Condensation
/ ˌkɒndɛnˈseɪʃən /
Also known as: surface condensation, condensation damp, cold bridging condensation
Definition
Condensation is the formation of liquid water on a surface or within a building element when warm moist air contacts a surface cooler than its dew point temperature. It is the most common form of dampness in UK homes - and the most commonly misdiagnosed, frequently attributed to penetrating or rising damp. It is caused by inadequate ventilation (allowing moisture-laden air to persist indoors) combined with cold surfaces (insufficient insulation causing surface temperatures to fall below the dew point). Surface condensation feeds black mould growth (Cladosporium, Aspergillus) on walls and ceilings. Interstitial condensation forms within the building element itself - see the separate entry. Approved Document F (ventilation) and Approved Document C (moisture resistance) are the relevant Building Regulations documents.
In practice
Condensation is overwhelmingly the most common cause of black mould on walls and ceilings in UK homes, but surveyors and homeowners frequently blame penetrating damp or rising damp, leading to expensive and unnecessary remediation works (hacking off plaster, installing chemical damp-proof courses) that fail to solve the problem. The distinction is usually straightforward: condensation appears at cold surfaces (corners, reveals, north walls), is worst in winter and in poorly ventilated rooms, and occurs regardless of rainfall. Penetrating damp follows defects and is often associated with heavy rainfall.
Remedying condensation requires reducing the indoor moisture load and raising surface temperatures. The most cost-effective first step is always improving ventilation - adding or upgrading trickle vents, fitting a positive input ventilation (PIV) unit in the loft, or installing mechanical extract fans in kitchens and bathrooms. Insulation improvements (cavity wall fill, internal wall insulation, loft insulation) raise surface temperatures and eliminate cold bridges. Simply applying anti-mould paint or bleaching mould treats the symptom, not the cause, and the mould will return within weeks.