Mould cleaning glossary
Plain-English explanations of the terms you will come across.
- Condensation
- Water that forms when warm, moist air touches a colder surface and cools below its dew point. It is the most common cause of mould in UK homes, especially on windows, external wall corners and ceilings.
- Dew point
- The temperature at which air can no longer hold its moisture and water starts to form on surfaces. The more humid the air, the higher the dew point, so cold surfaces get wet more easily.
- Fungicidal wash
- A cleaning solution containing a fungicide that kills mould growth on surfaces, rather than just removing its colour. It is applied, left for a contact time and then wiped or rinsed.
- HHSRS
- The Housing Health and Safety Rating System, used by councils in England to assess hazards in homes. Damp and mould growth is one of the listed hazards, and serious cases can lead to enforcement action.
- Hygrometer
- A simple instrument that measures relative humidity in a room. Cheap digital versions help householders see when moisture is building up and whether ventilation changes are working.
- Interstitial condensation
- Condensation that forms inside the layers of a wall, floor or roof rather than on the visible surface. It can cause hidden mould and timber decay, and is a design consideration in insulation work.
- Mould spores
- Microscopic particles released by mould to spread and grow elsewhere. They are present in most indoor air and only become a problem where they land on a surface that stays damp.
- Penetrating damp
- Water getting through walls, roofs or around windows from outside, often through defective gutters, pointing, render or flashing. It usually causes patches of damp and mould that worsen after rain.
- Positive input ventilation
- Often called PIV, a fan unit usually fitted in the loft that pushes filtered fresh air into the home. It slightly pressurises the house so moist air is pushed out, reducing condensation.
- Relative humidity
- How much moisture the air holds compared with the most it could hold at that temperature, shown as a percentage. Around 40 to 60 percent indoors is a sensible target for most homes.
- Rising damp
- Ground moisture drawn up through masonry where a damp-proof course is missing, failed or bridged. It shows as a tide mark low on walls, often with salts, and is less common than often claimed.
- Run-on timer
- A feature on an extractor fan that keeps it running for a set time after the light or switch is turned off, so steam is cleared after a shower or bath has finished.
- Sanitary silicone
- A flexible sealant for baths, showers, basins and worktops that contains a fungicide to resist mould. It is used when replacing old, mouldy sealant around wet areas.
- Thermal bridge
- A part of a wall, ceiling or window surround that conducts heat out faster than its surroundings, such as a concrete lintel or uninsulated corner. It creates a cold spot where condensation and mould often appear first.
- Trickle vent
- A small, adjustable slot vent usually built into the top of a window frame. It provides constant background ventilation to remove moisture and is designed to be left open.
Last updated 6 November 2026