The most common window used in home construction, the double-hung window is also the most difficult to seal with its two sliding sashes.

Newer windows have built-in weatherstripping that creates an excellent seal; older windows often need attention, however.

Installing spring bronze nail-on strips is the best way to weatherstrip wood windows. To form a complete seal, insert metal strips between the sashes and the jambs, and attach them to the top of the upper sash, the bottom of the lower sash, and to the bottom face of the upper sash.

Prepare the Window Before applying weatherstripping, prepare the window by scraping away loose paint or old weatherstripping. With double-hung windows, it's also a good idea to check the sash lock to make sure it's pulling the upper and lower sashes together to form a tight seal when closed. If it's not, you can correct this by repositioning one or both halves of the sash lock.
Attach the Vertical Strips Measure the side channels for both sashes and cut bronze spring strips to length with a tin snips. Slide the strips up between the sash and jamb so the nailing flange butts up against the sash stop. Push the strip up until it hits the top of the window or the pulley. If your window has pulleys, cut the strip into two pieces so the bottom strip is flush with the bottom of the pulley. Then cut a shorter strip to fit above the pulley. Attach the strips to the jambs with brads. If you notice a strip is not being compressed as the sash closes, you can increase the tension on the strip by slipping a putty knife under the open end and prying gently upward. Do this a little at a time until the strip compresses without causing binding.
Attach the Horizontal Strips Measure for the horizontal strips--three in all--and cut them to length so they extend the full width of the window. Attach one piece to the bottom of the lower sash so that the nailing flange is flush with the inside edge of the window. Hammer gently to prevent cracking the window. Attach the second piece to the top of the upper sash. The third piece is attached to the bottom face of the upper sash to create a seal between the sashes when the window is closed. Adjust the tension on any of these strips as you did for the vertical pieces.