How to Fix a Bowed Bottom Sash Rail

Install a piece of silicone weather stripping on the inside edge of the top rail of the bottom sash if there is a bow or if air is making its way between the top and bottom window panels when closed. Use your staple gun to fasten the stripping in place at the top edge of the sash, placing one staple every 2 inches or so. Use your hammer to drive the staples perfectly flat against the sash so they don't hang up on the upper sash during normal use.

Remove the existing center-mounted window lock with your screwdriver by turning its mounting screws counterclockwise until they release. Install two new window locks, at each end of the bottom sash, to replace the old single lock. Space the locks evenly and use your screwdriver and the hardware included to mount them. The tension of the new locks in their new positions will work against the bend in your sash to bring it back to a flush fit with the upper sash.

Inspect the underside of the bottom sash rail for a groove or indentation. This slot is a basic form of seal common to many modern windows, which allows for an air lock or sorts to take place when the window is closed. If air still passes through the gap between your sill and the warped bottom sash rail, try installing a piece of silicone weather stripping into the notch. Use stripping that's been designed for the purpose, with one tapered end and one wide end. The tapered end can be pressed into the existing groove with the end of your flat head screwdriver, leaving the wide end to block the passage of air. Close the window and test it.