The aluminium threshold on your doorway can retain moisture and condensation, causing rust and mould to accumulate. This can lead to rotting and deterioration of the door's wooden sill and even the subfloor adjacent to the threshold. The best way to correct the problem is to remove the old aluminium threshold and replace it with a thermal threshold composed of both aluminium and vinyl that protects the threshold from condensation. For added protection from condensation, install a strip of threshold tape on the wooden sill before installing your new aluminium thermal threshold.
Remove the old aluminium threshold on your door. Unscrew the screws holding the aluminium threshold to the door frame with a screwdriver. They are usually located underneath weather stripping that sits on top of the threshold. Lift off the old aluminium threshold.
Inspect the wood frame beneath where the threshold was and look for water damage or wood rot. If you find any, call a professional contractor to repair it.
Cut a piece of threshold tape with a utility knife the length of the threshold plus 2 inches so you have 1 inch extra on each side.
Peel off the protective backing on the threshold tape and press it to the wooden door sill. Trim the excess off on either end with the utility knife.
Install your new thermal threshold by screwing it into place with the screwdriver and screws where they were previously located. Follow the manufacturer's specific instructions for installation. Be sure to position your new threshold so it angles away from your house---this way water and moisture can run off.
Apply a bead of silicone caulk with a caulking gun where the threshold meets the door jambs.