A leaking windowsill can result in all kinds of harm to your home if it you don't take care of it quickly. The moisture can work its way down through the windowsill and into the walls or your home, causing water damage and mould. If you notice a puddle or dampness in or around a windowsill after rain, you should act quickly to seek out the cause of the water and take steps to fix the problem.
Scrape off old caulking around the window with a putty knife, and apply a new bead of waterproof caulking. Smooth the caulk to press it to the window.
Look for gaps on the outside of the windowsill where water can seep in. Cut rubber-coated insulation strips to fit the gaps, and slide them in place.
Look on the outside of the window around the frame, and locate gaps between the window and wall. Seal the gaps with a clear silicone caulk. Run a bead of caulk along the gaps, and spread it smooth with your fingers.
Seal gaps between the upper and lower window panes at the point where they slide past each other. Slip rubber-coated insulation strips in the gap between the panes. Leaks won't really occur here, but cold air seeping in through the gap could create condensation dripping down into the sill.