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First, determine if any air is flowing through your closed windows. A great way to do this is to hold a lit candle close to the window seams. If the flame bends, then it could be signs of a draft.
Also check the caulk on the outside window frame. Exterior caulk can dry out in the heat of summer. If you find a whole lot of gaps and cracks, it's time to re-caulk.
Use caulk softener to help you remove the old caulking. It should be applied at least two hours in advance to give it a chance to work. Once the old caulking becomes soft enough, it should come up fairly easily with a putty knife or five-in-one tool. You must remove as much of the old caulk as possible to ensure that the new caulking will adhere properly and give the window a good seal.
Polyurethane caulk works for both small and large gaps, so one tube should take care of a whole window. To begin, cut the tip off of the tube of caulk at a 45-degree angle so that the tip will fit nicely into the window seam, and load it into a caulking gun.
Clean the surface as best as you can and make sure there are no traces of old caulking still left. Push the caulk along the seam in a smooth motion (Image 1). When filling larger gaps, move more slowly to let the caulk adequately fill the space. Finally, use a wet finger to smooth out the caulk and give it a clean, finished look (Image 2).
Give the caulking 12-15 hours to dry and set, and your windows should be airtight for the season.