The forces that cause otherwise well-made windows to leak fall into three categories: house design, installation, and maintenance.
One of the main reasons to ensure your windows are watertight is to keep moisture out of the walls. Once inside your walls, moisture can cause rot, mold and pest problems. Keeping water at bay would be easier, of course, if our homes had no openings for doors and windows. But of course, houses do have what those in the building trades refer to as penetrations. And wherever there is a penetration, there had better be extra protection. (For best results, consult the manufacturer’s instructions.)
Drafty windows and air infiltration could have several causes. Improper installation is often the culprit but not always. If the windows were improperly installed, the defects can normally be corrected without removing or replacing the window.
Proper caulking and flashing should prevent perimeter air leakage. However, if the installation twists or racks the window frame causing the sash weatherstrip not to be able to seal the sash to the frame, removal of the window to correct the problem may be necessary. You can determine whether the air is coming in at the joint between the frame and the house or the joint between the sash and the frame. Hold a lighted candle or lighter in front of each joint when the wind is blowing and slowly move it along the joint. If the flame flickers strongly or goes out, the joint is probably leaking.
If the leak is between the frame and the sash and the window has been installed square, plumb, level and is not twisted or racked, the problem may be the construction of the window or a window design. If your windows are not efficient in your climate and particularly if it does not have insulated glazing, the air may not be leakage but simply cold room air flowing down the surface of the window.
As the window surface becomes colder, it cools the room air next to it. This cooled room air is then heavier than the warmer room air further into the room. Because the cooled room air is heavier it sinks toward the floor and is replaced by warmer room air, which is then cooled. This process can result in noticeable airflow but it is not air leakage from the outside. Replacement of the window by an energy-efficient window is the best solution here.