A Bathroom With No Window Is Not a Dark Room. It Is a Wet One.

Everyone with an internal bathroom describes the same complaint: it is dark. It is not dark. It has a light in it, and you can turn the light on. What it has is nowhere for four litres of steam to go twice a day, and almost every ugly thing in that room is a symptom of the water rather than the light.

Peeling paint above the shower. Black in the grout lines and around the sealant. A ceiling that has gone patchy in one corner. A door that swells and sticks in winter. A mirror that never fully clears. None of those are decorating failures and none of them are solved by decorating. They are all the same failure, which is that the air in the room stays wet for hours after anyone has used it.

Which is good news, in a way, because it means the room is fixable and the fix is mechanical rather than aesthetic. And once it is fixed, a windowless bathroom turns out to be the most freely decorated room in the house.

Find out whether the fan is doing anything at all

Two tests, both free, both worth doing before you read any further.

Hold a single sheet of loo roll flat against the fan grille while it runs. If it holds there on its own, the fan is moving air. If it drops, the fan is making a noise and achieving very little — which is extremely common, because grilles clog with dust and lint, and because a lot of fans were fitted with flexible ducting that has sagged, kinked, or come off entirely in the loft.

Then buy a small hygrometer. They cost very little and they will tell you the truth: relative humidity in a bathroom should climb during a shower and be back down under about 60 percent within twenty to thirty minutes of it ending. If it is still sitting at 75 percent an hour later, the room is wet all day, every day, and you now know exactly why the grout is black.

The numbers a fan has to hit

Extraction is rated in cubic feet per minute, and the working rule for a bathroom is roughly 1 CFM per square foot of floor, with a floor of about 50 CFM for anything up to 100 square feet. A small internal bathroom of 40 square feet still wants that 50 minimum. A larger room, or one with a separate enclosed toilet or a big tub, wants more — the usual approach is to add capacity per fixture rather than per foot.

Three details matter as much as the number:

It has to run on. A fan wired to the light switch stops the moment you leave, which is precisely when the room is at its wettest. A run-on timer of 15 to 20 minutes after the switch goes off does more than doubling the fan’s capacity would.

It has to go outside. Ducting that ends in a loft or a ceiling void has not removed the moisture from the building, it has moved it somewhere that cannot be inspected. Short, rigid, well-sealed duct to an external terminal.

It has to be quiet enough to leave on. Fan noise is rated in sones; anything under about 1.0 is quiet enough that nobody in the house resents it. A loud fan gets switched off, and a fan that is off has a capacity of zero regardless of what the box said.

And the free half of ventilation: air has to get in for air to get out. That is what the gap under the bathroom door is for, and it is why a thick new carpet in the hall or a draught excluder on that particular door can quietly stop a working fan from working.

Then choose finishes that are not arguing with the room

Once the air moves, the finishes stop being a losing battle — but they are still worth choosing deliberately.

Paint the ceiling in something made for bathrooms rather than standard matt emulsion, which is the surface that fails first because it is the coldest one in the room and condensation lands there. Sealant around a bath is a consumable, not a permanent fitting; when it goes black it is done, and cutting it out and redoing it is an afternoon rather than a project. Grout can be sealed, and in a room that stays wet it always eventually loses.

Consider fewer grout lines while you are at it. Large-format tile, a solid panel, a single sheet of material — the maintenance load of a wet room is roughly proportional to its linear feet of joint.

Now go dark, because you have nothing to lose

Here is the part that makes an internal bathroom enviable rather than unfortunate.

In a room with a window, a dark colour costs you something real: it absorbs the daylight you were given, and there is a genuine trade to weigh. A room with no window has no daylight to absorb. The entire argument against dark colour — the only serious one — does not apply. Pale paint in a windowless room does not make it brighter, because there is nothing coming in to bounce; it just makes it look like a room that was trying to be brighter and failed.

So paint it oxblood. Or a green so deep it reads black at the door. Tile it floor to ceiling in something glazed and slightly uneven so the light you do provide catches on it and moves. A small internal bathroom is the cheapest room in the house to be brave in — there is very little wall, so the expensive tile or the extravagant paper is affordable here and nowhere else — and it is the room where bravery has no downside, because the light in it is entirely yours to design.

Which means the light has to be good, and this is where the two halves of the room meet. A dark windowless bathroom lit by one ceiling fitting is a cave. The same room with a warm light beside the mirror, a second low light somewhere near the floor, and both on a dimmer is a jewel box, and the difference between those two outcomes is about the price of one weekend.

So: paper against the fan grille tonight, and a hygrometer this week. If the humidity is back under 60 percent half an hour after a shower, skip straight to the paint chart and enjoy yourself. If it is not, no colour you choose is going to survive long enough to be worth choosing — fix the air first, and then go as dark as you like.