Condensation
Plain English. Practical detail. UK context.
Condensation occurs when water vapour in air changes to liquid on or within a colder part of a building. Surface condensation can appear on glass, tiles or cold walls. Interstitial condensation occurs within a construction. Condensation is one possible moisture mechanism, not a diagnosis that rules out leaks or rain penetration.
Last reviewed: (editorial review against the public sources listed below).
Understand the moisture balance
Cooking, bathing and drying clothes add moisture, while ventilation removes some of it. Heating and insulation affect surface temperatures and the air’s capacity to hold moisture. A cold corner behind furniture can behave differently from the middle of a heated room. Thermal bridges can create local cold spots. Useful investigation considers where and when moisture appears, occupancy, ventilation operation, heating patterns and the building’s construction. It should not reduce every case to blaming the occupants.
How it is investigated
Look for patterns and gather evidence over time rather than taking one isolated humidity reading. Check extractor operation, air paths and obvious plumbing or rainwater defects. A hygrometer can support observation, but measurements need context and suitable placement. A thermal camera shows surface temperature patterns; it does not directly prove a moisture source. Some damp problems have several contributing causes. Traditional materials and retrofit alterations also affect how moisture moves and where it can dry.
Common mistakes
Painting over mould without addressing its cause usually leaves the underlying conditions in place. Blocking vents to stop draughts can worsen indoor air quality and moisture problems. A dehumidifier may help manage moisture temporarily but does not automatically replace necessary ventilation or repair a leak. Adding insulation internally without considering vapour movement, airtightness and junctions can introduce different moisture risks. Mould affecting health or extensive contamination needs prompt appropriate advice rather than an extended experiment with cosmetic products.
Remedies and related requirements
A proportionate response may combine repairs, effective extraction, improved background ventilation, suitable heating and targeted thermal improvements. The right approach depends on evidence and the household’s practical constraints. BS 5250 concerns moisture management in buildings. England’s Approved Document F provides ventilation guidance with applicable editions and transitional arrangements; other UK nations differ. Historic England’s moisture investigation guidance is important for traditional buildings. Agree monitoring and review after the initial measures so success is assessed from changed conditions, not just a newly painted surface.
Frequently asked questions
Is all black mould caused by condensation?
No. Damp and mould need investigation; water ingress and other mechanisms may contribute.
Will a dehumidifier cure the building?
It can reduce indoor moisture, but it does not repair leaks or automatically correct a deficient ventilation strategy.
Can insulation make condensation worse?
Poorly designed changes can move or concentrate moisture risks. Assess the whole construction and its junctions.
Is condensation proof that a wall needs damp-proof injection?
No. Injection targets a different moisture mechanism. Condensation requires assessment of moisture in the air, ventilation and surface temperatures rather than assuming rising damp.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.