Damp or condensation? How to tell which you've got
Most damp patches in English homes come from one of three places: moisture in the air settling on cold surfaces (condensation), rain getting through the walls or roof (penetrating damp), or water rising from the ground (rising damp). Where the patch is and when it gets worse will usually tell you which.
Getting this right matters because the fixes are completely different. Injecting a chemical damp course into a wall won't stop condensation, and a dehumidifier won't stop a leaking gutter.
The quick version
| What you see | Most likely cause |
|---|---|
| Water running down windows in the morning, black spots in corners, behind furniture and on bathroom ceilings | Condensation |
| A patch on an outside wall or ceiling that grows or darkens after heavy rain | Penetrating damp |
| A band of damp along the bottom of a ground floor wall, a tide mark with white salts, skirting boards rotting | Rising damp, or something that looks like it |
| A patch below a bathroom, around a pipe or under a sink, there all year | A plumbing leak |
Condensation
Warm air holds more water than cold air. Cooking, showers, drying washing indoors and simply breathing all add moisture. When that air touches something cold, such as a single-glazed window, an outside wall or the corner of a ceiling, the water comes out on the surface.
Signs that point to condensation:
- It's worst in winter, when windows stay shut and walls are coldest.
- Mould appears as black or grey spots, often in corners, behind wardrobes, around window frames and on the ceiling above a shower.
- It's in rooms where moisture is made, like the bathroom and kitchen, or rooms that are cold, like a spare bedroom kept unheated.
- Walls feel damp on the surface but the plaster underneath is sound.
Condensation is about air, heat and moisture, so the fixes are too: extractor fans that actually work, trickle vents left open, a little heat in every room, lids on pans, washing dried outside or in a vented space. The condensation and ventilation page covers what a specialist can add, such as better fans or a positive input ventilation unit.
Penetrating damp
This is water from outside getting into the structure. It usually comes through something that has failed: a cracked or blocked gutter, a leaking downpipe, worn mortar between bricks, cracked render, a slipped roof tile, or a window that's lost its seal.
- It tends to show as a patch rather than spots, and the patch can be at any height.
- It gets worse after heavy or driving rain, and may dry out in a long dry spell.
- It's on an outside wall, a chimney breast, or a ceiling under a roof or flat roof.
- Outside, directly behind the patch, you can often find the cause if you look.
Walk round the outside after rain and look at the gutters, pipes and brickwork behind the damp area. More on the penetrating damp page.
Rising damp
Water from the ground can rise up through brick and mortar if there's no damp proof course, or if the one there has failed or been bridged. Many older terraced houses in England were built before damp proof courses became normal, though plenty of them are dry.
- It only affects ground floor and basement walls.
- Damp sits in a band from the floor upwards, rarely higher than about a metre.
- There's often a tide mark at the top, with white, fluffy salts on the surface and plaster that crumbles or blows.
- Skirting boards and floor timbers next to the wall may be soft or rotten.
Before deciding it's rising damp, check outside. Soil, a path or a patio built up above the damp course will let water bypass it, and so will render taken down to the ground. Fixing that is often cheaper than any treatment. See rising damp.
A simple test you can try
Tape a square of kitchen foil, roughly 30cm across, tightly to the damp wall on all four sides. Leave it for a day or two, then peel it off.
- Wet on the room side of the foil: moisture from the air, so condensation.
- Wet on the wall side, or the wall under it is darker: moisture coming through or up the wall.
It's not a diagnosis, but it's free and it helps you describe the problem to a specialist.
When to get someone in
- The patch is growing, or plaster is crumbling, swelling or coming away.
- Timber is soft, or there's a mushroom smell, which can mean rot.
- You've dealt with ventilation and heating for a few weeks and nothing has changed.
- You're a landlord and a tenant has reported it. The landlords' page explains why you need a competent person to find the cause quickly.
On health, the NHS website now sends readers to the government's guidance on the health risks of damp and mould. It says some groups are more at risk, among them children, older people, pregnant women and people with asthma or other lung conditions. Worried about your own or a child's health? A GP or pharmacist can help.
Common questions
Can a house have condensation and damp at the same time?
Yes, and it's common. A wall that's already wet from a leak is colder, so moisture from the air condenses on it more readily. Fixing the leak and improving ventilation are often both needed.
Does black mould always mean condensation?
Usually, but not always. Black spotty mould grows wherever a surface stays damp, and condensation is the most common reason. It can also grow on a patch kept wet by a leak or by penetrating damp.
Will a dehumidifier fix it?
A dehumidifier helps with condensation because it takes moisture out of the air. It won't do much for water coming through a wall or up from the ground, because the source keeps topping it up.
Is the foil test reliable?
It's a rough guide, not a diagnosis. It can point you in the right direction, but a wall can show both results, and a specialist with a moisture meter and some experience will tell you more.
Should I get a damp survey?
If the signs don't clearly point to condensation, or you've tried the simple fixes for a few weeks and it's still there, a survey is worth having. Ask for one that finds the cause rather than one that comes with a treatment already attached. See damp and mould surveys.
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