If you do not think the damp comes from any of these causes, it is probably condensation. There is always some moisture in the air, even if you cannot see it. If the air gets colder it cannot hold all the moisture and tiny drops of water appear. This is condensation. You notice it when you see your breath on a cold day, or when the mirror mists over when you have a bath.
Condensation occurs mainly during cold weather, whether it is raining or dry.
It does not leave a 'tidemark'. It appears on cold surfaces and in places where there is little movement of air. Look for it in corners, on or near windows, in or behind wardrobes and cupboards. It often forms on north-facing walls. These four steps will help you reduce the condensation in your home.