Condensation on Bathroom Walls: Causes and Lasting Fixes

Water beading and running down bathroom walls is almost always condensation: warm, moist air from your shower or bath meeting a surface cold enough to make it drop its water. It is fixed by removing moisture faster (a properly sized, properly installed extractor fan), keeping walls warmer, and producing less steam in the first place. Wiping the walls only treats the symptom.

Below we explain how to confirm it is condensation, what causes it in Cardiff and Vale of Glamorgan homes, and which fixes work. We also cover the point where it stops being a housekeeping job and needs a fitter.

TL;DR

• Condensation needs three things: high humidity, cold surfaces and poor ventilation. Fix any one and the problem shrinks. Fix all three and it goes.
• A fan only helps if it is strong enough, ducted properly to outside, and used long enough after every shower.
• Keeping your home at 18°C to 21°C stops walls getting cold enough to collect moisture, but heat alone does not remove the water.
• Running cold water into the bath first and adding hot afterwards cuts steam at source.
• If walls stay wet with the fan on, or you see staining that does not follow the shower pattern, get a plumber or fitter to check for leaks and ducting faults.

Table of Contents

• Why bathroom walls sweat
• Is it condensation, or something else?
• The seven most common causes
• What actually stops it
• When the fix is a job for a fitter
• Our view: ventilation belongs in the design
• Fixing it during a refit
• A quick checklist for your next shower
• FAQ
• Sources

Why bathroom walls sweat

Warm air holds more moisture than cold air. When that air touches a cool surface, it can no longer hold all its water and the excess settles as droplets. The RICS consumer guide puts it in terms of dew point: condensation forms on any surface at or below the temperature at which the air’s water vapour turns to liquid.

A bathroom is the perfect setting for this. A hot shower releases a large burst of vapour into a small room, and the surfaces are often the coldest in the house: an external wall, a tiled window reveal, an uninsulated ceiling under a cold loft, or a wall behind a bath that gets no heat.

That is why the same bathroom can be fine in July and streaming in November. The NHS-backed West Yorkshire guidance notes condensation becomes a major problem in the colder months, when external walls drop in temperature.

Why “sweating” walls are not always a fault

Some fogging after a hot shower is normal. The question is how quickly it clears. If walls and mirrors are dry again within 20 to 30 minutes of the fan running, your ventilation is doing its job. If they stay wet for hours, or the same patch goes black with mould, something is wrong.

Is it condensation, or something else?

Condensation is often misdiagnosed as rising or penetrating damp. The distinction matters because the cures are completely different, and Wolverhampton Council’s damp and mould guidance says that if the problem is not a leak or a faulty or missing damp-proof course, it is probably condensation.

Use this table as a first check.

If you are not sure, test it. Dry a patch, run the shower with the door shut and the fan off, and watch. If the water appears on the surface within minutes, it is condensation. If a patch refills while nobody has used the bathroom, suspect a pipe. Our fitters check for hidden leaks as part of general plumbing and leak detection work in Cardiff.

The seven most common causes

1. The fan is too weak, or not on long enough

Under the Welsh version of Approved Document F, a bathroom needs mechanical extract of at least 15 litres per second if it uses intermittent extract, and a room with no openable window needs a fan with a 15-minute overrun. That is a minimum for new work, and an older or cheap fan can fall short of it. Most people switch the fan off when they step out of the shower, which is exactly when the room is fullest of vapour.

2. The fan is installed badly

A new fan is no guarantee. Practitioners regularly see fans venting into loft spaces, where the moisture just collects and drips back, and ducting with so many bends that airflow is choked. Rigid duct generally beats flexible hose, which sags and kinks.

Ducting through a cold loft has its own problem: warm moist air cools quickly and, if the duct is uninsulated, water forms inside the pipe and can run back into the fan or onto the ceiling.

3. The fan has failed

Sometimes the motor has simply died, or the grille is clogged with dust. If you can hold a tissue to the grille and it does not stick, the fan is not pulling air. We have a short guide hub on what to do when a bathroom fan stops working and on how to fix an extractor fan.

4. Cold walls

External walls, solid walls in older terraces and ceilings under cold roof spaces all sit below the temperature of the air. A heated towel rail and a warmed house raise surface temperatures, which is why heating matters. Wolverhampton’s guidance and the RICS guide both point to steady background warmth, rather than short blasts, as the way to keep walls above the dew point.

5. Better insulation, same ventilation

Draught-proofing and insulation save energy, but experts warn they can cause condensation if the home loses the airflow it used to have. If condensation started after new windows or a loft top-up, this is a prime suspect.

6. A recent refit that skipped the fan

When bathroom work is done, any retained fan has to be checked to confirm it still works. It is easy to miss, and if it is skipped you can end up with damp complaints that are far dearer to fix once new tiles are on the walls. A fresh, sealed, tiled bathroom with no extract is a condensation trap.

7. Too much steam at source

Long hot showers, a bath filled with hot water first, drying laundry on the radiator and a door left open to the landing all push moisture around the house.

What actually stops it

Step 1: Cut the steam

Run the cold tap first when filling a bath, then add hot. Wolverhampton Council’s guidance says this can reduce steam by up to 90%. Keep showers shorter if you can, and keep the bathroom door shut while you bathe so steam does not spread to colder rooms.

Step 2: Run the fan properly

Turn it on before you start the shower and leave it on for at least 15 to 20 minutes afterwards. A humidity-sensing or timer fan takes the memory out of it. Clean the grille a couple of times a year.

Step 3: Open the window, but sensibly

A window helps in summer, but in winter an open window in a cold bathroom drops wall temperatures and can make condensation worse elsewhere. Use it after the fan has done the heavy lifting, not instead of it.

Step 4: Keep the room warm

Heating alone does not remove moisture, but holding the home at roughly 18°C to 21°C stops walls from becoming cold enough for water to settle on them. RICS also notes that below 18°C there are health risks for vulnerable people, so if cost is the issue, look at targeted help rather than switching the heating off. A heated towel rail running through the morning is a sensible compromise.

Step 5: Wipe, then dry

Wipe wet surfaces with a cloth and a mild detergent, then dry them. Do not paint over mould spots. The cause has to go first. A squeegee on tiles and shower glass saves a lot of moisture from lingering.

What does not work

• A bowl of salt. It will absorb a tiny amount of water and will not stop a room that is producing litres of vapour.
• Closing the shower door or curtain tight. Leave it open so the moisture can disperse.
• A dehumidifier as the only fix. It can help in a room with no extract, but it should sit alongside ventilation and not replace it.
• Anti-mould paint on a wet wall. It sits on the surface and does not change the dew point.

When the fix is a job for a fitter

Housekeeping changes solve many cases. Call a professional when:

• the fan is on, strong and clear, but the room still runs with water;
• the duct goes through the loft and you see staining on the ceiling around it;
• there is no fan at all, or a fan with no overrun in a windowless room;
• you are about to retile or refit and want ventilation designed in;
• the wall is wet even when the bathroom is unused, which suggests a pipe or external defect.

A plumber can fit a correctly sized fan, run rigid insulated ducting to outside and check your pipework at the same visit. For black mould spreading through bedrooms or any household with young children, older people or anyone with asthma, public health guidance is clear that persistent damp raises the risk of respiratory infections and allergic problems, so do not leave it to the next decorating round. Rented homes are different: if you are a tenant, report it to your landlord in writing, and speak to the local council’s environmental health team if nothing happens.

Our view: ventilation belongs in the design

We fit bathrooms across Cardiff and the Vale of Glamorgan, and the pattern we see is consistent. Condensation complaints are rarely down to a single bad product. They come from a bathroom where the tiling, the fixtures and the fan were each chosen on their own, rather than as one system.

A stylish new room can look wonderful on day one and be wet by January if the fan is a bargain unit left over from the old suite, or the duct has been routed the shortest way instead of the right way. So when we plan a refit we treat extract as part of the plumbing and the design, not an afterthought. That means asking where the duct can run, whether the window opens, how cold the external walls are, and how the household really uses the room: a family of five with early-morning showers needs different extraction from a couple who bathe in the evening.

We also think layout matters. Pipework, shower position and the placement of the fan decide whether steam gets pulled out or pools in a corner. If you want to see how space planning shapes a small room, this Canadian guide to small bathroom layouts is a decent general read, and our own notes on how to lay out bathroom plumbing cover the plumbing side.

My advice to anyone with sweating walls is simple: check the cheap things first, the fan, the habits and the heat. If those do not solve it, get someone to look at the whole system, not just the symptom.

— James

Fixing it during a refit

A bathroom renovation is the cheapest moment to solve condensation for good, because walls are open, floors are up and the ducting route can be planned before tiles go on. We deliver bathroom renovation and installation as one accountable local team rather than a mix of subcontractors. Our Bathroom Renovation Cardiff page lists a published price of £2,500 to £4,000, one-off.

If you are in the Vale, see bathroom fitters in Barry, or for a wetter, more open design, ask about wet room conversions, where extraction and waterproofing need extra care. Finishes matter too: tiling and vinyl flooring that suit a damp-prone room are part of the same job. If you just want a fan checked or a leak traced, our Cardiff plumbers can do that without a full refit.

Before you sign anything for new work, it is worth reading our list of what to ask a fitter about bathroom warranties, and asking directly how the extract will be tested.

A quick checklist for your next shower

1. Fan on before you start.
2. Cold water first in the bath.
3. Door shut during, shower door open after.
4. Squeegee the tiles and glass.
5. Fan stays on 15 to 20 minutes, longer in winter.
6. Check the grille is clear once or twice a year.
7. Keep a steady background temperature rather than heating in bursts.

If walls are still wet after a month of this, the issue is the fan, the duct or the building, and it is time for a professional look.

FAQ

Why does my bathroom have condensation?

Because steam from bathing meets cooler surfaces and turns back into water. It gets worse where the fan is weak, blocked, badly ducted or switched off too early, and where external walls are cold.

Does putting the heating on get rid of condensation?

Not by itself. Heat does not take moisture out of the air, but it keeps walls warmer so less water settles on them. Pair steady heating at around 18°C to 21°C with a working fan.

How long should I leave the extractor fan running?

For a bathroom with no openable window, current Welsh building guidance requires a 15-minute overrun on the fan. That is a good habit for any bathroom, and longer in winter if walls are still damp.

Can a new fan still leave my walls wet?

Yes. If it vents into a loft, uses long or kinked flexible ducting, or is undersized for the room, moisture lingers. Poor installation is a common reason new fans disappoint.

Is condensation the same as damp?

They overlap but are not the same. Condensation is moisture from the air, mostly fixed by ventilation and habits. Other damp, such as leaks or penetrating water, usually needs structural or plumbing repair.

Sources

• Approved Document F Volume 1: Ventilation (Wales, 2026)
• RICS: Damp and mould consumer guide
• Wolverhampton Council: Damp and mould
• Wolverhampton Council
• West Yorkshire Healthier Together: Dealing with damp and condensation
• Welsh Government: Considering summertime relative humidity in older properties
• BookaBuilderUK: Replace a failed bathroom extractor fan
• BookaBuilderUK: Ventilation in UK bathrooms and kitchens
• Landlord Today: Where bathroom money goes furthest in a rental property