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Picture of How to Recaulk Window Panes
Over time, wood dries out and shrinks. If you have wooden window frames in an older house, the wood might have shrunk enough to pull away from the caulking that seals in the window panes. Even if you have metal or vinyl windows, caulking snafus can happen, and the caulk itself could be old or misapplied.

Aside from letting the warm air out of your home, old or bad caulk can cause paint to peel and can let moisture into the window frame, which can lead to wood rot.

Recaulking an old window isn’t difficult. You’ll need a caulking gun loaded with the right kind of sealer (ask an employee at your local hardware store for a recommendation), a putty knife, some paper towels or rags that you can pitch afterward and a steady trigger finger.

Source: Cure for the Common Cold Room

Photo via Dean in SF

Step 1: Clear out old caulk.

Picture of Clear out old caulk.
Use the putty knife to clean out any old caulk from around the window or frame. You want to start with a work surface that’s empty and fairly clean.

Photo via Wikipedia Commons
 

Just so everyone is clear, you should use glazing putty between the window glass and the window frame, if the frame is wood. Caulk is not a good option as it doesn't leave as clean of a seam and will wear out far sooner than the glazing putty will. I'm currently reglazing some windows that were installed in 1927, so if you do it right, it should last at least that long.