There are many other areas besides doors and windows where leakages can take place including attic hatches, mail slots, pipes and wires, window frames, baseboards, fireplaces, air conditioning systems, and more. Typically, the attic will be the biggest problem because hot air rises but this leakage can be fixed by plugging the holes. If there are gaps where the attic walls meet the floor, you should find ways to seal these holes and this will stop leakages. Also, you can seal the attic hatch itself with weatherstripping. Even if you have pull down stairs for your attic, weatherstripping can be used to seal the gaps and prevent valuable heat and money from escaping.
Furthermore, darkened insulation means that the interior is relatively dusty and the air will leak through small holes. When the weather is cold, you may start to see frost on the insulation and this is where the hot and warm air collides. When the weather is warmer, the insulation can become damp for the same reason. If you see electrical wires or vent pipes, use caulk or expanding foam to seal the openings. When the caulk is dry, you can then cover the area with insulation and prevent air from escaping.