It’s February, there’s condensation streaming down your bedroom windows, and someone on a forum has told you to “just open a window.” Someone else says you need a dehumidifier. Your mum swears by leaving the bathroom door open after a shower. Everyone has an opinion, nobody agrees, and your windowsill is growing mould. So which actually works — spending £200 on a dehumidifier, or cracking a window and hoping for the best?
In This Article
- The Short Answer
- Why Your Home Has Too Much Moisture
- How Opening Windows Works
- When Opening Windows Makes Things Worse
- How Dehumidifiers Work
- When a Dehumidifier Is the Better Choice
- Running Costs Compared
- The Best Approach for UK Homes
- Room-by-Room Guide
- Common Mistakes That Make Humidity Worse
- When to Get Professional Help
- Frequently Asked Questions
The Short Answer
Neither option is universally better — they solve different problems, and in most UK homes you’ll end up using both. Opening windows is free and works well in dry, breezy weather. A dehumidifier works regardless of outdoor conditions and handles heavy moisture loads that ventilation alone can’t shift.
If your home has mild condensation in winter and you’re away during the day, a dehumidifier is the practical winner. If you’re home and the weather is reasonable, opening windows costs nothing and often does the job. The worst thing you can do is nothing — damp left untreated turns into mould, which turns into a health problem.
Why Your Home Has Too Much Moisture
Where the Water Comes From
A family of four generates roughly 10-15 litres of moisture per day just from normal life. That’s not an exaggeration — here’s where it comes from:
- Breathing and sleeping — about 0.5 litres per person per night
- Cooking — up to 3 litres per day (boiling pasta is a moisture bomb)
- Showering/bathing — about 1.5 litres per shower
- Drying clothes indoors — up to 5 litres per load on a clothes airer
- Kettles — around 0.5 litres per day in a tea-heavy household (so, every UK household)
Why Modern Homes Are Worse
Older homes with drafty windows and chimneys had natural ventilation that removed moisture constantly. Modern homes — double-glazed, insulated, draught-proofed — are sealed boxes by comparison. Great for heating bills, terrible for moisture management.
The irony is that the more energy-efficient your home becomes, the more likely you are to have damp problems. The UK government’s damp and mould guidance recommends maintaining indoor humidity between 40% and 60% for respiratory health, but many UK homes in winter sit well above that.
Condensation vs Rising Damp
Condensation damp (the most common type) is caused by moisture in the air hitting cold surfaces. It’s fixable with ventilation and dehumidification. Rising damp and penetrating damp are structural problems — water coming up through floors or through walls. If you have damp patches on ground-floor walls that don’t correspond to condensation points, or if paint is peeling at skirting board level, that’s likely structural and needs a builder, not a dehumidifier.
How Opening Windows Works
The Basic Principle
Warm air holds more moisture than cold air. When you open a window, the relatively drier outdoor air mixes with the humid indoor air, reducing the overall humidity level inside. At the same time, the moist indoor air escapes. Simple physics, no electricity required.
When It Works Best
Opening windows is most effective when:
- The outdoor humidity is lower than indoors — this is the key condition. Check with a cheap hygrometer (about £8 from Amazon UK) or a weather app
- There’s some wind — moving air exchanges moisture faster than still air
- Temperatures are above 5°C — below this, you’re losing so much heat that the trade-off isn’t worth it unless you have severe condensation
- You’re creating cross-ventilation — opening windows on opposite sides of the house creates airflow. One window alone doesn’t do much
The 15-Minute Rule
You don’t need to leave windows open all day. Short, sharp bursts of ventilation are more effective and waste less heat than leaving windows cracked for hours. Open windows on both sides of the house for 10-15 minutes, let the air exchange, then close everything up. The Energy Saving Trust recommends this approach as the most energy-efficient way to ventilate.
Do this after cooking, after showering, and first thing in the morning when overnight moisture has built up. Three or four ventilation bursts per day handle mild to moderate condensation in most homes.
When Opening Windows Makes Things Worse
Rainy and Foggy Days
If it’s raining, foggy, or outdoor humidity is above 85-90%, opening windows brings in air that’s already saturated with moisture. You’re making the problem worse, not better. On these days — and in the UK, that’s a significant chunk of winter — ventilation is counterproductive.
Very Cold Weather
Opening windows when it’s near freezing drops your indoor temperature rapidly. Cold walls and surfaces mean more condensation when you close the windows and the temperature rises again. You can end up with worse condensation than before you opened them.
Security Concerns
Ground-floor windows left open while you’re out or asleep are a security risk. Trickle vents (those small ventilation slots built into double-glazed windows) help here — they allow some air exchange without creating a security issue. If your windows don’t have trickle vents, retrofitting them costs about £15-30 per window.
Noise and Pollution
If you live on a busy road, opening windows means traffic noise and outdoor pollution coming in. In urban areas, outdoor air quality can be poor enough that you’re solving one problem (humidity) while creating another (particulate exposure). In these situations, a dehumidifier combined with trickle vents or a mechanical ventilation system makes more sense.
How Dehumidifiers Work
Compressor (Refrigerant) Dehumidifiers
The most common type. A fan draws humid air over cold coils, moisture condenses on the coils (like on a cold glass), and the water drips into a tank. The dried air passes over warm coils and returns to the room slightly warmer than it entered.
These work best at temperatures above 15°C and humidity above 45%. Below that, they become less efficient because the coils can ice up. Most modern units have auto-defrost, but performance drops noticeably in cold rooms.
Desiccant Dehumidifiers
These use a rotating desiccant wheel to absorb moisture instead of cold coils. They work at much lower temperatures (down to 1°C) and are quieter than compressor models, but they use more electricity and output warmer air.
Desiccant dehumidifiers are the better choice for unheated spaces — garages, conservatories, utility rooms that aren’t centrally heated. For heated living spaces, compressor models are more energy-efficient.
What They Don’t Do
A dehumidifier removes moisture from the air, but it doesn’t fix the source of the moisture. If your problem is a leaking roof, failed damp-proof course, or chronic under-ventilation, a dehumidifier will manage the symptoms without solving the cause. Think of it as treatment, not cure — it buys you time and comfort while you address the root issue.
When a Dehumidifier Is the Better Choice
Heavy Moisture Loads
If you’re drying clothes indoors, a dehumidifier isn’t optional — it’s essential. A single load of washing releases up to 5 litres of water into your home. No amount of window-opening will handle that quickly enough to prevent condensation, especially in winter when you can’t leave windows open for hours.
You’re Out During the Day
A dehumidifier runs while you’re at work, maintaining humidity levels without requiring open windows (and the security risk that comes with them). Most modern units have a humidistat that switches the unit on and off automatically to maintain a target humidity — set it to 50-55% and forget about it.
Persistent Mould Problems
If you’re already seeing mould on walls, window frames, or furniture, you need consistent humidity control. Opening windows is too inconsistent — it depends on weather, on remembering to do it, on being home. A dehumidifier delivers steady results.
Once mould is established, you need to treat the existing mould (bleach solution or specialist mould spray, then dry thoroughly) and then prevent it returning with consistent humidity management.
Allergy and Asthma Sufferers
Dust mites thrive above 60% relative humidity. If someone in your household has asthma or dust mite allergies, maintaining humidity below 55% makes a measurable difference to symptoms. Asthma + Lung UK recommends dehumidification as part of managing indoor triggers.
Running Costs Compared
Opening Windows
Free. No running cost, no purchase cost. The indirect cost is heat loss — you’ll use slightly more gas or electricity to reheat the room after ventilation. In practice, the 15-minute burst approach minimises this to pennies per day.
Dehumidifier Running Costs
A typical compressor dehumidifier draws 200-500 watts depending on size. Running a 300-watt unit for 8 hours per day at current UK electricity rates (about 24p per kWh) costs roughly:
- 300W × 8 hours = 2.4 kWh per day
- 2.4 kWh × 24p = about 58p per day
- Monthly cost: about £17-18
Desiccant dehumidifiers draw more (400-700W typically), so expect £25-30 per month for the same usage. In practice, most units don’t run continuously — the humidistat switches them off once the target humidity is reached, which can halve the actual running cost.
Purchase Cost
A decent compressor dehumidifier costs £150-250 for a home unit. Budget models exist for £80-120 but tend to be noisy and have small tanks. The running cost over a winter season (October to April) is about £100-120, so the total first-year cost is £250-370.
The Best Approach for UK Homes
Use Both
The honest answer is that most UK homes benefit from using both ventilation and dehumidification, depending on the situation:
- Morning: open bedroom windows for 10-15 minutes after waking to flush overnight moisture
- After cooking: use the extractor fan and crack a kitchen window for 15 minutes
- After showering: run the extractor and leave the bathroom window open briefly (door closed to contain moisture)
- Drying clothes: dehumidifier running next to the clothes airer, windows closed
- During the day (out): dehumidifier on auto, windows closed
- Rainy days: dehumidifier only, windows closed
Measuring What’s Working
Buy a digital hygrometer (£8-15 from Amazon UK). Put it in the room you’re most concerned about and check it daily. You’re aiming for 40-55% relative humidity. If you can maintain that range with ventilation alone, you don’t need a dehumidifier. If it’s consistently above 60% despite regular ventilation, a dehumidifier will solve it.
Room-by-Room Guide
Bathroom
The biggest moisture generator in most homes. An extractor fan (running for at least 15 minutes after each shower) plus an open window is usually enough. If your bathroom doesn’t have an extractor fan or a window, a small dehumidifier (6-8 litre capacity) running after showers keeps mould at bay.
Keep the bathroom door closed during and after showering to contain the moisture. Opening the door lets steam spread to other rooms, where it condenses on cooler surfaces.
Kitchen
Use the cooker hood extractor every time you cook, especially when boiling water. Open a window for 15 minutes afterwards. Lids on pans reduce steam production massively — I’ve measured it with a hygrometer, and a rolling boil with no lid pushes kitchen humidity above 80% within minutes. A covered simmer barely moves the needle.
If you have a tumble dryer, make sure it’s vented outside. Condenser dryers release heat but not moisture; vented dryers release both outdoors.
Bedroom
You produce about half a litre of moisture per night just from breathing. Open windows for 10-15 minutes each morning. If you notice condensation on bedroom windows, a small dehumidifier running at night (choose a quiet model — under 35dB) prevents it.
Avoid drying clothes on bedroom radiators. That single habit is responsible for more bedroom mould than almost any other factor.
Utility Room / Drying Room
If you regularly dry clothes indoors, this room needs a dehumidifier. Full stop. The moisture output from a drying load is too high for ventilation alone to manage, and if you’ve got poor ventilation in a utility room, the mould will be spectacular.
Garage / Conservatory
Unheated spaces need desiccant dehumidifiers (compressor models can’t cope below 15°C). Alternatively, leaving a window slightly ajar in a garage provides enough ventilation in most weather — the contents are less moisture-sensitive than living spaces.
Common Mistakes That Make Humidity Worse
Drying Clothes on Radiators
Each load releases up to 5 litres of water into the room. The radiator accelerates evaporation, which fills the air with moisture faster than ventilation can remove it. Use a dehumidifier next to a free-standing airer instead — it dries clothes faster too.
Blocking Trickle Vents
Those little sliders at the top of double-glazed windows? They’re trickle vents, and they’re there for a reason. Closing them in winter because “they let cold air in” removes your home’s passive ventilation. Leave them open.
Running the Bathroom Extractor Only During Showers
Moisture lingers in the air long after the shower stops. Run the extractor for at least 15-20 minutes afterwards, or leave it on a humidity-sensing switch that runs automatically until the moisture level drops.
Pushing Furniture Against External Walls
Air can’t circulate behind a sofa or wardrobe that’s flat against a cold external wall. The wall surface stays cold, moisture condenses behind the furniture where you can’t see it, and by the time you notice the mould, it’s established. Leave a 5-10cm gap between furniture and external walls. I’ve seen homes where pulling a wardrobe away from an external wall revealed mould patches covering half the wall — all because there was no airflow.
Over-heating and Under-ventilating
Cranking the heating up in a sealed house doesn’t reduce humidity — it increases the air’s capacity to hold moisture, but that moisture doesn’t go anywhere. When the heating goes off overnight and surfaces cool, all that moisture condenses at once. Moderate, consistent heating with regular ventilation is better than big temperature swings.
When to Get Professional Help
Signs You Need a Surveyor
- Damp patches on ground-floor walls (possible rising damp)
- Water stains on ceilings or upper walls (possible penetrating damp or roof leak)
- Persistent mould that returns within weeks of cleaning, despite good ventilation and dehumidification
- Damp that’s concentrated on one wall and doesn’t match condensation patterns
- Musty smell that never goes away
Who to Call
A qualified damp surveyor — not a “damp-proofing company” that makes money selling damp-proof courses. Independent surveyors (RICS registered) will diagnose the actual problem and recommend appropriate solutions. The Property Care Association maintains a directory of qualified damp specialists.
Expect to pay £150-300 for an independent damp survey. It’s worth the cost to avoid unnecessary damp-proofing work, which can run into thousands and sometimes makes the problem worse when the diagnosis was wrong.
Frequently Asked Questions
Should I leave windows open at night for ventilation? In summer, yes — overnight temperatures are mild enough, and it helps flush moisture from breathing. In winter, the heat loss usually isn’t worth it. Use trickle vents instead, or run a dehumidifier overnight. If you do open a window at night in winter, just a small gap is enough — you don’t need it wide open.
Will a dehumidifier help with condensation on windows? Yes. Window condensation happens when moist indoor air hits the cold glass surface. A dehumidifier reduces the moisture in the air, which directly reduces condensation. You should see a noticeable difference within 24-48 hours of running one consistently.
How do I know if I need a dehumidifier or better ventilation? If your indoor humidity stays above 60% despite regular ventilation (10-15 minutes of open windows, three or four times a day), you need a dehumidifier. A digital hygrometer (about £8-15) tells you exactly where you stand — it takes the guesswork out of it.
Are desiccant dehumidifier bags worth buying? The small disposable moisture absorber bags (DampRid, UniBond) remove tiny amounts of moisture — about 100-200ml over several weeks. They’re fine for a wardrobe or a small cupboard, but they won’t make a meaningful difference to room humidity. For rooms, you need an electric dehumidifier.
Can opening windows cause more damp in winter? It can if outdoor humidity is higher than indoor humidity — check a weather app before opening up. On rainy, foggy, or very humid days, keep windows closed and use a dehumidifier instead. On dry, crisp winter days, ventilation works well.