Damp in Older Houses: Causes and Genuine Fixes
Damp is diagnosed badly more often than almost any other building problem, largely because the companies diagnosing it also sell the treatment.
There are three distinct causes with three different remedies, and getting the diagnosis right determines whether the money you spend achieves anything.
Condensation
By far the most common, and the one most frequently misdiagnosed as rising damp. Warm moist air meets a cold surface and water forms on it.
The signs are mould in corners, behind furniture and around windows, worse in winter, worse in bedrooms and bathrooms, and generally at high level as much as low.
The fix is ventilation, heating and reducing moisture generation. Extractor fans that actually vent outside and run on a timer, trickle vents left open, drying washing outside where possible, lids on pans. Improving insulation raises surface temperatures and helps considerably.
This costs very little and no chemical treatment addresses it.
Penetrating damp
Water entering through a defect: failed pointing, a cracked render, a blocked gutter discharging down a wall, a leaking downpipe, defective flashing, or a windowsill without a drip groove.
The signs are localised patches that worsen after rain and correspond to something outside. Following the water backwards usually finds it.
The fix is repairing the defect. This is genuine building maintenance and it works permanently, unlike treating the symptom indoors.
Rising damp
Moisture drawn up through masonry from the ground by capillary action, where there is no damp proof course or it has been bridged.
It is real but considerably rarer than the volume of damp proofing sold would suggest. The classic sign is a tide mark at a consistent height above floor level, with salt deposits.
Before accepting a diagnosis, check the obvious bridges: ground level raised above the damp course by a new patio or flowerbed, render carried down past the course, or debris in a cavity. Correcting those is cheap and frequently resolves it.
On chemical injection
Injected damp proof courses are widely sold, often with a long guarantee that sounds reassuring. Independent building conservation bodies have long questioned how often they are necessary.
If a survey recommends injection, get a second opinion from an independent surveyor who does not sell treatment. The survey costs a few hundred and can save several thousand.
Be particularly cautious about replastering specifications that accompany injection, which are often the larger part of the cost.
Older buildings behave differently
Solid-wall buildings without cavities are designed to absorb and release moisture. Sealing them with impermeable modern renders, plasters and paints traps water and makes damp worse.
Lime-based materials allow the wall to breathe. Using cement render on a period property is a common and damaging mistake, and reversing it is expensive.
Sensible sequence
Rule out condensation first, since it is most likely and cheapest to address. Then inspect outside thoroughly in the rain, which shows you what a dry-day survey will not. Only then consider rising damp, and get independent advice before committing.
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