Steamy showers meet winter air, and you reach for the latch. What follows in that small room can haunt your walls.
Across the UK, households crack a window after a hot wash, hoping to clear the steam. New guidance suggests that habit can backfire on cold days, driving condensation and feeding stubborn mould in busy family bathrooms.
Why your open window backfires on cold days
A hot shower pumps vapour into a small, tiled box. When that warm, moist air hits cold glass and chilly plaster, water condenses fast. Those droplets are not harmless. They wet grout, silicone, paint and plasterboard. Damp patches turn into a food source for mould spores that already float in indoor air.
One 10-minute shower can push bathroom humidity close to 90%. Give mould 24–48 hours of damp, and it spreads.
Bathrooms carry higher risk than other rooms. International health data show they are two to three times more likely to host visible mould than bedrooms or lounges. In a family home with toddlers splashing in the tub and teenagers taking long showers, that risk is multiplied by use. Open a window on a cold, damp afternoon and you may accelerate the problem by chilling surfaces further.
The dew-point trap explained
Condensation starts when air cools to its dew point. At 21°C and 60% relative humidity, the dew point sits around 13–14°C. Many UK bathrooms sit below that on winter mornings, especially around windows and on external walls. Cold air from an open window lowers surface temperatures even more. Warm steam then meets a cold wall and dumps liquid water.
That is why a window works on a warm, dry day but often fails in November. You remove some moisture, yes, but you also cool the very surfaces you are trying to keep dry. The result is wetter walls after you close the window and the temperature rises again.
What to do instead: a three-step routine that works
Ventilate, contain, and dry. This simple routine beats the chill-and-drip cycle.
- Run an extractor fan five minutes before the shower, during, and for 20–30 minutes after.
- Keep the bathroom door closed while showering to stop steam drifting into cooler hallways and bedrooms.
- Squeegee tiles, screens and the bath. Follow with a quick towel wipe on ledges, grout lines and the window frame.
- Warm the room briefly. A heated towel rail or low underfloor setting raises surface temperature and reduces condensation.
- Lift soggy mats and hang towels to dry. Mop visible puddles so liquid water is not left to evaporate back into the air.
Fans, not windows, remove moisture without chilling surfaces. The drier the surface, the harder it is for mould to root.
Set your fan up properly
Many fans are too weak, too dirty or switch off too soon. In the UK, performance of 15 litres per second (about 54 m³/h) is a sensible minimum for a bathroom. A run-on timer of 15–30 minutes clears lingering moisture. Check the backdraft shutter opens freely. Clean the grille every few months to restore airflow. If the fan sounds like a jet, it may be vibrating against the wall; a quiet, efficient unit encourages routine use.
Consider a humidity-sensing model. It ramps up when steam rises and winds down when relative humidity drops below a target level, often around 50–60%.
Cold window versus extractor fan: what actually changes
| Scenario | Moisture removal | Surface temperature | Condensation risk | Heat loss |
|---|---|---|---|---|
| Cold day, window wide open | Moderate if windy | Drops sharply | High on walls and glass | High, feels chilly |
| Fan on, door closed | Consistent, controlled | Stays warmer | Low if squeegeed | Low to moderate |
| Warm, dry day, window ajar | Good | Stays mild | Low | Low |
Health and home costs you can avoid
Mould spores aggravate asthma, eczema and rhinitis. Young children and older adults feel the effects sooner. Black spots on grout often signal hidden damp behind silicone and skirting. Paint flakes. Sealant lifts. A small bathroom can need new grout and sealant within a year if condensation goes unchecked.
Energy and repair costs also add up. Throwing a window wide open for 30 minutes twice a day in winter can waste dozens of kilowatt-hours of heat over the season, especially in electrically heated homes. Depending on tariffs and how often the room is used, households can lose tens of pounds on heat, and spend more again on mould removers, repainting and replacement sealant.
Smart options for busy households
Dehumidifiers offer a safety net in small, windowless bathrooms. A 200 W unit running for two hours uses about 0.4 kWh. On a 28p/kWh tariff, that is roughly 11p. Place it just outside the bathroom with the door ajar after you have run the fan and wiped down. Moisture absorbers help in cupboards but are overwhelmed by daily showers.
Tired of foggy mirrors? A pea-sized smear of shaving foam buffed onto the glass leaves a temporary film that reduces misting for days. It does not remove moisture, so keep the fan running.
When opening the window still helps
On warm, dry days, fresh air is your ally. If the outside temperature is close to indoor levels and the weather is dry, a window opened a crack can clear moisture quickly without chilling surfaces. Use both window and fan for five minutes, then close the window and let the fan finish the job.
A cheap digital hygrometer takes the guesswork out. Aim to get the bathroom back below 60% relative humidity within an hour of showering. If it lingers higher for much longer, review your routine.
A quick checklist for your door
- Switch on the fan five minutes before showering.
- Close the bathroom door and keep it closed until the fan finishes.
- Limit showers to eight minutes where possible.
- Squeegee tiles and glass; towel the window frame and sill.
- Hang towels to dry; lift mats; mop splashes.
- Leave the fan running for 20–30 minutes after use.
- Open the window only on warm, dry days.
Useful numbers and a simple test you can try
How much vapour are you dealing with? A single shower can add 0.5–1.5 litres of water to the air and surfaces. If you wipe down and remove half of that as liquid, the fan has far less to extract. That small change shortens drying time and protects grout lines.
Try this practical test this week. Take a hygrometer reading before a shower. Shower with the fan on, the door shut, and no window open. Squeegee and towel surfaces, then leave the fan running for 30 minutes. Check the reading again. Repeat another day with a cold window open and the fan off. The higher final reading tells you which habit leaves your bathroom wetter. Most homes will see the fan-first method win by a clear margin.
Target: back to under 60% relative humidity within an hour. Faster drying means fewer black spots and less musty smell.
If you are renovating or renting
Planning a refit? Choose a quiet, efficient fan with a run-on timer or humidity sensor, seal external vents properly, and route ducting with gentle bends to maintain airflow. Specify mould-resistant grout and sealant, and consider a small underfloor warming mat in front of the shower.
Renting and stuck with a weak fan? Report it and keep a simple routine: short showers, door closed, fan on, and wipe-downs. A compact desiccant dehumidifier in the hallway after showers can help until the fan is upgraded.
Isnt this just common sense? My gran always cracked the window and never had mould—maybe it’s more about cleaning than “dew points”.
If my fan is rated 85 m³/h but vents through ~3 m of flex duct with two bends, am I actually hitting the 15 L/s target? Any quick way to test airflow without fancy gear?