How do I reduce damp in a house?
How to tell condensation, penetrating damp and rising damp apart, and what actually fixes each.
- Difficulty
- beginner
- Time
- 2 hr
- Read
- 2 min
Short answer
Identify which of the three types you have first: condensation appears on cold surfaces and is worst in winter; penetrating damp is a patch that worsens after rain; rising damp is a tide mark low on a wall. Condensation is a ventilation fix, the other two need the water source stopped.
Damp is three different problems that look similar and have almost nothing in common. Treating one as another wastes money and time — a dehumidifier does nothing for a leaking gutter, and repointing does nothing for a bathroom with no extractor. The diagnosis matters more than the remedy.
Step by step
- Note where the damp is and when it's worst.High up and worst in winter: condensation. Localised patch that worsens after rain: penetrating. Low on the wall with a tide mark: possibly rising.
- For condensation, deal with moisture production, ventilation and surface temperature.Extractors, lids on pans, laundry outside, steady background heat. See the condensation guide.
- For penetrating damp, go outside and look at what's above the patch.Blocked or leaking gutters, cracked render, failed pointing, a slipped tile, a blocked weep hole or a raised path bridging the damp course.
- Clear gutters and downpipes and check them during actual rain.A gutter that overflows only in heavy rain is invisible on a dry day and is a very common cause.
- Check that external ground level is at least 150mm below the damp-proof course.Raised patios and flowerbeds bridging the DPC are a frequent cause of what looks like rising damp.
- For a suspected rising damp problem, get an independent assessment before paying for treatment.True rising damp is far less common than it's diagnosed. Many 'rising damp' surveys are done by firms selling the treatment.
- Once the source is fixed, let the wall dry out fully before redecorating.Masonry takes months to dry. Decorating too early traps moisture and the problem reappears through the new paint.
Tips
- A cheap moisture meter gives a useful relative picture across a wall, but the readings are easily misled by salts in plaster — treat them as a comparison, not a verdict.
- Photograph damp patches with a date and a ruler for scale. Whether a patch is growing is the single most useful piece of evidence.
- Damp that appears in only one room and only in winter is almost always condensation, whatever anyone selling injection damp-proofing tells you.
- Blocked or painted-over air bricks are a common cause of damp in suspended timber floors.
Common mistakes
- Treating condensation as rising damp — Expensive, invasive, and doesn't address the actual cause.
- Redecorating before the wall has dried — Traps moisture and the stain comes straight back through the new paint.
- Sealing the wall with waterproof paint — It moves the problem rather than solving it, often pushing damp further along the wall.
Questions people ask
How do I tell rising damp from condensation?
Rising damp shows as a tide mark up to about a metre from the floor, is present year-round, and often leaves salt deposits. Condensation appears on cold surfaces, is worst in winter and in unventilated rooms, and often shows as mould rather than a stain.
Do dehumidifiers fix damp?
They manage condensation and help a wall dry after a leak is fixed. They do nothing about the cause of penetrating or rising damp.