Condensation is worse in cold weather and appears on cold surfaces. Penetrating damp is worse after rain and is usually a localised patch. Rising damp does not change much either way and is genuinely rare.
- Condensation. By far the most common. Warm moist air meets a cold surface and the water comes out of the air onto the wall or the glass. Worst in winter, worst behind furniture on external walls, worst in bedrooms and bathrooms. The fix is ventilation, heating and insulation.
- Penetrating damp. Water getting in from outside: a failed gutter, cracked render, a blocked air brick, a slipped tile. Usually a defined patch, and it gets visibly worse after rain.
- Rising damp. Water drawn up from the ground through the masonry. Shows as a tide mark up to about a metre with salt deposits. It is much rarer than the industry selling treatment for it suggests.
The timing is what separates them. Keep a note of when the damp appears and what the weather was doing, because that record is the diagnosis.
If somebody offers a free damp survey and then diagnoses rising damp and sells you injection treatment, get a second opinion from an independent surveyor who does not sell the remedy. This is the single most common expensive mistake in British housing.
All three look similar on a wall, which is why misdiagnosis is so common, but they have nothing in common underneath. One is an air problem, one is a building fabric problem and one is a ground water problem.
Household chemicals can be dangerous in combination. Never mix them, keep the room ventilated, and test anything on a hidden patch first. Full disclaimer.