Relative humidity and how it can promote condensation

Relative humidity and condensation are closely linked. Condensation forms when moist air meets a surface cold enough to cool that air to its dew point, and the higher the relative humidity indoors, the smaller the drop in temperature needed for this to happen. Understanding relative humidity, dew point and surface temperature explains why condensation and mould appear where they do, and what actually controls them.

This guide explains the science in plain English and sets out practical measures for homeowners, tenants and landlords.

What is relative humidity?

Relative humidity (RH) is the amount of water vapour in the air, expressed as a percentage of the maximum amount the air could hold at that temperature. At 100% RH the air is saturated and cannot hold any more water vapour.

Warm air can hold much more water vapour than cold air. That means RH depends on temperature as well as on how much moisture is present:

• If a room cools and no moisture is removed, its RH rises
• If a room is heated and no moisture is added, its RH falls

A reading of 50% RH, for example, means the air holds half the water vapour it could hold at its current temperature.

Two important points follow. First, RH is not a measure of how much water is in a room, a wall or a floor. Second, a high RH reading on its own should not be used to infer a leak. It needs to be read alongside temperature, dew point and the building itself.

What is the dew point?

The dew point is the temperature at which air becomes saturated and water vapour starts to condense into liquid. It depends on how much moisture the air contains.

For air at 20°C:

So in a room at 20°C and 70% RH, any surface colder than about 14°C is likely to collect condensation. At 50% RH, the surface would need to fall to about 9°C. This is why higher humidity makes condensation so much more likely: the dew point moves closer to the temperature of the room and its surfaces.

Why surface temperature matters

Condensation does not form in the middle of a room. It forms on surfaces, and specifically on the coldest ones. The air touching a cold surface cools, its RH rises, and if it reaches the dew point, water is deposited.

The coldest surfaces in a home are usually:

• Single glazing and window frames
• Window reveals, lintels and other areas around openings
• External corners and wall-to-ceiling junctions
• Solid external walls, especially on north-facing elevations
• Areas with missing or damaged insulation, including where loft insulation stops short of the eaves
• Walls behind wardrobes and furniture, where warm air cannot circulate

These cold spots are often called thermal bridges. On a thermal image they show clearly as cooler areas, which is why thermal imaging is a standard part of our damp and mould surveys.

Relative humidity, condensation and mould

Mould does not need visible droplets of water to grow. What matters is the humidity at the surface, over time.

A room’s RH can be moderate while the RH right at a cold wall surface is much higher, because the air there is cooler. It is commonly cited in building guidance that mould can grow where the relative humidity at a surface remains at around 80% or above for a sustained period, and that room humidity persistently above about 70% can lead to those surface conditions on colder external walls. Short peaks, such as after a shower, are not usually the problem; it is humidity that stays high for days or weeks, combined with cold surfaces.

This explains why mould so often appears in the corners of bedrooms, behind furniture and on window reveals, even when there are no visible water droplets.

Damp and mould can harm health, particularly for people with respiratory conditions, weakened immune systems, children and older people. The government has published guidance on this for landlords: Damp and mould: understanding and addressing the health risks for rented housing providers.

What affects condensation risk?

The British Standard for managing condensation and moisture risk in buildings is BS 5250:2021, “Management of moisture in buildings. Code of practice”. In practical terms, the risk of condensation in a home depends on the following factors:

• Moisture production – cooking, bathing, washing, drying clothes indoors and even breathing add water vapour to the air
• Ventilation – without extraction and background ventilation, that moisture stays inside and RH rises
• Heating – cold or intermittently heated rooms have colder surfaces and higher RH
• Insulation and thermal bridges – poorly insulated or bridged areas are colder and reach the dew point first
• Air movement – still air in corners and behind furniture lets surfaces stay cold and humid
• Occupancy – more people in a home means more moisture

Practical ways to control relative humidity and condensation

Reduce moisture at source

• Cover pans when cooking
• Dry clothes outside where possible, or in a room with the door closed and window open or extraction running
• Make sure tumble dryers are vented to the outside or are condensing types that are emptied regularly

Remove moisture where it is produced

• Use kitchen and bathroom extractor fans during and after cooking and bathing, and check they actually work
• Keep kitchen and bathroom doors closed while in use so moisture does not spread to colder rooms
• Keep trickle vents open and clear

Keep surfaces warmer

• Heat the home steadily, not in short, intense bursts. The NHS advises heating the rooms you use regularly, such as the living room and bedroom, to at least 18°C where possible.
• Leave a small gap between furniture and external walls to let air circulate
• Improve insulation at cold spots where practical

Aim for a moderate humidity range

A range of roughly 40% to 60% RH is widely suggested for homes. A simple digital hygrometer can help occupiers see how humidity changes through the day. Dehumidifiers can help in the short term but do not address underlying causes such as defective extraction or cold surfaces.

Fix the building

If condensation keeps returning despite sensible heating and ventilation, the building may be contributing. Missing insulation, faulty extractor fans, blocked vents or penetrating damp making walls colder all need investigation. Our guide to the types of damp explains how condensation differs from other sources of moisture.

Relative humidity readings in a damp survey

During a damp and mould survey, we record ambient temperature, relative humidity and dew point, and compare them with surface temperatures. This shows whether conditions favour condensation and helps separate condensation from penetrating damp, rising damp and leaks. Find out more in what happens during a damp survey.

For landlords, the distinction matters. Condensation is often linked both to how a home is heated and ventilated and to the building itself, such as poor insulation or ineffective extraction. We currently act as expert witness for social housing providers. In housing disrepair claims, establishing the true cause of damp and mould is often central. See our damp and mould expert witness service. Social landlords in England should also be aware of their duties under Awaab’s Law.

Frequently asked questions

What is a good humidity level for a home?

Around 40% to 60% RH is widely suggested. Occasional higher readings after cooking or bathing are normal; persistently high readings, especially in cold rooms, increase the risk of condensation and mould.

Why do I get condensation on my windows every morning?

Overnight, rooms cool and occupants add moisture by breathing. Windows are usually the coldest surface, so they reach the dew point first. Background ventilation, consistent heating and extraction in wet rooms help.

Will a dehumidifier stop condensation?

It can lower RH and reduce condensation, but it treats the symptom. If the cause is poor ventilation, cold surfaces or a building defect, the problem will return when the dehumidifier is switched off.

Get expert advice on condensation and damp

If condensation or mould keeps coming back, our RICS Chartered Surveyors can find out why. Book a damp and mould survey by calling 0151 374 0905 or emailing office@lansdownassociates.co.uk.