Condensation
| Condensation | |
| Type | Moisture source |
|---|---|
| Cause | Warm moist air meeting a cold surface |
| Standard | BS 5250 (management of moisture in buildings) |
| Distinguishing feature | Seasonal and surface-wide, not a point source |
Condensation is the formation of liquid water when air containing water vapour is cooled below its dew point, typically at a cold surface. It is the most common cause of dampness in United Kingdom housing and the most common thing mistaken for a plumbing leak. BS 5250 is the code of practice for the management of moisture in buildings.[1]
How it presents[edit]
Condensation appears on or near surfaces rather than emerging from a point. It concentrates on the coldest parts of a room: window reveals, external corners, behind furniture placed against an external wall, and at thermal bridges. Mould growth follows, favouring the same locations.
It is strongly seasonal, worsening in cold weather, and responds to occupancy: drying laundry indoors, cooking without extraction and blocked or absent ventilation all raise indoor humidity and increase it. A damp problem that improves markedly in summer and worsens in winter is behaving like condensation, not like a leak.
Distinguishing it from a leak[edit]
The distinctions are practical and reliable. A plumbing leak has a source and a gradient pointing back to it; condensation is distributed across the coldest surfaces. A leak does not care what the weather is doing; condensation follows it. A leak on a pressurised system shows at the meter or as pressure loss; condensation shows at neither.
Moisture meters read both, which is why a reading alone does not settle the question. The informative measurement is where the readings are highest and whether that pattern points to a source or to a cold surface. Thermal imaging is useful here for the same reason: it shows the cold areas where condensation is expected, and a warm plume where a heating leak is not.
See also
References
- ^BS 5250: Management of moisture in buildings. Code of practice. BSI. Retrieved 2026-09-05