Short answer: condensation is moisture from indoor air settling on cold surfaces. A leak, or penetrating damp, is water getting in from outside or from plumbing. Where the damp is, and what it follows, usually tells you which. Damp that follows rain suggests an outside defect, but condensation is usually worse in rainy weather too, so rain alone does not settle it. Damp that follows cooking, bathing or drying washing, on the coldest surfaces, points to condensation.
The quick comparison
- Condensation: droplets on window glass, cold pipework and plaster beside windows. Mould tends to form around windows, on external walls and on ceilings near external walls. Common causes are drying clothes indoors and not ventilating when cooking or bathing.
- Penetrating damp or a leak: water coming through an external defect or from plumbing. Moisture is generally fairly uniform and gets stronger towards the point where water gets in, and it does not usually carry salts unless they were already in the structure.
- Rising damp: a horizontal tide-mark with salts and discolouration below it, which can reach 900mm or more. It is often misdiagnosed. A failed original damp-proof course is described as extremely rare.
Signs of condensation
Condensation forms when a surface is at or below the dew point. Window glass is the obvious place, but cold external walls, corners and ceilings near external walls are affected too. Porous surfaces such as plaster absorb it rather than showing droplets, so the material can stay damp without anything running down it.
It is often worse in cold weather, when surfaces are colder and indoor humidity is high. Ask what is going on in the home: how many people live there, when the heating is on, whether washing is dried indoors and how well rooms are ventilated. Moisture sources are often not visible from outside.
Signs of a leak or penetrating damp
Look outside first. The usual external suspects are blocked or leaking gutters, roof flashings and tiles, open chimney pots, poor external pointing, cracked render, raised ground levels and failed drains. Inside, plumbing leaks and broken shower or bath seals add to the list. Checking outside while it is raining, for example whether gutters overflow, is a simple first step.
If a patch appears or grows after rain and eases in dry weather, an outside defect is the likely cause. A plumbing leak does not follow the weather, so damp that stays the same in dry spells is worth checking against nearby pipes and seals. Our page on damp patches caused by failed pointing covers that case in detail, and cracked mortar letting water in shows what the joints look like. Around baths, showers and kitchens, see bathroom and kitchen leaks and black mould.
Is it rising damp?
Look for a horizontal line with salt staining, low on the wall. Rain at low level can look similar. If the moisture gets stronger towards one point rather than sitting evenly along the base, water getting in from outside is worth checking before anything is blamed on the ground.
The foil test, and what it cannot do
Historic England describes a simple test: fix a piece of metal foil to the damp area. Droplets forming on the surface of the foil point to condensation. If the damp is underneath the foil, the moisture is coming through from the wall. The test helps separate the two, but it does not show where the moisture in the wall is coming from.
Why a moisture meter is not enough
In masonry, electrical moisture meters give qualitative, comparative readings only. Salts can affect them, and they cannot tell one moisture source from another. RICS advises being wary of anyone who diagnoses damp from a meter alone, and SPAB says rising damp is widely misdiagnosed from high meter readings. Use a meter as one clue, not the answer.
When to get a specialist in
RICS says finding the cause can sometimes be complex, and advises a specialist building surveyor with experience of building pathology, with an inspection that complies with the RICS Joint Position Statement 2022 (JPS22). RICS also says large areas of mould should be handled by qualified professionals. For an older property, Historic England's repointing guidance advises an independent condition survey by an architect, surveyor or architectural technologist who is familiar with older buildings.
Where NSJ fits
NSJ repairs failed pointing and sealant, and we replace worn window and door gaskets. We do fix condensation when failed gaskets are the cause. Cold air getting past a tired gasket can chill the glass edge, the frame, the reveal and the wall beside the window, and moisture then settles there. In that case window and door gasket replacement deals with the cause, and our pages on perished window and door seals and draughts through windows and doors show the signs to look for. Droplets between the panes of a double-glazed unit are a different fault, a failed sealed unit, and that is a glazing job rather than a gasket one.
Condensation that comes from everyday humidity or poor ventilation is not something we fix. For that, the RICS damp and mould consumer guide (listed under Sources below) is a sensible place to start. Our inspections are not damp or condensation surveys. If the damp is widespread or keeps coming back, a damp surveyor is the right first call. If you can see one external defect, or the damp is a local patch, a building leak and water ingress inspection fits. Inspections start from £125. If you are not sure, send photos and a short description and we will say whether an inspection is the right first step.
Sources
These are plain references so you can check them yourself. They are general guidance, not NSJ policy, and not a promise about any particular property.
- RICS, Damp and mould consumer guide (rics.org/consumer-guides/damp-and-mould)
- SPAB, What is rising damp? (spab.org.uk/advice/what-rising-damp)
- Property Care Association, Code of Practice: Control of Dampness in Buildings (January 2023), including sections 4.2.2 and 5.1.6
- Property Care Association, The Use of Moisture Meters to Diagnose Dampness in Buildings
- Historic England, Assessing Damp in Historic Buildings (foil test, paraphrased)
- Historic England, Repointing Brick and Stone Walls (independent condition survey advice, paraphrased)