We can easily attribute 25% of all heat loss to poor quality windows. Consider these three easy solutions:
Seal vertical slider windows with « V » or tubular shaped weatherstripping. Also weatherstrip around the frame.
Transparent film that shrinks with the heat of a hairdryer considerably increases the window's air tightness and also increases its thermal resistance. The film helps prevent frost from forming and leaves the window nice and clear.
If you don't use your patio door in the winter, cover it with a sheet of shrink film. Removable sealing products are also available for patio doors.