Damp, Condensation & Mould Risk Checker UK
Check whether your home conditions suggest a low, moderate, high or very high moisture risk. Use Quick Check for simple screening, or Advanced Check for dew point and cold-surface calculations.
Check your home in under 60 seconds
Get a practical explanation, see what could improve the result, and only see dehumidifier suggestions when your answers indicate one may be useful.
How the checker works
The Quick Check combines several practical signals rather than relying on one humidity number. It looks at indoor humidity, room temperature, window condensation, visible mould, ventilation, indoor laundry, how often the problem occurs and how long the dampness persists. This matters because a short humidity spike after a shower is different from a bedroom that stays humid and develops wet windows every morning.
Advanced Check adds a dew-point calculation. If you also enter a cold wall or window-surface temperature, the tool estimates the relative humidity immediately beside that surface. A room can show a fairly ordinary humidity reading in the middle of the room while a much colder surface is close to saturation.
What humidity level increases mould risk?
UK housing guidance commonly uses roughly 40–60% relative humidity as a desirable indoor range for normal occupied homes, allowing for short-term fluctuations. Persistent higher humidity does not prove that mould will grow, but it can increase the risk when combined with cold surfaces, low air movement and repeated moisture production.
The tool therefore avoids telling you that a single room-humidity reading equals a precise percentage chance of mould. Its risk bands are a screening aid that combines several conditions. When exact measurements are missing, the confidence level is reduced instead of pretending that the result is more precise than it is.
Why condensation forms
Warm air can hold more water vapour than cold air. When moist indoor air meets a sufficiently cold window, wall or thermal bridge, the local air temperature can reach the dew point. At or below the dew point, condensation can form on that surface. Mould-friendly surface conditions can develop before visible water droplets appear, because the relative humidity at a cold surface can be much higher than the room reading.
Advanced mode uses the Magnus approximation to calculate dew point from indoor temperature and relative humidity. If you enter a measured surface temperature, it also estimates surface relative humidity and the room-humidity level at which that surface would reach saturation.
When a dehumidifier can help
A dehumidifier can be useful when excess airborne moisture is a major contributor. Common examples include regular indoor laundry drying, repeated morning condensation, rooms that remain above the preferred humidity range and homes where sensible ventilation alone is not keeping moisture under control.
Product suggestions are deliberately shown only when the result indicates that a dehumidifier may be relevant. The tool first identifies whether the likely problem is moisture in the air, a cold-surface issue or a possible building/water-ingress problem. It then suggests a product class or a small shortlist to compare rather than automatically pushing a purchase.
When a dehumidifier is not the first fix
A dehumidifier cannot repair a failed roof, plumbing leak, rain penetration or another building defect. If you select signs such as a patch that worsens after rain, a known leak or low-wall tide marks, the tool suppresses the normal product route and advises investigating the moisture source first.
Persistent or severe damp, extensive recurring mould or suspected building defects may need a qualified professional assessment. This checker is not a building survey and does not diagnose a health condition.
How to take better measurements
For a useful room reading, place a hygrometer away from radiators, open windows, direct sunlight and very local moisture sources. Give it time to settle. If you have an infrared thermometer, measure the coldest representative part of the problem surface rather than a warmer section nearby. A measured surface temperature gives a higher-confidence Advanced result than the simplified surface estimate.
What “What would improve this result?” means
Advanced mode can show how the estimated surface humidity changes if the room were brought to 60% or 55% RH, and it estimates the surface temperature needed to stay below the tool’s mould-risk screening threshold. This helps distinguish a mainly moisture-driven problem from one dominated by a very cold surface. The figures are guidance for understanding the conditions, not a guarantee that a particular action will remove mould.
Methodology and limitations
The Advanced calculation uses the Magnus approximation for dew point. Surface relative humidity is estimated from indoor vapour pressure and the entered or estimated surface temperature. A sustained surface RH around 80% is used as an indicative screening threshold for elevated mould risk. Duration and recurrence are included because persistent conditions matter more than a brief one-off event.
The simplified surface-temperature estimator uses broad surface-type assumptions only when a measurement is not available, so it is labelled lower confidence. It is not a U-value calculation and does not replace a thermal survey. Quick Check scores are also intentionally shown as bands rather than a fake “73% chance of mould” style number.
Sources
- UK Government guidance: understanding and addressing damp and mould in the home
- Centre for Sustainable Energy: condensation, damp and mould
Methodology version: V1.0 · Last reviewed: October 2026.
Frequently asked questions
Is 60% humidity too high?
Brief periods around 60% can occur in normal homes. If humidity remains above this level for long periods, especially beside cold windows or walls, the condensation and mould risk can increase.
Can mould grow without visible condensation?
Yes. A cold surface can have a much higher local relative humidity than the centre of the room, so mould-friendly conditions may exist before liquid water becomes obvious.
Will a dehumidifier stop mould?
It can reduce airborne moisture where high humidity is an important contributor, but it cannot remove every cause of mould and it cannot repair leaks or building defects.
What is dew point?
Dew point is the temperature at which air with its current moisture content reaches saturation. A surface at or below that temperature can develop condensation.
What size dehumidifier might I need?
Use this checker to understand the problem first, then use the Airmestra Dehumidifier Size Calculator to narrow down capacity.
What if I do not know my humidity?
Quick Check can still give a basic screening result, but its confidence will be lower. A simple hygrometer can make the next check much more useful.
Why does the tool sometimes recommend no product?
Because a dehumidifier is not always the right first response. If the answers point towards a leak, rain penetration or another building moisture source, the tool prioritises investigating that cause instead of forcing an affiliate product recommendation.
Useful next steps for damp and condensation
If you need to monitor humidity, read our UK humidity meter guide and ideal indoor humidity explainer. To choose the right appliance, see dehumidifiers for condensation or our mould prevention guide. You can also use the free dehumidifier size calculator.
A risk screening result is not a medical diagnosis or building survey. Investigate leaks and persistent mould at their source.