How to Stop Condensation and Black Mould in a Flat

If you have ever woken up on a January morning to find the bedroom window running with water, you are in very large company. Condensation is the single most common damp problem in British flats, and every winter it produces the same results: water pooling on sills, paint lifting off reveals, window frames rotting quietly from the bottom corner up, and black mould creeping along the edge of a wardrobe pushed against an outside wall.

It is also the damp problem most often blamed on the building when the cause is sitting inside the flat — and, occasionally, the reverse. So here is what condensation actually is, how to tell it apart from a genuine building fault, and what works.

First: is it actually condensation?

This matters more than anything else in this article, because the fix and the person responsible are completely different depending on the answer.

Condensation appears on cold surfaces, is worst in winter, worst in the early morning, and worst in bathrooms, kitchens and bedrooms. It shows up on glass, on window reveals, in corners, behind furniture and on the cold outside wall of the flat. There is no single wet patch with a centre to it.

A leak or penetrating damp looks different. A defined wet patch, a tide mark, damp that is worst after heavy rain or worst directly under a bathroom above, staining on a ceiling, or damp that is there in July as well as January — none of that is condensation. That is water getting in, and in a block it is very often a communal matter: the roof, a gutter, a cracked render, a failed window seal, or a pipe in the flat above.

If it looks like the second kind, do not spend the winter wiping windows. Report it. Our guide to what to do when your ceiling starts leaking covers who pays and what happens next, and if we manage your building you can report a problem with photos in a couple of minutes. Photographs taken while the wall is actually wet are worth more than any description written a fortnight later — as we set out in why accurate fault reporting saves time and money.

What condensation actually is

Warm air holds more water vapour than cold air. When warm, moisture-laden air touches a cooler surface, the air at that surface cools, can no longer hold all the moisture it was carrying, and releases the surplus as liquid water.

The temperature at which that happens is the dew point. Everything below is about one of two things: putting less moisture into the air, or keeping surfaces above the dew point.

In a flat, the cold surfaces are predictable — single-glazed or older double-glazed windows, the reveals around them, concrete lintels, external corners, the wall behind a wardrobe, and any part of the structure that bridges inside and outside. Those are exactly the places condensation turns up first.

Where all that moisture comes from

People are usually surprised by how much water a household puts into the air just by living. The figures below are the ones that appear in local-authority condensation guidance across the country, for a five-person household over a single day:

That is just over 10 kg — around 10 litres of water a day — and none of it has anywhere to go unless you let it out.

Two things make it dramatically worse:

• Flueless gas and paraffin heaters. Burning a litre of paraffin puts roughly a litre of water vapour into the room, on top of the combustion products. In a block of flats these are almost always prohibited outright — by the lease, and by the building’s fire risk assessment — and for good reason. Do not use them.
• A newly built or newly refurbished flat. Mortar, screed and plaster carry a great deal of water, and it has to leave through the air in the room. Nine to eighteen months is the figure usually quoted for a new build to dry out fully. If that is your flat, expect to heat and ventilate harder than you otherwise would for the first winter or two. It is not a fault.

The mould problem

Mould is condensation’s most visible consequence: black speckling along sealant, around window reveals, in the top corner of an external wall, and on the back of anything stood tight against cold plaster. It needs three things — moisture, a surface and time — and it does not need dirt. Clean plaster is perfectly good food.

It is not only a cosmetic problem. Mould spores can trigger and worsen respiratory problems, allergies and asthma, and the risk is highest for children, older people and anyone with an existing lung condition. That is why current national guidance on damp and mould treats it as a health issue rather than a housekeeping one.

Dealing with what is already there:

• Wipe small areas down with a proprietary fungicidal wash, following the instructions on the bottle. Wear gloves, open a window and keep the room ventilated while you work.
• Do not dry-brush or vacuum mould. It sends spores straight into the air you are breathing.
• Wash affected fabrics and shampoo affected carpets; dry-clean anything that cannot be washed.
• Anti-mould paints are worth using, but only after the surface has been treated, and only as a delaying tactic.
• Do not simply paint over it. It comes back, usually in the same month next year.

The only permanent cure is to stop the condensation. Everything else buys you a season.

The three rules that actually fix it

1. Ventilate — get moist air out of the flat before it reaches a cold surface.
2. Insulate — keep surfaces above the dew point.
3. Heat steadily — keep the structure of the flat from getting cold in the first place.

All three work together. Ventilating a freezing flat just makes it colder; heating a sealed one just moves the moisture around.

What to change this week

Most flats are transformed by the cheap end of that list:

• Leave a low background heat on through cold spells rather than blasting the flat for two hours in the evening. A structure that never fully warms up is a structure with cold surfaces on it all winter, and the stop-start approach frequently costs more, not less.
• Close the bathroom door before you shower, not after, and vent the room directly outside — window open or extractor on — for a good while after you have finished. A humidistat extractor that runs on by itself is one of the best twenty-minute upgrades in a flat.
• Put a lid on the pan and run the kitchen extractor while you cook, then for a few minutes afterwards.
• Dry washing outside if you possibly can. If you cannot, put it in one room with the door shut and the window open or the extractor running — never spread across the living room, and never on the radiators.
• If you use a vented tumble dryer, make sure the hose actually goes outside. A hose venting into the room is a 5 kg-a-day moisture source with a plug on it.
• Leave the trickle vents in your windows open. They are there precisely for this, they cost almost nothing in heat, and taping them up is the single most common self-inflicted damp problem we see.
• Pull furniture 50 mm off external walls and leave a gap at the back of wardrobe and cupboard shelves so air can move. Door ventilators help on built-in cupboards.
• Consider a dehumidifier, particularly for a flat where washing has to be dried indoors. Domestic units draw roughly 200–600 W depending on type — a few pence an hour — and will pull a startling amount of water out of a flat in a day.
• Never use LPG, bottled gas or paraffin heaters indoors — see above.

When the simple fixes are not enough

If you have done all of that and the windows are still streaming, the problem is the fabric of the flat, and the work gets bigger. Before you plan any of it, read the next section — in a flat, almost none of this is yours to just get on with.

• Windows. Replacing tired single glazing or failed double glazing is the single most effective change, and secondary glazing is a much cheaper halfway house. Whatever you fit, keep some deliberate ventilation — trickle vents or an openable pane. A perfectly sealed flat is a wet flat.
• Cold walls. Thermal lining paper or insulated plasterboard on a cold external wall lifts the surface temperature and can stop mould on that wall outright. Older guidance recommended lining walls and ceilings with expanded polystyrene: do not do that. Polystyrene wall and ceiling tiles are a serious fire-spread risk, and no responsible managing agent or fire risk assessor will sign them off today.
• Cold floors. On a solid floor, carpet with a good underlay, or cork or cushioned flooring, will feel and behave far better than vinyl over concrete. Check your lease first — most leases in a block require carpet or a specified acoustic underlay in the living areas precisely to stop noise complaints downstairs.
• The roof over a top-floor flat. An uninsulated roof will produce condensation on the ceilings below it, and this is the one on the list that is usually not the leaseholder’s to fix. In most blocks the roof and the void above the top floor are communal, so insulating them is a building matter to be raised with your managing agent or the directors of your management company, budgeted from the service charge or reserve fund, and quite possibly consulted on under Section 20. Grant schemes for insulation exist but come and go and are mostly aimed at whole buildings or lower-income households — worth checking, never worth planning around.

Get permission before you start

This is where flats differ completely from houses, and where good intentions turn into a breach of lease.

Almost every flat lease bars alterations to the structure and to the exterior without the freeholder’s prior written consent — a Licence to Alter. Windows are the classic trap: in many leases the window frames and the glass are not even part of your demise, so replacing them is not your decision at all. Our guide to what is actually yours in a flat explains where that line usually falls, and planning renovations in your flat covers the consent process itself.

On top of the lease, new windows may need planning permission if the building is listed or in a conservation area, and replacement glazing is notifiable under the Building Regulations. A block-wide change of window also has to look like a block-wide change of window — one flat in a different frame colour is a problem for everybody.

The short version: ask your managing agent, freeholder or RMC directors before you instruct anybody. It is a short email, and it is far cheaper than taking windows back out.

What to do next

• Work out first whether it is condensation or water getting in. If there is a defined wet patch, a tide mark or damp in summer, report it rather than ventilating it.
• Do the cheap things properly for a fortnight — lids, extractors, trickle vents, background heat, furniture off the wall — before spending anything.
• Treat existing mould with a fungicidal wash rather than paint.
• Get written consent before changing windows or anything else structural.

If we manage your building, report a problem or use the Gena Go app and send us photographs — we would far rather look at a damp patch in November than a rotten window frame in March. And if you are a director of a management company wrestling with damp complaints across several flats, talk to us: working out which of them are building faults and which are not is exactly the sort of thing a managing agent should be doing for you.