Damp: Identifying the Type Before Paying for a Cure
Damp is frequently misdiagnosed, and the wrong treatment is expensive and ineffective because the cause continues.
Condensation
By far the most common. Warm moist air meets a cold surface and water condenses on it.
Signs: black spotted mould, worst in corners, behind furniture, on cold external walls, around windows, and in bathrooms and bedrooms. Usually worse in winter.
Cause: moisture generated indoors combined with insufficient ventilation and cold surfaces.
Remedies: ventilation, particularly extraction during cooking and bathing; consistent low background heating rather than short intense bursts; moving furniture off external walls; and insulation to raise surface temperatures.
It is a building physics problem, not a defect requiring injection or treatment.
Penetrating damp
Water entering from outside through a defect.
Signs: localised patches that worsen after rain, frequently at any height rather than confined to the base of walls.
Causes: failed pointing, cracked render, blocked or leaking gutters, defective flashing, damaged roof coverings, or a bridged cavity.
Remedy: find and fix the external defect. Internal treatment without doing so hides the symptom while the structure continues to get wet.
Rising damp
Groundwater rising through masonry by capillary action.
Genuinely uncommon compared with how often it is diagnosed.
Signs: a tide mark typically no more than a metre above ground level, salt deposits, and it does not vary much with rainfall.
Frequently confused with a raised external ground level bridging the damp course, or with a blocked air brick, both of which are cheaper to fix than a chemical course.
Getting a diagnosis
An independent damp surveyor who does not sell treatment has no incentive to find rising damp.
Contractors offering a free survey are selling a product, and their diagnosis reflects that.
Electrical resistance meters read salts and other conductive material, not only moisture, so a high reading is not proof.
Check the obvious external causes yourself first: gutters, ground levels, air bricks and pointing. They explain a large proportion of damp problems and cost little to correct.
Why treating the wrong cause is expensive
Injected damp courses and internal tanking are sold as remedies and treat only one of the three causes.
Applied to condensation or penetrating damp, they fail, because the water continues to arrive.
Waterproof renders can make matters worse by trapping moisture in the wall and driving it higher.
The cost of doing nothing
Persistent damp damages plaster, decoration, joinery and eventually structure.
Mould has health implications, particularly for anyone with a respiratory condition or for young children.
Both the damage and the cost of correction rise with time, which is the argument for a proper diagnosis early rather than a cheap treatment now.
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