If a brown patch appears in the middle of a wall or on an wall not in contact with the ground.
This will be a leaking pipe and will need immediate attention from a plumber to find the source. If you do not cure this problem ASAP then at worst a ceiling could collapse! In the meantime a dehumidifier will help to dry out the effected area.
Anything else is caused by condensation and lots more information can be found here
Once you have identified and dealt with the causes of the damp then the smell will go as well. It is vital to remove the cause as soon as possible because the mould spores that come with damp are damaging to your health.
Regardless of the cause of your damp problem it all comes back to an excess of moisture and a dehumidifier can either solve the issue completely or bring the moisture under control until you have an opportunity to bring the structural problem under control. Details of our domestic dehumidifiers for the home can be found here.
So to solve the problem you have three choices, one increase the temperature of the glass so that it is the same temperature as the wall, two reduce the air temperature of the room so that the difference between the window temperature and the air temperature is not so great and three reduce the amount of moisture in the air.
How to remove the moisture from the air?
This is what a dehumidifier does and this is what you want to achieve. Better still because the air is drier the air will feel warmer (no damp chill effect) and it will easier to heat (less water molecules to heat up).
Do I need a dehumidifier in every room?
No, one dehumidifier positioned centrally will do the job. Wherever you put the dehumidifier it will become the driest point in the house and the excess moisture from the different rooms will migrate towards the dehumidifier in order to balance out this inequality. The dehumidifier draws the air through itself and removes the excess moisture.