Various contaminants can build up in masonry walls which eventually become hygroscopic, i.e. they attract and hold damp. This is especially true of chimneys where contaminants from the combustion process are produced in volume. Combining this with the reduced use of fireplaces can lead to chimneys becoming permanently damp. Eradication of this type of damp is problematic and often requires the use of wall membranes. In all cases attention must be paid to keeping the chimney open, ventilated, regularly swept, and clear of debris.
Hygroscopic damp can be caused by long term rising damp drawing salts up from the ground and depositing them in masonry walls. These salts must be removed and/or neutralised, often through removal and replacement of contaminated plaster.