How do I tell what kind of damp I have?
Distinguishing condensation, penetrating damp and rising damp, since each has a completely different remedy.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 2 min
Short answer
Condensation is the most common by a wide margin — it appears on cold surfaces, in cold weather, with black mould. Penetrating damp follows rain and appears as patches on external walls. Rising damp is rare, stops at about a metre, and often shows a tide mark.
Misdiagnosis is expensive here. Condensation is a ventilation and heating problem costing very little; the treatments sold for rising damp are costly and frequently applied to walls that had condensation all along.
What you'll need
- Torch
- Moisture meter (optional)
- Hygrometer (optional)
Step by step
- Note where and when it appears.Cold external walls, window reveals, behind furniture, in winter, worse in the morning — that pattern is condensation.
- Look for black spotty mould.Black mould in corners and on window frames is almost always condensation. Other damp types produce staining and salts rather than that particular mould.
- Check whether it follows rain.A patch that darkens during and after heavy rain and dries out afterwards is penetrating damp — a defect in the external fabric.
- Look at the height of the damp.Rising damp stops at about a metre, where the water can rise no further, and often leaves a horizontal tide mark with salt deposits.
- Measure the humidity in the room.A cheap hygrometer. Consistently above 60% indoors means condensation is likely, and it identifies the cause immediately.
- Check ventilation before anything else.Extractor fans working, trickle vents open, air bricks unblocked. Modern draught-proofing without added ventilation is a common cause of a sudden condensation problem.
- Check the external fabric for penetrating damp.Overflowing gutters, cracked render, failed pointing, blocked weep holes, ground level above the damp course, or a blocked cavity.
- Be sceptical of a free damp survey from a company selling treatments.An independent surveyor with no product to sell will tell you if it is condensation. Rising damp is genuinely rare.
Tips
- Simple changes fix most condensation: extractor fans used properly, lids on pans, drying washing outside or in a ventilated room, and keeping background heating on.
- Leaving a 5cm gap behind furniture on external walls prevents the cold pockets where mould appears first.
- Damp always has a cause. Treating the surface — a mould spray, a coat of anti-damp paint — without fixing the cause guarantees it returns.
Common mistakes
- Treating condensation as rising damp — It leads to expensive and unnecessary chemical treatments while the actual ventilation problem continues.
- Sealing a house without adding ventilation — Draught-proofing plus new windows without trickle vents traps moisture indoors and causes a condensation problem that was not there before.
- Painting over mould — It grows back through the paint within weeks. The moisture source has to be removed first.
Questions people ask
How do I tell condensation from rising damp?
Condensation appears on cold surfaces at any height, in cold weather, usually with black spotty mould. Rising damp stops at about a metre with a tide mark and salt deposits, and is far less common.
Is rising damp common?
No — it is rare, and frequently diagnosed where the actual problem is condensation or penetrating damp. Get an opinion from a surveyor who is not selling a treatment.