The call came in on the second Tuesday of January, which is when they always come. A woman in Catford, two children, a ground-floor conversion flat, and black spotting across the top corner of the back bedroom that had appeared over Christmas. She wanted a mould clean and she wanted it that week.

My first question was what she’d set the thermostat to. Fourteen, she said, a bit sheepishly, because the December bill had frightened her and everyone tells you to turn it down.

That’s the answer. Not the whole answer, because nothing in a damp flat ever has one cause, but it’s most of it. There’s a temperature below which a London flat in January stops being a home you’re heating and starts being a surface that collects water, and for most conversions it sits between fifteen and sixteen degrees.

What Temperature Does the Condensation Actually Start At?

Whatever temperature your coldest wall is, rather than the number on the thermostat.

Air holds moisture in proportion to how warm it is. Cool it down and at some point it can’t hold what it’s carrying, so the water comes out onto whatever did the cooling. In a flat, that’s never the middle of the room. It’s the plaster on a north-facing solid wall, the corner where two external walls meet, and the strip of reveal around a window where the brickwork comes straight through.

Those surfaces sit three to five degrees below the air temperature in the room, sometimes more in a solid-wall Victorian conversion with no insulation. So a bedroom reading 16°C on a thermostat by the door has a corner behind the wardrobe at 11 or 12, and the wall gets there first.

The two numbers that matter are a room above 16°C overnight and humidity under 60 per cent. A hygrometer costs about £12 and tells you more about your damp problem than I can from the doorway. I’ve stopped guessing and started asking people to buy one before I visit.

The Window You Wipe Every Morning

Streaming windows are not the problem. Glass is the coldest surface in the room, so it collects first, and in that sense it’s doing you a favour by showing you where the moisture load is before anything grows.

The trouble starts when the glass is double-glazed and reasonably warm, because then the coldest surface in the room is a wall, and walls don’t run into a puddle on the sill where you can see them. They absorb, and they hold it, and six weeks later something black appears in a corner.

A flat that stopped streaming after new windows went in and started growing mould instead is one of the most common stories I hear.

Where Is All That Water Coming From In the First Place?

A family of four puts something like ten to twelve litres of water into the air of a flat every day, and almost none of it comes from anything you’d call wet.

Breathing and sweating overnight accounts for a couple of litres between two adults in a closed bedroom, which is why that room is usually the worst in the flat by February. Cooking adds more, particularly boiling pasta and rice uncovered. Showers, obviously. A bath left to cool with the door open puts its entire surface into the hallway.

Then there’s the big one. A single load of washing dried on an airer indoors releases roughly two litres, and in a January flat with no outside space that airer goes up four times a week. It usually stands in the hallway or the bedroom, which are the two places you’d least want it.

The Airer in the Hall

I’ve seen the same arrangement in twenty flats. Airer in the hallway because the living room’s too small, door to the back bedroom open because the flat needs the warmth to circulate, and a ceiling-height patch of black in that bedroom by March.

Move the airer into the bathroom, shut the door, and run the extractor fan for an hour. The room already has a fan and a tiled surface that doesn’t care. That single change has fixed more damp complaints for me than any product I’ve ever sprayed.

If you have a tumble dryer and it’s a vented one, the hose has to go outside. Not into a bucket, not into a wall void, not draped out of a window that’s then closed around it.

Is Turning the Heating Down Really What Caused This?

Turning it down, no. Turning it off and on twice a day in an uninsulated flat, yes.

Two hours of heat at seven in the morning and three in the evening warms the air and barely touches the plaster. Solid brick takes a long time to come up to temperature and gives it back slowly, so a flat heated in short bursts has permanently cold walls with intermittently warm, moist air washing over them. That’s the exact recipe. The wall gets there first, every time, and it does it twice a day.

A low background temperature holds the surfaces up. For a Victorian conversion I’d put the thermostat at 15 or 16 overnight and through the day, then boost it when people are home, rather than letting it swing between 21 and 11.

Whether that costs more depends on your flat, and anyone who tells you otherwise with confidence hasn’t paid your bill. In a well-insulated 2010s block, constant low heat is usually cheaper and certainly drier. In a draughty top-floor conversion with single glazing at the back, it can cost you more, and I’m not going to pretend it won’t. What I’d say is that a professional mould strip, a repaint and a ruined mattress will comfortably exceed the difference, and so will the conversation with a landlord who decides it was lifestyle.

The Trickle Vents Painted Shut

Look at the top of your window frames. Most London flats fitted with uPVC since the nineties have small slotted vents there, and a good proportion of them have been painted over, snapped shut, or blocked with a folded bit of card by a previous tenant who felt a draught.

Open them. All of them, all winter. They’re there to let the wet air out at the rate it’s being produced, and they lose you less heat than the thirty seconds with a window wide open that people do instead.

Which Rooms Go First, and Why Always That Corner?

The unheated one with the door shut. Every time.

A spare room or box room with the radiator off and the door closed becomes the cold end of the flat, and moist air from the rest of the home migrates into it and settles on the coldest thing in there. People close the door to save money on heating a room nobody uses, and then store the suitcases and the winter coats in it, against an external wall, with no air behind them.

After that it’s the main bedroom’s external corner, the wall behind a sofa that’s pushed flat against an outside wall, and the inside of a built-in wardrobe on an external wall. That last one catches people badly, because the first they know of it is a smell in the clothes rather than a mark on a wall.

Leave 50mm behind any large piece of furniture on an external wall. It sounds trivial and it raises the surface temperature of that plaster by a couple of degrees, which is often the whole margin.

The Wardrobe in Harlesden

Fitted units along the back wall of a first-floor bedroom, built tight to the plaster in 2018 by someone who did a tidy job of it. Two winters later the client rang me about a smell in her coats and a grey bloom on the shoulders.

Nothing wrong with the building. The back panels of those wardrobes were the coldest surface in the flat and they had no air moving across them at all. We drilled a row of vent holes through the plinth and the back rail, and she leaves the doors ajar from November to March. Four winters clear since.

When Is It the Building Rather Than Your Habits?

There’s a version of this article that blames tenants for everything, and I’d rather not write it.

Condensation is seasonal, sits on cold surfaces, shows up in corners and behind furniture, and covers a broad patch with fine black speckling. Penetrating damp tracks the weather – a patch that darkens after three days of rain and dries back is water coming through the fabric from a failed gutter, a cracked render bay, or a blocked downpipe. Rising damp is rare, is confined to the bottom metre, and leaves a tide mark with salting in the plaster.

If the patch is on an internal wall, appears regardless of weather, or keeps returning in the same spot after proper treatment, you’re looking at a building fault and no amount of ventilation discipline will fix it.

Renters have more standing here than most people think. The Homes (Fitness for Human Habitation) Act 2018 covers damp and mould as a hazard, and social landlords now work to statutory response deadlines under the rules brought in after Awaab Ishak’s death. Report it in writing, with dates and photographs, and keep the thread.

What I’d Put in the Email

Room by room, when it started, what you’ve changed, a photograph with something in it for scale, and a note of the hygrometer readings over a fortnight. Ask for a surveyor’s visit rather than a mould wash. That framing matters, because a wash is what most managing agents offer first and it’s a decorator’s solution to a building question.

Here’s the part my invoicing dislikes. Mould washes are the job I turn down most often. Somebody rings, wants the black gone before a viewing or a visit from a relative, and what they’re buying is a fortnight – I’d be spraying a surface and leaving the cold wall exactly where it was. I’d rather sell nobody a fortnight. Anti-mould paint is the same deal with a longer warranty on the packaging; the wall gets there first regardless of what colour it is.

The woman in Catford put the thermostat to 16 and moved the airer into the bathroom, and rang me in April to say the corner had stayed clean but the flat had cost her about eleven pounds a week more over the winter. Both those things are true and she’d still rather have the eleven pounds.