Quick answer

When to start using a dehumidifier in the UK is not a date: it is the first morning you find water on the inside of your windows, anywhere from late September to late November depending on the building. A hygrometer above 60 per cent through the day says the same weeks earlier. Check it is condensation first: water on cold glass on cold mornings is. A patch still damp in July, at a fixed height, or worsening after rain is a building fault, and a dehumidifier makes it worse.

Table of contents

The signals that say the season has started

Search for a start date and almost every answer is a month. October, usually. Not wrong so much as not yours: a solid-wall 1930s semi and a well-insulated flat two streets away start weeks apart.

The window check

The trigger is not a temperature outside but a temperature on a surface inside. Air holds more water vapour before it saturates when warm than when cold, so when a heated room's air meets glass that has shed heat all night, the air at the pane falls below its dew point and the vapour turns back to liquid.

A room at 20C holding 60 per cent relative humidity has a dew point near 12C, so any colder surface beads up. Drop the room to 50 per cent and the dew point falls to about 9C, which most double glazing stays above on a mild October night. Single glazing is closer to that line than most people assume, but not usually below it: at 20C inside and 8C outside the middle of the pane is near 11C, and it drops under only when the night turns properly cold. The glass edge and an uninsulated aluminium frame run colder, and that is where the first water shows.

So the check takes ten seconds. Before the curtains open, run a finger down the inside of the coldest window in the house. Dry glass, not started. Mist, it just has. Water on the sill, a while back.

What a hygrometer adds

A hygrometer tells you what the glass cannot: whether the air is loaded all day, or only in the hour after everyone wakes up.

CIBSE, the UK building services institution, points at 40 to 60 per cent relative humidity in dwellings. Meaco's managing director Chris Michael, quoted by Expert Reviews, separately gives 50 to 55 per cent and puts the onset of mould growth around 68 per cent: a manufacturer's figures, not a standards body's. Our reading: 55 is the winter working target, 60 is where to start paying attention, above 65 for days is a mould risk whether or not a spot has appeared.

Where the meter sits matters more than which one you buy: a warm living room reads fine while the corner behind the wardrobe runs ten points higher. Put it in the room that smells.

The other four signals

  • Washing that used to dry overnight on the airer now takes two days.
  • A musty smell in one room, usually the coldest bedroom or a spare room kept shut.
  • Black speckling in the top corners of external walls, or a dark bloom behind a sofa or wardrobe.
  • Cold metal going damp: window catches, radiator valves, the inside face of an external door.

One of those justifies a hygrometer. Two together and something is wrong, but the musty smell, the dark bloom and the slow-drying washing come just as readily from a leak or from damp coming through the wall. Settle which of those you have in the next section before you spend anything.

Condensation, or a problem no machine will fix

A dehumidifier only addresses one of the four kinds of damp. The UK government's damp and mould guidance for rented housing splits them like this.

Condensation is moisture generated inside the home landing on colder parts of the building: window frames, corners, walls behind sofas and wardrobes. The guidance calls it the most common form, and it is the one a dehumidifier helps with: seasonal, worst in the coldest rooms, obvious in the morning.

Penetrating damp is water getting in from outside through a defect in a wall, roof, window or floor. Rising damp is ground moisture drawing up through walls and floors in contact with the ground, usually in older properties and, the guidance notes, often misdiagnosed. Traumatic damp is a leak: pipework, an overflowing bath, a burst main, a failed tank.

The tell is where and when. Condensation forms on cold surfaces on cold mornings and fades in summer. Aim a dehumidifier at a patch still damp in July and you dry the room air, pull more water out of the wall to replace it, and leave the defect working all winter.

New builds are their own case. A home finished in the last year or two is still giving up construction water from concrete, screed and plaster, and it streams through its first heating season whatever the household does. Expect that rather than a fault, and hold the machine at a moderate setting rather than a very low number: drying fresh plaster too fast cracks it. Still streaming in a second winter is one for the developer, not a bigger machine.

In a rented home, report it in writing and keep a copy, and name anyone in the house who is in one of the at-risk groups below: that can move it onto the 24-hour emergency limb rather than the 10-working-day one.

  • England, social tenants. Awaab's Law, in force since 27 October 2025: investigate within 10 working days; then a written summary within 3 working days and the relevant safety work completed within 5, those two running concurrently rather than in sequence; emergencies investigated and made safe within 24 hours. Safety work that cannot be completed in time obliges the landlord to secure suitable alternative accommodation at their expense until it has been. From 30 November 2026, subject to parliamentary approval, the same timescales cover further hazards including excess cold.
  • England, private tenants. Not yet. The Renters' Rights Act 2025 provides for extending it to the private sector but sets no date, so the route meanwhile is the council, which holds the enforcement powers.
  • Scotland. From 6 October 2026 the Investigation and Commencement of Repair (Scotland) Regulations 2026 cover private residential tenancies as well as Scottish secure tenancies, add being substantially free from damp and mould to the repairing standard, and set 10 working days to investigate, 3 for the written summary and 5 to commence relevant safety work. England says completed; Scotland says commenced. Enforcement is the First-tier Tribunal (Housing and Property Chamber), not the council.
  • Wales and Northern Ireland. Different again: start with your council or local authority.
  • Owner-occupiers get no route at all, which is the argument for an independent damp survey when the diagnosis is not obvious. Use a surveyor who does not also sell the treatment.

Health comes before hardware. The NHS names babies, children, older people, people with existing skin problems such as atopic eczema, people with respiratory problems such as allergies and asthma, and people with a weakened immune system as most affected; the government's guidance separately adds pregnant women, people with a mental health condition, and people who are bedbound or housebound. It also calls it "totally unreasonable to blame damp and mould in the home on 'lifestyle choices'".

Coughing or wheezing is a GP appointment, not a wait to see whether a machine helps. A wheeze or breathlessness getting worse, or a reliever inhaler not lasting as long as it should, is a same-day appointment or 111 out of hours. A reliever not working at all, or someone who cannot finish a sentence, is 999. A dehumidifier manages a symptom, not a building.

Month by month through a British damp season

The condensation season runs roughly October to March, the stretch when outdoor temperatures hold indoor surfaces below the dew point of indoor air.

August and September

Nothing yet in most homes, and that is the honest answer. Warm nights keep glass above dew point, so use the time: buy the hygrometer, check the extractors pull by holding loose paper to the grille, leave the trickle vents open, pull furniture clear of external walls. If the paper barely moves, or the fan does not run, that is a repair rather than a habit, and in a rented home it is the landlord's: report it in writing. Where a flat has a whole-home ventilation system rather than a fan per room, do not seal vents to help a dehumidifier along, have the system checked.

One kind of home should not wait. A ground-floor or coastal flat, a bungalow, a suspended timber floor or anything with a cellar can hold above 60 per cent through a warm August, with a musty back room and no misted glass anywhere, because the water comes up from below rather than condensing on cold glass. If that is your meter reading now, your season has already started.

October

The month everyone names, and for many UK homes roughly right. Once the heating cycles and overnight lows drop into single figures, the first misted panes appear, usually in a bedroom: bedrooms run cooler and two sleeping adults put out a lot of vapour. Start on the first morning you see it, not the first of the month.

November

Peak. Cold enough that every external surface is a candidate, wet enough that opening a window stops appealing, dark enough that washing goes on the airer indoors. The month a dehumidifier earns its place.

December to February

The coldest surfaces of the year, and where the type of machine decides the outcome. An unheated garage, conservatory or shut-up back bedroom can sit below 10C, and a standard compressor collects very little there. So can a whole house heated for a few hours a day, or barely at all. In a room that really is at 18C to 21C, leave the target at 55 per cent and let the humidistat cycle; in a home at 12C to 15C throughout, a compressor will disappoint you whatever you set it to, and a desiccant is the machine that works.

March onwards

Ease off, but not on a date. The signal reverses the way it arrived: a fortnight of dry glass every morning, with the hygrometer under 55 per cent unaided, means your season is over. That can be early March on the south coast and late April in a Pennine cottage. Three summer exceptions justify switching back on: drying washing indoors through a wet spell, a room just plastered, and the aftermath of a leak.

Setting one up so it actually works

Most disappointment comes from running the machine in a way that could not have worked.

Match the technology to the room temperature

This is the specification that decides whether a dehumidifier does anything in a British winter. Compressors chill a coil and condense water onto it, so the room has to be warm enough for air to give up moisture at that surface. Desiccants pass air across a moisture-absorbing wheel and warm it to drive the water off, so cold rooms do not trouble them.

Meaco, a UK manufacturer, publishes the thresholds plainly: below 10C use a desiccant, between 10C and 15C a desiccant or a large compressor, above 15C either. Meaco puts compressor draw at roughly 150 to 260 watts against about 330 for a desiccant, though that 330 is a desiccant's lowest setting: its own DD8L Pro is listed at 231, 437 and 623 watts across three fan speeds. A January garage is desiccant territory, where the warm discharge air is a bonus rather than a nuisance.

Buying one machine for a home that really does sit at 18C to 21C with the heating on, we would take a compressor with a humidistat and a laundry mode. Check that with a thermometer first, because plenty of UK homes are not that home: a solid-wall cottage, a house heated one room at a time, or any household keeping the heating down can run at 12C to 15C all winter, and by the bands above that is desiccant territory, not compressor territory. A desiccant does draw more power per hour, which is the honest cost of one that collects anything in a cold room.

Run it as a closed loop

  1. Pick the room that shows the problem first, not the largest room in the house.
  2. Shut the door and the windows. A dehumidifier drawing on outdoor air runs forever and achieves nothing; it needs a closed room. Trickle vents are not the same as an open window and stay open. Never block a permanent air vent or air brick: if the room has a gas fire, back boiler or stove, that vent feeds it, and covering it risks carbon monoxide. Fit a carbon monoxide alarm in any room with a fuel-burning appliance.
  3. Stand it roughly central, clear of walls and furniture, intake and outlet unobstructed.
  4. Set the humidistat to 55 per cent and let it cycle. Chasing 40 wastes electricity and dries out timber, plaster and skin for nothing extra.
  5. Empty the tank before bed and again first thing; a machine that filled at three in the morning missed the coldest hours. Continuous drainage is better where the layout allows, but a hose only runs downhill, so the unit must stand higher than whatever it drains into, and most UK bedrooms have nothing to drain into. Route any hose where nobody will walk into it in the dark, and never put the machine in a bathroom: UK wiring rules do not allow a 13A socket in one.
  6. For laundry, use the laundry mode in the smallest room you can shut, airer beside the machine.

Warm the surface as well as drying the air

A dehumidifier attacks one half of the dew point equation. The other half is surface temperature: a bedroom held at 16C overnight rather than switched off keeps its glass above dew point when an unheated one will not. Which way round is cheaper depends on whether that room can be heated on its own. Most UK homes are a single heating zone, so holding one bedroom at 16C all night means heating the whole house all night, which will generally cost more than a dehumidifier cycling at 150 to 260 watts. Where the room can be heated by itself, warming the surface treats the cause rather than the symptom. Smart thermostatic radiator valves are the usual route to per-room control, though fitting them to older UK radiators has a catch worth reading first, and re-valving a house is not a cheap afternoon.

The rest is free: lids on pans, the extractor left running after a shower, an extra spin before anything goes on the airer. Worth saying plainly: two adults putting a few litres of vapour into a home each day is normal occupancy, and a building that cannot cope with normal occupancy has a fabric or ventilation fault. These habits shave the load; they do not fix a cold wall.

Where people get this wrong

  • Using it against a leak. If the wall is wet while the air is not, the machine is the wrong tool and the defect is still spreading.
  • Running it with a window open. Ventilation and dehumidification both work, just not in the same room at the same time. The one exception is the mould clean below: while you are cleaning, that window is open for your lungs and the machine is off.
  • Blocking a vent to seal the room. Sealing means the door and the windows, never a permanent air vent or air brick, which may be the air supply for a gas fire, back boiler or stove.
  • A compressor in a cold space. In a 6C garage it spends more of its time defrosting its own coil than dehumidifying, so it drips a little and collects almost nothing. That is the technology working as designed rather than a fault, and exactly why buying one for a cold home is the expensive mistake.
  • Chasing a very low number. Below roughly 40 per cent you get static, cracked lips, shrinking timber and no extra protection against mould.
  • Cleaning mould yourself past the point where you should. Brushing or hoovering a dry patch throws spores around the room. Government guidance is that a tenant may reasonably remove only very small amounts, larger areas belonging to qualified professionals, so a patch bigger than your hand is one to report rather than clean. Within that limit: an FFP2 or FFP3 mask, not cloth or surgical, plus gloves and goggles; that room's window open during the clean and afterwards, door shut so spores do not travel; a mould and mildew product in preference to bleach, never two cleaners mixed; a damp cloth, then bin it. If you are in one of the at-risk groups above, ask someone else to do it.
  • Never turning it off. Running through a dry summer out of habit burns electricity for nothing. The spring signal is the autumn one read backwards.

Frequently asked questions

Should I run a dehumidifier all night?

Check the manufacturer's instructions first: Electrical Safety First advises against running a dehumidifier overnight or while you are away from home unless the maker says it is safe for unattended use. If yours is cleared for it, plug it straight into a wall socket rather than a coiled extension reel. Then the seasonal logic applies: the coldest hours are the ones that produce the condensation, so a machine that only runs while you are awake misses them. If it is not cleared, run it in the evening with the door shut and switch it off at bedtime.

Is it too late to start in December or January?

No. The damp season still has months to run, and a saturated room takes days to bring down whenever you begin. You are only starting from wetter air.

Which is better for a UK winter, a compressor or a desiccant dehumidifier?

Room temperature decides it, not the season. Meaco gives three bands rather than one line: below 10C a desiccant, 10C to 15C a desiccant or a large compressor, above 15C either. Meaco also says the line has blurred, and that its large low-energy compressors now compete with desiccants at 15C. Below 10C the case for a desiccant is still clear-cut.

My windows are wet but the hygrometer reads 55 per cent, what is going on?

The meter reads air in the middle of the room while condensation forms at the glass, several degrees colder. At 20C and 55 per cent the dew point is near 11C, so a pane at 8C still streams.

How long before a dehumidifier makes a difference?

Windows usually stay clearer within a day or two once the room holds under about 55 per cent. Wet plaster or a wall drying after a leak takes weeks, and mould stains still need cleaning.