If the French doors aren't hanging properly, you may have gaps that let in drafts. Inspect the doors to look for visible gaps, particularly where the two doors meet and along the sides where the doors are hinged to the frame. Look at the edges of the door to see if they are straight. Lifting at the knob helps determine if the hinges are loose. If the door moves up and down, tighten the screws in the hinges. Tightened hinges allow the doors to align better for fewer gaps. Some models of French doors have hinges that allow you to adjust the horizontal and vertical alignment of the door after it has been installed, too.
Weatherstripping is another way to fill the gaps in a French door to reduce drafts. You'll find different types of weatherstripping available at a home improvement store. Wrapped foam is a common type of weatherstripping. The strips come with either a wood or metal covering so you are able to choose a style that works best with your doors. These strips screw or nail to the edges of the French doors to seal the gap. Install weatherstripping on your French doors if they don't have it already. If the doors already have weatherstripping, inspect the current weatherstripping to ensure it isn't damaged or cracked. Replace old weatherstripping for maximum draft protection.
Door sweeps go along the bottom edge of French doors to reduce the amount of wind that comes in at that area. These metal pieces are screwed to the inside bottom edge of the door. When the door is closed, the door sweep should rest securely against the threshold, blocking air without interfering with the functionality of the door. Open and close the doors several times after installing the sweeps to ensure they still work properly.
A temporary solution to fixing drafts is to place items over the door to block the drafts. If you notice air seeping in along the bottom edge of the French doors, place a draft blocker at the edge. These long, narrow items are typically made from fabric and are flexible so you are able to press them right against the door. If you don't use the door often, hanging heavy drapes or a blanket over the door helps reduce and trap the wind that makes it inside.