash channels.
If the channel is squeezing against the window, widen the channel by inserting a wood block where it binds and tapping it with a hammer.
If grime is blocking the channel, use a sharp wood chisel to remove the blockage. Then, using medium sandpaper wrapped around a wood block, sand the channel. Finally, lubricate the channel with a coating of wax.
If you can’t budge the window, use a utility knife to cut the painted edges of the sash. Then work a 3-inch-wide putty knife between the sash and frame and tap it with a mallet.
From outside, wedge a prybar between the sill and sash; work alternately at each corner so the sash moves up evenly. Protect the sill with a wood block.
» Double-Hung Window Is Painted Shut
If you have a double-hung window that has been painted shut, you can usually get it working again by cutting through the paint that is sealing the sash around its perimeter. Try this:
Use a utility knife—or, better yet, a special little tool that looks like a serrated metal spatula called a window saw to cut through the paint all around the movable sash. Be careful not to leave any areas uncut both inside and out and also take care not to damage the painted surfaces.
Make sure the window is unlocked. Also make sure that there are no nails or screws securing the sash to the jamb; if there are, remove them. Then try to force the window open with abrupt force, using the heels of your hands.
If that doesn’t work, try to free the window with a wood block and a hammer or mallet, but be careful not to jar the window so hard that you break the glass! Once you get the window open, brush away the loose paint.