Relative Humidity Explained: RH, Vapour Pressure & Mould Risk

Relative humidity explained — and why 80% at a surface means mould.

Relative humidity is the most quoted — and most misunderstood — number in building moisture. It is not a measure of how much water is in the air; it is a measure of how close the air is to saturation, which changes with temperature even when the actual moisture content does not. Understanding that distinction is the key to reading humidity data correctly.

Relative humidity (RH) is the ratio of the amount of water vapour currently in the air to the maximum it could hold at that temperature, expressed as a percentage. 50% RH means the air is holding half the moisture it could at its current temperature. Because the maximum the air can hold rises steeply with temperature, the same quantity of water gives a high RH in cold air and a low RH in warm air.

Why RH changes with temperature alone

This is the crucial behaviour. Take a parcel of air at 20 °C and 50% RH. Cool it — without adding or removing any water — and its RH rises, because cold air can hold less. Keep cooling and it reaches 100% RH at the dew point, where condensation begins. Warm the same air and its RH falls. The moisture content never changed; only the air's capacity did.

Same moisture content, different temperatures (illustrative)

Air temperature	Relative humidity	Status
20 °C	50%	Comfortable
15 °C	~68%	Rising — capacity falling
12 °C	~83%	Mould-risk territory at this surface
9.3 °C	100%	Dew point — condensation begins

This is exactly what happens at a cold wall. The room air may be a comfortable 20 °C / 50% RH, but the thin layer of air touching a cold external corner is chilled — so its local RH at that surface climbs into mould territory or beyond, even though the room as a whole reads 50%. This is why 'surface RH' matters more than 'room RH' for mould.

The 80% surface-RH mould threshold

Mould does not need liquid water. The spores of common indoor moulds can germinate and grow when the relative humidity at a surface stays above roughly 80% for sustained periods (the exact threshold and the time required depend on the species, the substrate and the temperature — described by 'isopleth' growth curves). This threshold sits well below the 100% RH of visible condensation, which is why mould so often appears on cold surfaces that never look wet.

What's a healthy indoor RH?

For comfort, health and building protection, indoor relative humidity is best kept in a moderate band — broadly 40–60% RH:

Below ~40% RH: air feels dry; can aggravate respiratory comfort and increase static and dust irritation.
40–60% RH: the comfort and health 'sweet spot' — also suppresses dust mites and many microbes.
Above ~60–65% RH: dust mites thrive, mould risk rises on cold surfaces, and the dew point climbs so more surfaces are at risk.
Persistently above 70% RH: strong mould and condensation risk; indicates inadequate ventilation relative to moisture generation.

Occupant activity adds a surprising amount of moisture — cooking, showering, washing and drying clothes, and simply breathing can release the equivalent of several litres of water a day into a home's air. Controlling indoor RH is therefore a balance between that moisture generation and the ventilation removing it.

Using RH data in diagnosis

On a moisture survey we log room temperature and RH over a representative period (ideally days, capturing the daily cooking/showering/sleeping cycle), not just a single spot reading — because RH swings hour to hour. Combined with surface temperatures from thermal imaging, the logged data lets us calculate dew point and surface RH, distinguish a humidity-driven (condensation) problem from a liquid-water (penetrating/rising) one, and size the ventilation needed to bring the air back into the healthy band.

Common questions from clients.

My hygrometer reads 55% — so why do I have mould?

Because that reading is the room air, not the surface. At a cold wall, corner or thermal bridge, the local surface relative humidity can be far higher (80%+) even when the room reads 55%, because the air touching the cold surface is chilled and its RH rises. Mould forms there. The fix is to warm the surface and/or lower the room humidity.

Will a dehumidifier solve my damp problem?

A dehumidifier lowers room RH and can give relief, but it treats the symptom, not the cause, and runs at a cost. If the underlying issue is cold surfaces (thermal bridges, poor insulation) or inadequate ventilation, the durable fix is to address those. Dehumidification is a useful stop-gap and a diagnostic aid, not usually the end solution.

Why is condensation worse in winter?

In winter the surfaces of external walls, windows and corners are much colder, so they sit closer to — or below — the dew point of the indoor air. At the same time we ventilate less (windows shut) and generate similar moisture, so indoor RH and dew point rise. Colder surfaces plus higher humidity is the perfect condensation recipe.