There are two main causes of damp in chimneys: condensation and ingress of rain.

Condensation Moisture which condenses can cause a problem in flues that remain in use. The burning of fuel produces water vapour. If the flue is very tall, wide or particularly cold, the flue gases may cool to the point where the moisture vapour condenses within the flue. This happens particularly when wet fuels such as freshly cut timber are used. Condensation-related damp can usually be reduced by introducing a flue liner, as this both adds some insulation to the flue and reduces its surface area.

Ingress of rain

There are three likely routes by which rain can enter the structure: simply down the flue and into the building; around defective flashings between the chimney and the roof; or through the wall of the chimney stack itself where the fabric is too thin or too porous to prevent penetrating rain from getting around the flashings.

Rain can usually be prevented from coming down the flue by introducing a fairly discreet capping. Types are available for flues which are no longer in use (providing ventilation only) and for flues still in use. If it is still in use, the draught may be affected by the capping, causing the fire to smoke, so some experimentation may be required. When a flue has been relined, rainwater which had previously been soaked up by the old parging may run down the new flue as if it were a drainpipe. Rainwater in the fireplace may be a problem after a flue has been relined.

Where flashings are defective, these should be thoroughly inspected and repaired in accordance with usual good practice and the recommendations of the Lead Sheet Association.

Damp can cause problems whether the flue is in use or redundant. Moisture will combine with sulphates deposited inside the flue as a result of combustion to form weak acids. These acids attack the lime in the parging and the mortar joints, as well as the brickwork itself. They also form hygroscopic salts (salts which attract atmospheric moisture).

In a well-ventilated flue, warm air rises constantly, drying out the chimney. Moisture levels increase rapidly in a redundant flue if either the fireplace or the chimney is sealed. It is therefore essential to maintain ventilation to all flues, including redundant ones in particular, top and bottom.

Often damp will penetrate the brickwork to form tarry stains on plaster and wallpaper and often adjacent ceiling plaster inside the building. Remedial treatment depends on the severity of the staining. All chimneys should be swept and those which are not longer used capped or cowled and ventilated at both stack and fireplace level. Those which are to remain in use should have the flue lined. Small patches of damp can be treated with aluminium primer sealer, but if there are large areas of damaged plaster it will be necessary to remove the plaster from the chimney breast and to allow the wall to dry out as much as possible before and to replaster in accordance with the APP specification but using a sulphate resisting Portland cement. If staining is severe a plaster lath e.g Deltalath should be used to isolate plaster from the masonry.

Further information

A very good article on chimneys.
http://www.buildingconservation.com/articles/services/chimney.htm

National Association of Chimney Sweeps with links to other chimney trade associations.
http://www.chimneyworks.co.uk/index.html

Details for flashings etc
http://www.leadsheetassociation.org.uk

Inserts for redundant flues

http://www.hepworthterracotta.co.uk/zone_terracotta/default.asp

http://www.wtknowles.co.uk/products/chimneypots/

http://www.thechimneypotshop.com/index.htm