The one window you should crack open before every shower: plumbers explain how it stops black mould in the grout

Steam fogged the mirror before the water even hit your shoulders. By the time you stepped out, the tiles were slick, the air was dense, and the bathroom felt more like a low-budget spa than a room in a semi. A week later, the grout line behind the shampoo bottles had gone from bright white to faint grey. A month on, there were black specks marching along the corners like ants.

When the plumber turned up to look at a “leaking shower”, he didn’t start with the pipes. He walked straight over to the small top window, the one that hadn’t been cracked open in months, and ran a finger over the frame. Dust, no draught, faint tide mark of condensation on the paint. Then he said the bit people never want to hear.

“The best mould remover is a window you open five minutes before every shower.”

Why this one small window matters more than bleach

Every hot shower puts several litres of water into the air as vapour. In a typical British bathroom - small, tiled, often internal or half-buried in the middle of the house - that moisture has nowhere to go if everything is shut tight. It hangs, it cools, and it settles back out as condensation on the coldest surfaces it can find: tiles, plasterboard, metal trims and especially grout.

Cleaners attack what you can see. Ventilation stops the bit you can’t see ever getting started. Plumbers and bathroom fitters notice the same pattern again and again: homes where the top bathroom window or trickle vent is cracked open before and during showers have far less black mould in the grout, even if the cleaning routine is nothing special.

That “one window” doesn’t have to be flung wide. What matters is which window you choose and when you open it.

Ideally, it’s:

• The highest opening section in the bathroom (the little top-hung light, not the big sash).
• Opened a small amount before you turn the water on, not just afterwards when the room is already a sauna.

Warm, moist air rises. Give it a high-level escape route and it leaves quietly, instead of having to cool, stick to the walls and feed a mould colony.

What plumbers see in bathrooms every week

Ask any plumber who replaces silicone or re-grouts showers for a living and they’ll tell you they can read the ventilation habits of a house from the walls alone. The same hotspots show up:

• A dark halo where the ceiling meets the tiled area.
• Black streaks in corners and along the bottom grout lines.
• A musty smell from the shower tray even when it looks “clean”.

Then they glance up and see either a sealed window or an extractor fan coated in fluff that only comes on when the light is on - and gets switched off the second someone leaves the room.

People often say:

• “We never open the window, it makes the room freezing.”
• “The fan’s annoying at night, so we turn it off at the pull-cord.”
• “I just wipe the tiles down at the weekend; that should be enough.”

From their point of view, none of that solves the basic issue: air that gets wet but never gets replaced.

The physics hiding in your grout lines

Grout and tile adhesive are slightly porous. They soak up a bit of moisture every time you shower, like a stone left in a puddle. If the room stays humid for hours, that moisture never really leaves. The surface may dry to the touch, but just under the skin stays damp.

Damp, rough, slightly warm surfaces with poor airflow are exactly what black mould (Stachybotrys and its friends) like best. Bleach will whiten the surface for a while, but the roots sit deeper in the grout. Without airflow to actually dry the material, they come straight back.

Ventilation changes that equation. A cracked top window and a working fan don’t “kill” mould; they starve it. They shorten the time your grout spends wet after each shower, so spores that land there never get enough of a foothold to bloom into those familiar black dots.

How to use that window to stop black mould

The trick isn’t dramatic cross-breezes and icy towels. It’s small, consistent changes in timing and where the air moves.

Plumbers who see the best-kept bathrooms tend to recommend a simple routine:

1. Five to ten minutes before your shower

   • Crack open the top bathroom window or trickle vent by about 5–10 mm.
   • If you have an extractor fan, switch it on early if it has a separate pull-cord.
   • Close the bathroom door so the moisture will be drawn towards the outside, not down the hallway.

2. During the shower

   • Keep that top window slightly open the whole time.
   • Let the fan run - it works with the window, not against it. The fan pulls moist air up and out; the window gives it a low-resistance escape point.
   • Stand away from the direct draught if you hate cold air; the top vent pulls steam above head height rather than across your skin.

3. For 20–30 minutes afterwards

   • Leave the window on a crack and the fan running while you get dressed and make a brew.
   • After ten minutes you can open the bathroom door a little to let drier air from the rest of the house in, which helps dilute the last of the humidity.
   • Only close the window once the mirror has cleared and the sill is dry, not just when you feel finished.

Think of that top window as a pressure valve. You’re not trying to turn your bathroom into a wind tunnel; you’re just giving the steam a preference: outwards, not into your grout.

Let’s be honest: nobody will do this perfectly before every shower. But doing it most days, especially in winter, changes the long-term state of your walls dramatically.

What if your bathroom has no window?

Plenty of UK bathrooms are internal. They rely entirely on extract fans and whatever fresh air leaks in under the door. In those cases, plumbers point to a different “one window”: the nearest external window outside the bathroom.

Use this pattern instead:

• Make sure your extractor fan vents outside, not just into the loft.
• Before showering, open the nearest external window on the landing or bedroom a crack.
• Keep the bathroom door slightly open after your shower (while keeping privacy in mind) so the fan can draw in fresher, drier air from the rest of the home and push moist air out.

If your bathroom does have a window but you hate opening it wide in winter, there’s usually a small, more forgiving option:

• Use the trickle vent at the top of modern frames.
• Or open the top-hung section rather than the big lower sash.

A centimetre gap high up can be more effective - and more comfortable - than swinging a large cold pane all the way open at face level.

A simple daily rhythm that keeps grout clear

Once you understand that mould is fed by hours of trapped moisture, not minutes of steam, the preventative routine becomes straightforward.

You can think of it as a four-step cycle:

• Pre-ventilate: Crack the highest window or nearby external window before you start.
• Capture and release: Run a decent extractor fan during and after your shower.
• Strip surface water: Use a quick squeegee or microfibre cloth on tiles and glass so less liquid has to evaporate into the room.
• Dry the room, not just yourself: Give the bathroom 20–30 minutes of airflow before you seal it back up.

Here’s how those habits stack up in practice:

Habit in the bathroom	Effect on mould over time
Never opening the window, fan off after you leave	High risk: grout stays damp for hours, frequent black spots and peeling silicone
Window open after shower only, fan intermittent	Medium risk: some drying, but condensation still builds up on cold mornings
Top window cracked before, during and after shower, fan left running	Low risk: surfaces dry quickly, mould struggles to get started

You don’t need specialist sprays to join the bottom row of that table. You mostly need consistency with a five-millimetre gap.

When bleach still has a place

Ventilation is prevention. Cleaning is catch-up. Plumbers are blunt about this: if the grout is already black, you’ll have to clean or replace it and fix the airflow, or you’ll be back at the same point by autumn.

Reasonable steps:

• Use a bathroom-safe mould remover on existing spots.
• Rinse thoroughly so chemicals don’t sit in the grout and break it down prematurely.
• Once it’s dry, start the window-and-fan routine straight away to stop regrowth.

“We’re not selling magic sprays,” as one installer put it. “We’re selling less steam on your walls.”

Bleach is a reset button, not a solution. The small top window, opened at the right time, is the long-term fix.

FAQ:

• Which exact window should I open if my bathroom has two? Use the highest one - usually the small top opener. Warm steam rises, so a high-level gap lets it escape quickly without chilling you as much as a low, wide-open sash.
• Won’t opening the window waste heat in winter? A slightly open top window for 20–30 minutes loses less heat than you think, and it protects your walls, paint and grout from damage that’s more expensive to fix later.
• Do I still need the extractor fan if I open the window? Yes. The fan creates a steady pull; the window gives the moisture an easy way out. Together they clear humidity faster than either one alone.
• How long should the fan run after a shower? Aim for at least 20 minutes. Many modern fans have overrun timers you can set; if yours doesn’t, use a pull-cord or switch and make it part of your post-shower routine.
• What if mould keeps coming back even with ventilation? Check that your fan actually vents outside and isn’t blocked, look for hidden leaks around the tray or pipes, and consider re-grouting or using an anti-mould grout in badly affected areas. If in doubt, a plumber or damp specialist can check for underlying issues.