Condensation and damp glossary
The words surveyors, installers and building guidance use, explained in plain English.
- Absolute humidity
- The actual amount of water vapour in the air, usually in grams per cubic metre (g/m³). Unlike relative humidity, it doesn't change when air warms or cools, so it's the right way to compare indoor and outdoor air.
- Air brick
- A perforated brick that ventilates a suspended timber floor void or an old fireplace. Blocking air bricks can lead to damp and timber decay.
- Air changes per hour (ACH)
- How many times the air in a room is replaced with outdoor air each hour. Higher rates remove moisture faster but lose more heat.
- Condensation
- Water vapour turning to liquid when air cools to its dew point, usually on a cold surface. See what causes condensation.
- Cavity wall
- Two layers of masonry with a gap between them, common in UK homes built from the 1920s–30s onwards. The cavity may be empty or filled with insulation.
- Damp-proof course (DPC)
- A waterproof layer built into a wall just above ground level to stop moisture rising from the ground. Bridging it — with high paths, soil or render — can cause low-level damp.
- Desiccant dehumidifier
- A dehumidifier that absorbs moisture with a drying material rather than a cold coil. It works better than compressor models in cold rooms but usually uses more power.
- Dew point
- The temperature at which air becomes saturated and water starts to condense. Any surface colder than the dew point of the room air gets wet. Try our dew point calculator.
- dMEV (decentralised mechanical extract ventilation)
- Quiet extractor fans that run continuously at low speed in wet rooms and boost when needed.
- fRsi (temperature factor)
- A number from 0 to 1 describing how close an inside surface stays to room temperature. 1 is as warm as the room; 0 is as cold as outside. UK guidance recommends at least 0.75 at junctions in homes to limit mould risk. See thermal bridges.
- Hygrometer
- A device that measures relative humidity, usually alongside temperature. Loggers record readings over time.
- Hygroscopic salts
- Salts left in old plaster and masonry (from groundwater, chimneys or past damp) that absorb moisture from the air, keeping walls damp-looking even after the original cause is fixed.
- Interstitial condensation
- Condensation that forms inside a wall, roof or floor, where warm moist air meets a cold layer. It's hidden, so it's prevented through design — vapour control layers and breathable materials — rather than spotted.
- Mould
- Fungi that grow on surfaces kept at high humidity — around 80% or more for long periods. Visible black or grey spots are usually condensation-related. See black mould.
- MVHR (mechanical ventilation with heat recovery)
- A whole-house system that extracts stale, moist air, supplies fresh air, and transfers heat between them so less warmth is lost.
- PIV (positive input ventilation)
- A fan unit, often in the loft, that gently pushes filtered air into the home, displacing moist air through gaps and vents. See ventilation explained.
- Rain penetration
- Water entering a building from outside through defects such as blocked gutters, cracked render, failed pointing or window seals. It needs repair, not condensation control.
- Relative humidity (RH)
- How much water vapour air holds compared with the maximum it could hold at that temperature, as a percentage. The same air reads a higher RH when cooled and lower when warmed. See ideal humidity.
- Rising damp
- Groundwater drawn up through masonry by capillary action. It exists but is often misdiagnosed; low-level damp has several other common causes.
- Sealed unit
- The double- or triple-glazed glass panel in a window frame. Misting between the panes means its seal has failed.
- Solid wall
- A wall with no cavity, typically brick or stone, common in homes built before about 1920. Usually colder on the inside than a filled cavity wall.
- Thermal bridge
- A spot where heat escapes faster than through the surrounding structure — corners, lintels, junctions — leaving a colder inside surface prone to mould.
- Trickle vent
- A small adjustable slot, usually in the top of a window frame, providing constant background ventilation. Best left open.
- U-value
- How quickly heat passes through a wall, window or roof, in W/m²K. Lower is better insulated — and gives a warmer inside surface.
- Vapour control layer
- A membrane on the warm side of insulation that limits moisture moving into the structure, preventing interstitial condensation.