Your windows can become stuck for many different reasons. Sometimes the wood can expand or contract, a moving part may get painted over, or sometimes they are fused together. Here are some of the most common problems solved for stuck windows:

If the joint is painted over then you can cut the paint with a window zipper. This tool is designed to do exactly that. You can also use a putty knife to accomplish this task. Try holding the blade against the sash and then push the edge into the joint while you also draw the tool over the surface at the same time.

If paint has built up than that can cause a lot of friction to occur. Use a paint scraper to get rid of the old paint from the window stop, blind stop, and parting strip. If you lower and raise the sash while doing this it helps. If the sash is low then you might want to take the window stop out and sand and scrap the edges of the window. If you can't do any of these things you might have to do something more complicated. This means you need to take out the sashes and strip all the paint off the wood. Afterward you can repaint the sashes and then install them again when the paint is dried.

If there is too much friction you can lubricate the sash channels. Use candle wax or baby powder for this job. This will also help the surfaces that are painted from sticking together. If you need to reduce tension caused by spring metal weather stripping in the sash channels than just use a hammer and a wooden block to flatten them out.

If it is stuck you can try to whack it on the center rail. Do this near the place where the window locks. This can break a bond between the painted pieces and allow you to open the window up. Use your hand to do this task, but if doesn't work then try a mallet. A rubber one is best. You can also try to tap a block of wood against each side of the sash as well.

If you are worried about preventing heat loss, then you can install friction channels. If you want to do this then take out the sashes, weights, and pulleys. Use fiberglass insulation into the openings of the weigh cavities. Work your way down from the top with a rod or stick. Create new channels using a hammer and chisel. Follow these tips and never find a window you can't open again!