Almost every wasted pound spent on damp goes on treating the wrong mechanism, so the diagnosis matters far more than the product.
Telling the three types apart
Penetrating damp appears after rain, is worst on the exposed elevation, and often traces back to a defective gutter, a cracked render panel or a failed pointing joint. Condensation shows on cold surfaces and in corners, is worst in winter, and comes with mould rather than salt staining.
Rising damp is far rarer than the marketing suggests. It shows a tide line at a consistent height with salts in the plaster, and it can only be confirmed once the other two are ruled out.
What usually needs doing
Rainwater goods and ground levels come first, every time. A blocked gutter or a path bridging the wall base causes more damp in Kent housing stock than any failure of a damp course.
Then breathability: cement render and plastic paint on a solid wall trap moisture behind them. Removing them and going back with lime render and a mineral paint lets the wall work as it was designed to. Internally, lime plaster and better ventilation do more than any injected chemical.
Work we would not recommend
Injected damp-proof courses into a wall that has no rising damp. Tanking a room that has a condensation problem. Waterproof cement render on a soft brick or ragstone elevation. Each of these can move the moisture rather than remove it.
If we can't find a mechanism that explains what you're seeing, we'll say so and suggest monitoring rather than sell a treatment.
Common questions
Do I need an injected damp course?
Rarely. Injection only addresses true rising damp, which is much less common than penetrating damp and condensation in older properties.
Why did damp appear after my walls were rendered?
A dense cement render stops a solid wall drying outward. Moisture that used to evaporate through the face now moves inward and shows up internally.
Is lime plaster worth the extra?
On a solid wall, yes. It lets moisture move and dry rather than trapping it behind a gypsum skin, and it handles the salts that come with older masonry.
Will better heating solve condensation?
Heating helps, but only with ventilation. Warm damp air with nowhere to go still condenses on the coldest surface in the room.