Quick comparison
| Condensation | Penetrating damp | Rising damp | |
|---|---|---|---|
| Where | Cold corners, windows, behind furniture, ceilings | Any height on external walls, near windows, chimneys | Bottom metre of ground floor walls |
| When worse | Winter, mornings | After rain | All year |
| Looks like | Black mould spots, wet windows | Damp patches that spread, blown plaster | Tide mark, white salts, crumbling plaster |
| Usual fix | Ventilation, heating, mould treatment | Outside repair: gutters, pointing, render | Remove bridging, damp proof course, replaster |
The foil test
Tape a square of kitchen foil, about 30cm across, tightly onto the damp area and leave it for 24 to 48 hours. If moisture forms on the room side of the foil, it is condensation. If the wall side is wet, moisture is coming from inside the wall, which points to penetrating or rising damp, or a leak.
Watch the weather
Note when the patch gets worse. If it grows after heavy rain, think penetrating damp. If it is worse on cold mornings and after showers and cooking, think condensation. If it stays the same all year and sits low on the wall with a tide mark, think rising damp.
Check the outside
Stand outside and look at the wall behind the damp patch. Overflowing gutters, cracked render, missing pointing, a leaking downpipe or soil piled up against the wall are all clues. Penetrating damp nearly always has a visible cause outside.
When to get a survey
If you have been quoted for damp proofing, if the tests are unclear, or if you are buying a property with damp flagged, a damp survey with proper readings is worth it before you spend more.
Questions people ask
Can you have condensation and damp at the same time?
Last reviewed 15 November 2026 by the Home Damp and Mould team. Read our editorial policy.