Damp in an Old House: Diagnose Before You Spend

Damp is the most commonly misdiagnosed problem in older houses, largely because the free survey is usually performed by the company selling the treatment.

The three kinds

Condensation. By far the most common. Appears in corners, behind furniture on external walls, around windows, as black spotted mould. Worse in winter, worse in bedrooms and bathrooms. Caused by moisture-laden air meeting a cold surface.

Penetrating damp. A defined patch that gets worse after rain. Caused by a failed gutter, cracked render, a bridged cavity, defective pointing or a leaking pipe. The patch is usually roughly where the defect is, though water can travel.

Rising damp. Genuinely exists, and is genuinely rare. Shows as a tide line up to about a metre, sometimes with salt deposits, at the base of a wall with a failed or absent damp-proof course. Before accepting this diagnosis, check the external ground level — a raised patio or a flowerbed above the DPC bridges it, and lowering the ground is the fix.

Diagnose it yourself first

• Tape a square of clear plastic tightly to the damp patch and leave it 48 hours. Moisture on the room side of the plastic means condensation; moisture on the wall side means water coming through the wall.
• Check the gutters in heavy rain. Stand outside with an umbrella and look. Overflowing gutters cause an enormous share of "mystery damp".
• Check external ground levels against the DPC line — it should be at least 150 mm above the ground.
• Look for a leak: under baths, at waste pipes, at the boiler condensate.

Fixing condensation

Ventilate, heat gently and consistently, insulate cold spots. In practice: extractor fans that actually vent outside and run on a timer, ten minutes of window opening morning and evening, do not dry washing on radiators in a closed room, and keep furniture 50 mm off external walls so air can move.

Before injecting anything

Chemical DPC injection is expensive, disruptive and frequently sold for problems it cannot solve. Get an independent damp survey from someone who does not carry out the remedial work. It costs a few hundred and it regularly saves thousands.