Damp patch on my bedroom chimney breast which has worsened with the re

Chris Plastow

If the chimney is not properly ventilated, it will become damp from condensation, particularly in cold weather or during rain. It is caused by a chemical called ammonium sulphate, which is hygroscopic. If the chimney is correctly ventilated, there is rain water penetration, down the flue or through the brickwork.