Do I need a humidifier, and what does it actually do?

Humidity in a British house is usually a problem of excess, not shortage. That is why the country's damp and condensation guidance is all about removing moisture. The exception is real but narrow: a well-sealed, well-heated home during a cold spell, where heating cold outside air that was already carrying little water drives the indoor relative humidity down. In that specific situation a humidifier makes the room noticeably more comfortable. Outside it, adding water to indoor air in a British winter is how you get streaming windows and mould in the corners.

Safety

What you'll need

• A hygrometer, before anything else
• Clean water as specified by the manufacturer
• Distilled or filtered water, in a hard water area (optional)

Step by step

1. Measure the humidity before deciding anything.A cheap digital hygrometer, left in the room for a day. Comfortable and healthy is broadly 40 to 60 percent relative humidity. Above 60 sustained is where dust mites and mould thrive. Below about 35 is where dry-air symptoms start. Without this reading the whole decision is guesswork.
2. Check whether your symptoms actually indicate dry air.Genuine dry-air signs: static shocks off door handles and carpets, a dry nose or throat that eases outside the house, timber furniture or a wooden instrument developing cracks, houseplant leaf tips going brown and crisp. A dry throat alone is not enough — heating, sleeping with the mouth open and a cold all do the same thing.
3. Rule out the far more common opposite first.Wet windows in the morning, black spotting in corners or behind furniture, a musty smell, or washing taking days to dry all mean you have too much moisture. Adding more will make every one of those worse. If any of these are present, stop here.
4. Try the free measures before buying a machine.Turning the heating down a degree raises relative humidity without adding any water. So does drying a little washing indoors, leaving the bathroom door open after a shower, and keeping a few houseplants. In marginal cases these are enough.
5. Choose the type on safety and maintenance rather than on features.Evaporative: a fan blows through a wet wick, self-limiting so it cannot over-humidify a room, no white dust, wick needs replacing. Ultrasonic: silent and cheap to run, but throws minerals from hard water into the room as fine white dust and will over-humidify if left unattended. Warm mist: boils the water so it is the most hygienic, uses far more electricity, and is a scald hazard around children.
6. Set a target and use a humidistat, not a timer.Around 45 percent is a sensible target. A machine with a built-in humidistat stops there. One without will keep going until the room is saturated and the windows run — which in a British winter takes surprisingly little time.
7. Treat the tank as the main risk and clean it accordingly.Standing warm water grows bacteria, and an ultrasonic or evaporative machine then aerosolises it into the room. Empty and dry it daily when in use, clean and descale it weekly as the manual directs, and never top up on top of old water.
8. Use the right water for your area.Hard tap water in an ultrasonic model produces the fine white mineral dust that settles on every surface in the room and is drawn into the lungs. Distilled or filtered water, or an evaporative model, avoids it. This is a genuine reason to pick evaporative in a hard water area.
9. Watch the windows for the first few nights.Condensation on the inside of the glass in the morning means you have gone too far, whatever the hygrometer said in the middle of the room. Turn the target down, and remember the coldest surface in a room always reaches saturation first.
10. Stop using it when the weather turns.The need is seasonal, and generally limited to a cold, dry, hard-heating spell. Empty it, dry it fully and put it away rather than leaving water in it for months.

Tips

• The hygrometer is the purchase worth making whether or not you buy a humidifier. It settles both this question and the dehumidifier one, and it costs very little.
• For a single sleeper with a dry throat, a bowl of water on a radiator or simply a lower bedroom temperature is often enough and cannot over-humidify anything.
• Evaporative models are self-limiting, which is why they are the safer choice in a house where the machine may be left running unattended.
• In a bedroom, keep the machine well away from the bed and off soft furnishings. Fine mist landing on a mattress or headboard is how a humidifier causes mould at very close range.

Common mistakes

• Buying a humidifier without measuring the humidity — Most British rooms are above 50 percent already. Adding moisture to a room that has enough causes condensation on cold surfaces and mould in corners, which is a far harder problem to undo than a dry throat.
• Using hard tap water in an ultrasonic model — The dissolved minerals are aerosolised as fine white dust that coats the room and is breathed in. Use distilled or filtered water, or buy an evaporative machine instead.
• Topping up the tank without emptying it — Old standing water is where bacteria and biofilm grow, and the machine then sprays that into the air you breathe. Empty, rinse and refill.

Alternatives

• Turn the heating down one degree: Raises relative humidity immediately at no cost and with no machine, because the same water in the air is a larger fraction of what cooler air can hold.
• A bowl of water on a radiator, or drying a little washing indoors: Adds moisture slowly, cannot over-humidify a room, needs no power and cannot grow anything that gets sprayed at you.
• Deal with draughts instead: Much dry-air discomfort in winter is actually cold draughts across the face. Draught-proofing addresses it and saves money at the same time.

Questions people ask