Damp & Condensation Glossary

Plain-English definitions of the damp, condensation, mould, ventilation and timber-decay terms used in damp surveys — each linked to a free tool where one applies.

Damp & moisture

Rising damp: Moisture that travels up through a wall from the ground by capillary action, typically to about 1 m, often leaving a tide mark and hygroscopic salts. Controlled by a damp-proof course.
Penetrating damp: Moisture entering through the building fabric from outside — via defective pointing, render, roofs, flashings or seals — usually worse after rain and at high level as well as low.
Damp-proof course (DPC): A barrier built into or injected into a wall to stop rising damp passing up from the ground into the structure.
Hygroscopic salts: Salts (e.g. nitrates, chlorides) drawn from the ground or masonry that attract moisture from the air, keeping a wall damp and complicating diagnosis of rising damp.
Tanking (BS 8102): Waterproofing of below-ground or earth-retaining structures (e.g. basements) by barrier, drained-cavity or integral protection, to the grades of BS 8102.

Condensation & psychrometrics

Condensation: Water deposited when moist air is cooled to its dew point — the most common form of domestic damp, occurring on cold surfaces (surface condensation) or within construction (interstitial).→ tool
Dew point: The temperature at which air becomes saturated and condensation begins. Any surface at or below the dew point will collect condensation.→ tool
Interstitial condensation: Condensation forming within the layers of a building element when warm, moist air reaches its dew point inside the construction — assessed by the Glaser method (BS EN ISO 13788).→ tool
Glaser method: The steady-state assessment of interstitial condensation risk in BS EN ISO 13788, comparing vapour pressure and saturation vapour pressure across a construction month by month.→ tool
Relative humidity (RH): The amount of water vapour in air as a percentage of the maximum it can hold at that temperature; high RH at a cold surface drives condensation and mould.→ tool
Surface water activity (aw): The relative humidity at a surface; mould can germinate where surface aw is sustained above roughly 0.75–0.80, even before liquid condensation forms.→ tool
Sd value: The water-vapour-diffusion-equivalent air-layer thickness of a material (metres) — how much a layer resists vapour. High-Sd layers act as vapour control layers.→ tool
U-value: The rate of heat transfer through a building element (W/m²K); a lower U-value means better insulation. Calculated to BS EN ISO 6946.→ tool

Ventilation & air quality

Ventilation rate (Part F): The airflow required to control moisture and pollutants under Approved Document F — including whole-dwelling rates and minimum extract rates for kitchens, bathrooms and utility rooms.→ tool
Air changes per hour (ACH): How many times the full volume of air in a room is replaced each hour — ACH = (flow in l/s × 3.6) ÷ room volume in m³.→ tool
MVHR / MEV: Mechanical Ventilation with Heat Recovery / Mechanical Extract Ventilation — continuous mechanical systems that ventilate a dwelling (MVHR also recovers heat from extract air).→ tool
Indoor air quality (IAQ): The condition of indoor air assessed via temperature, relative humidity, CO₂ and VOCs — key to occupant health and to controlling condensation and mould.→ tool

Timber & decay

Wet rot: Timber decay by fungi requiring high moisture content (typically over ~50%), confined to the wet area; the most common form of timber decay in buildings.
Dry rot (Serpula lacrymans): A serious timber-decay fungus that can spread through masonry and behind finishes, capable of conducting moisture to drier timber; requires prompt, thorough treatment.
Woodworm: The larvae of wood-boring beetles (e.g. common furniture beetle) that tunnel through timber; activity is judged from fresh bore dust ("frass") and flight holes.

Compliance

Awaab’s Law: A duty requiring landlords to investigate and remedy prescribed hazards — most prominently damp and mould — within fixed statutory timescales after a tenant reports them; extended to the private rented sector by the Renters’ Rights Act 2025.→ tool
HHSRS damp & mould hazard: Damp and mould growth is a prescribed hazard under the Housing Health and Safety Rating System (Housing Act 2004); a Category 1 rating triggers a local-authority duty to act.
Healthy Homes Standards (NZ): New Zealand’s mandatory standards for private rentals (since 1 July 2025) covering heating, insulation, ventilation, moisture ingress & drainage and draught stopping.