Why are my windows draughty and will new windows stop the draught?

Draughty windows and air infiltration could have several causes. Improper installation is often the culprit but not always. If windows are improperly installed, the defects can normally be corrected without removing or replacing the window. Proper sealing around the window 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 by holding 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.