Damp and mould

What causes condensation, and how to minimise it

When indoor temperatures drop or humidity is high, water vapour in the air turns into liquid droplets on the coldest surfaces in a room. That is condensation — and where it is left to sit, mould growth follows. This guide explains where the moisture comes from, what makes it worse, and the practical steps that reduce it.

How To Minimise It

Condensation forming on the inside of a window in a UK home

What causes condensation?

Warm, moist air meeting a cold surface.

When indoor temperatures drop or humidity is high, water vapour in the air turns into liquid droplets on the coldest surfaces in a room. This is commonly seen as condensation on windows, walls or other surfaces, and over time it can lead to mould growth forming on them.

Around windows

Glass and frames are usually the coldest surfaces in a room, so they are where water vapour turns to liquid first — and where mould appears on reveals, sills and sealant.

On external walls

External walls sit colder than internal ones, particularly behind furniture, in corners and on north-facing elevations where the surface never really warms up.

On ceilings close to external walls

The junction between ceiling and external wall is a cold path through the fabric, so condensation and mould collect along that line before anywhere else in the room.

What makes it worse

Things that increase condensation in your home.

Most of these are everyday household activities rather than faults. They matter because they add water vapour to the air faster than the home can remove it.

Drying clothes over radiators

A single wash load releases litres of water into the air as it dries, and drying it indoors puts all of that moisture into the rooms you live in.

Leaving kitchen and bathroom doors open

Not closing doors, or not opening a window, during cooking and bathing or showering lets moisture spread from the room that produced it into colder rooms elsewhere.

Cooking with pots uncovered

An uncovered pan puts steam straight into the room. A lid keeps most of that water in the pan rather than on your walls and windows.

Tumble dryers and cooker hoods that vent indoors

Incorrectly vented tumble dryers and recirculating-only cooker hoods direct the moisture back inside the home rather than outside it.

Poor ventilation

Especially in bathrooms and kitchens, where the moisture is produced fastest and has the least opportunity to escape.

Not heating the home sufficiently

Cold surfaces are what the water vapour condenses onto. An under-heated home has more of them, so the same amount of moisture causes far more condensation.

Overcrowding

People produce moisture simply by being there, so the more people living in a home, the more moisture is produced within it.

How do I minimise condensation?

Nine steps, in the order worth taking them.

Start by finding where the moisture is coming from and putting right any building defects. Ventilation and heating then keep the moisture that remains from settling on cold surfaces.

First identify the source of the moisture

Finding the source requires a methodical approach over time, and you may need professional help to do it. Until the source is known, the remedy is guesswork.

Fix defects in the property

Condensation is not always the whole story. A leak, a failed gutter, a defective roof covering or penetrating damp adds moisture that no amount of ventilation will remove, so building defects need putting right first.

Use windows and extractor fans for ventilation

This is especially important after cooking, bathing, showering or drying clothes.

Windows do not need to be open all the time — around 5 minutes for every hour you are in a room is enough.
Trickle vents, small devices often fitted at the top of a modern window, allow fresh air to circulate naturally through a room.
A secure window latch is another way to allow a small, safe amount of ventilation.
Opening windows does not always work when humidity outside is high, so use extractor fans as well.
Always use extractor fans while cooking, bathing or showering.

Check the ventilation systems in your home

Ventilation only helps if it can cope with the peak periods: cooking, washing, bathing, showering and drying clothes indoors.

Fit bathrooms and kitchens with mechanical air extractors sized for those peaks.
Check extractors are the right size and working well, and clean them regularly.
If you use a tumble dryer, check it is fitted with an external vent or a condensate system.
Do not block air vents in bedrooms and cupboards — they let fresh air circulate and reduce condensation.

Keep kitchen and bathroom doors closed and windows open when in use

Containing the moisture in the room that produced it, and giving it a route straight outside, stops it reaching colder rooms where it will condense.

Cover pots and pans where possible during cooking

One of the simplest changes available, and it removes a significant share of the water vapour a kitchen produces.

Maintain internal heat levels

Keep your home well ventilated and maintain internal heat levels, ideally between 18°C and 21°C. This reduces the likelihood of condensation and mould spores forming.

Internal temperatures below 18°C may affect the health of pre-school-aged children, the elderly and those with existing health conditions.

Consider upgrading your windows

If you own your home, you can address heat loss by upgrading your windows — particularly from single glazing to double, triple or secondary glazing. Remember that this does not address the source or the amount of moisture in your home, and modern windows have tighter air seals than older draughty ones, so you may also need to increase ventilation.

Finally, regularly wipe up condensation

Wipe it off daily with a dry cloth or sponge around cooler places, such as window areas and tiled wall surfaces in bathrooms and kitchens. This helps prevent mould growth.

Fuel poverty

For those struggling to pay heating bills, there is advice from various platforms to heat the person, not the home. However, not heating your home adequately can contribute to condensation and mould issues. If you can only heat yourself, remember that your home still needs ventilation.

If you are renting your home

Report any concerns to your landlord or managing agent, and keep a dated record of what you have reported along with photographs of the affected areas.

Ventilation being checked in a home affected by condensation and mould

Where a surveyor helps

When the source is not obvious, measurement settles it.

Condensation is manageable once you know it is condensation. Where the pattern does not fit, or where it keeps returning after every remedy, an inspection establishes what is actually happening in the building.

Whether it is condensation at all

Condensation, penetrating damp, rising damp, a plumbing leak and bridged construction can all leave a similar-looking patch. Establishing which one you have decides everything that follows.

Where the cold surfaces are

Thermal imaging shows the cold spots, thermal bridges and missing insulation that decide where in a room the moisture will condense first.

Whether the ventilation can cope

We measure extract rates and airflow rather than assuming a fan works because it makes a noise, and check that dryers and hoods actually vent outside.

An independent view on what to do

We are consultants, not contractors, and we do not sell ventilation systems, treatments or remedial work. The recommendation reflects what the building needs.

Common questions

Condensation at a glance.

What causes condensation in a home?

When indoor temperatures drop or humidity is high, water vapour in the air turns into liquid droplets on the coldest surfaces in a room. That is condensation, and it is most commonly seen on windows, walls and other cold surfaces. Left on those surfaces, it allows mould growth to form over time.

Where does mould from condensation usually appear?

Typically around windows, on external walls and on ceilings close to external walls. Those are the coldest surfaces in a room, which is why moisture settles there first — often behind furniture and in corners where air movement is poorest.

What everyday activities increase condensation?

Drying clothes over radiators, cooking without lids on pans, not closing kitchen and bathroom doors or opening windows during cooking and bathing, incorrectly vented tumble dryers and recirculating-only cooker hoods, poor ventilation in bathrooms and kitchens, not heating the home sufficiently, and overcrowding — people themselves produce moisture, so the more people in a home the more moisture is produced.

How much ventilation does a room actually need?

Windows do not need to be open all the time. About 5 minutes for every hour you are in a room is enough, or you can allow a small amount of continuous ventilation through trickle vents on modern windows or a secure window latch. When humidity outside is high, opening windows may not help, so extractor fans should be used as well — and always while cooking, bathing or showering.

What temperature should I keep my home at?

Ideally between 18°C and 21°C, alongside good ventilation. That reduces the likelihood of condensation and mould spores forming. Internal temperatures below 18°C may impact the health of pre-school-aged children, the elderly and those with existing health conditions.

Will new windows stop condensation?

Not on their own. Upgrading from single glazing to double, triple or secondary glazing addresses heat loss and cold surfaces, but it does not address the source or amount of moisture in your home. Modern windows also have tighter air seals than older draughty ones, so ventilation usually needs increasing at the same time.

I rent my home — what should I do?

Report any concerns to your landlord or managing agent. Alongside that, the day-to-day measures still help: ventilate while cooking and bathing, keep those doors closed, avoid drying clothes on radiators where you can, and wipe up condensation as it appears.

Find the source

Still getting damp patches after ventilating and heating?

We measure moisture, temperature and airflow together, so you find out whether you are dealing with condensation, a defect or a leak — before spending on a remedy that does not fit the cause.

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