Steam on the Glass? A Room-by-Room Plan to Stop Condensation Taking Hold This Winter

November has a habit of announcing itself on the inside of your windows. The heating comes on, the kettle goes on, and by mid-morning the bedroom glass is wearing a film of water that was nowhere to be seen in September.

That is condensation, and it is rarely a sign that anything is broken. It is a sign that moisture is being produced faster than your home can shift it. Once you accept that, the fix stops being mysterious and starts being a matter of routine.

Why the glass streams and the walls follow

Warm air holds more water vapour than cool air. Every shower, every pan of pasta, every load of laundry dried on a radiator and every breath you take adds to the total. In a busy family home you are looking at something in the region of ten to fifteen litres of water vapour a day, which is a small bucket's worth floating around the place.

When that moist air meets a cold surface it cools, gives up the water it can no longer hold, and leaves it behind as droplets. Windows are simply the coldest thing in the room, so they get it first. The real worry is what happens on the surfaces you cannot see: the corner behind the wardrobe, the plaster above the window reveal, the skirting on a north-facing wall. Keep those damp for long enough and black mould moves in for the season.

Start by knowing your numbers

Before you buy anything, buy a hygrometer. They cost about the same as a decent sandwich and take the guesswork out of the whole exercise. Aim for somewhere between 40 and 60 per cent relative humidity in living spaces. Below about 40 per cent and the air starts feeling dry and scratchy; consistently above 70 per cent and you are watering the mould.

Take your readings on a cold morning, an hour after the heating has come on. That is when the problem is at its most visible.

The kitchen: catch it at the hob

Extraction is the single most underused bit of kit in a British kitchen. Put the fan on before you start cooking, not halfway through when the windows fog up, and let it run for ten minutes after you have finished.

Two checks worth making this week. First, does the extractor actually vent outside, or is it a recirculating model quietly pushing steam back into the room? Second, if it is ducted, is the duct clear and the filter washed? A clogged grease filter will halve the flow and double the noise.

Pans with lids and a window cracked a couple of centimetres while you cook will handle the rest. A big Sunday roast deserves a full purge: doors and windows wide for five minutes afterwards, then shut everything down again.

The bathroom: twenty minutes of discipline

Run the extract fan during the shower and for a solid twenty minutes after. Keep the door shut while it does its work, otherwise you are simply moving the moisture into the hallway.

Hold a piece of tissue against the fan grille: if it stays there, you have decent extraction; if it flops back, the fan is decorative. Cheap fans are noisy, and noisy fans get switched off, which is why humidity-sensing or overrun-timer models are worth the extra twenty pounds. If your fan duct runs through a cold loft, make sure it is insulated, or the steam will condense inside the pipe and drip back down through the ceiling.

Squeegee the shower screen, leave the bath to cool with the door ajar, and consider a lick of mould-resistant paint rather than blaming the plaster when it goes black.

Bedrooms: the surprise culprit

People are perplexed when a bedroom, with no shower and no hob, still mists up. Two adults breathe out a fair amount overnight, and plenty of us dry laundry on the radiator in there as well.

Keep beds and wardrobes a hand's width away from external walls so air can move behind them. Leave the trickle vent open through the night, even in cold weather; that slow, steady exchange of air is doing real work. In the morning, throw the window wide for ten minutes while you get dressed. It clears the night's moisture in one go and costs far less heat than leaving a window ajar for eight hours.

Living rooms and hallways: the forgotten corners

These are the rooms where furniture sits against cold walls and where trickle vents get covered by curtains, cushions or, in some memorable cases, a bookcase. Pull the sofa forward, unhook the curtains, and check that every background vent in the house is actually open rather than jammed shut by a previous owner worried about draughts.

If the hallway is the coldest spot in the house, it is also the spot where moist air from every other room will dump its water. A small decentralised heat recovery unit, fitted into an external wall and running at a trickle, can quietly transform a hallway without any duct runs.

Laundry is the elephant in the room

Drying clothes indoors adds several litres of water to a home in a single day. That is more than the bathroom and kitchen put together. If you have outdoor space or a dryer, use it. If you have a condenser dryer, check the tank; if you have a vented one, check the hose goes outside and not into the loft.

For the unavoidable rainy-week wash, shut it in one room with the extract fan running or a dehumidifier on, rather than spreading it across a whole flat.

Cracked windows, extract fans or continuous ventilation?

All three work. They work at different scales and different price points, and most homes end up using a mixture.

Crack a window is free and instant, brilliant for a quick purge after cooking, and useless the moment you leave the house or the weather turns horizontal. It also costs you heat, which is why it should be short and sharp rather than a permanent two-finger gap.

Extract fans are targeted and cheap to run. A modern bathroom fan pulling twenty watts for half an hour a day is comfortably under a pound a year in electricity. They only fix the room they are in, though, and only if the duct goes somewhere useful.

Continuous ventilation is the grown-up option. Positive input ventilation in a loft pushes filtered air through the landing; a decentralised unit does the same job for one room; whole-house mechanical ventilation with heat recovery handles the whole property properly and recovers most of the heat from the air it expels. Costs run from a few hundred pounds for the simplest unit to several thousand for a fully ducted system, and it needs filters changing and commissioning done properly.

The heat loss question, answered honestly

Every air change takes warmth with it, so it is fair to ask why you would deliberately open a vent in January. The answer is that a slow, controlled trickle of background ventilation moves a lot less heat than people assume, and it is far cheaper than dealing with mould, redecorating and the health costs of persistently damp air. Heat recovery ventilation deals with the objection almost entirely, since it hands most of the warmth back to the incoming air.

Set against that, a dehumidifier drawing three hundred watts for hours on end is not the bargain it looks like. Use ventilation as the strategy and a dehumidifier as the occasional tool.

A five-minute daily routine

• Trickle vents open in every habitable room, all winter
• Fan on before cooking, twenty minutes after a shower
• Window thrown wide for ten minutes each morning
• Lids on pans, laundry outside where possible
• Furniture pulled clear of external walls
• Glass wiped down, grilles dusted, hygrometer glanced at

Do those and the windows will still mist on the coldest mornings, because physics is physics. But they will clear, the walls will stay dry, and you will not spend February scrubbing black spots off the paintwork.