Ventilation or a dehumidifier: which one your flat needs

Follow the water and the difference is obvious

Ask one question of each machine: where does the water end up?

With ventilation, it ends up outside the building. Outdoor air is admitted in the dry rooms — bedrooms and living room — crosses the flat, and is drawn out through the wet rooms, the bathroom and the kitchen. The vapour leaves with the air that was carrying it, along with everything else that air had picked up.

With a dehumidifier — deshumidificador on the box in a Catalan or Spanish shop — the water ends up in a tank in your hallway. The machine pulls in the air that is already in the room, passes it over a coil cold enough for part of the vapour to condense, drips that water into the tank, and returns the same air to the room a little drier and a little warmer. Nothing enters the room and nothing leaves it, except when you carry the tank to the sink.

An exchange, then, against a closed loop. Everything that follows is a consequence of that one difference: in a home with people in it, ventilation is the work and the dehumidifier is a helper; in a space with nobody in it, that can invert completely.

Water is only one of the things you want gone

A cold coil is very good at condensing water vapour and useless against anything that stays a gas at that temperature. Carbon dioxide stays a gas. So do the smell of last night's cooking, the fumes off a newly painted wall, and the flat, used quality of air in a bedroom that has been closed since midnight. All of it goes round the loop and comes back out at the other end of the machine.

This is why Spain's building code approaches the subject as renewal rather than drying. The Código Técnico de la Edificación deals with dwellings in the section written on plans as DB-HS3, and it organises a home into rooms where outdoor air is admitted and rooms from which air is extracted, with transfer openings in between so that the air can travel from one to the other. The kitchen additionally needs its own extraction provision for cooking fumes, separate from the general ventilation of the dwelling. There is no point in that arrangement where an appliance that gives the air back could stand in.

Put practically: a dehumidifier can hold a closed bedroom's humidity down all night, and the room will still be stale in the morning. Staleness is not made of water.

Which one leads, situation by situation

Situation	What leads	Why
A flat that is lived in — showers, cooking, people	Ventilation	Moisture and everything else are being produced continuously, so they have to be removed continuously
A second home shut between visits on the coast	Dehumidifier	Nobody is producing CO₂ or smells; the only variable is water, and the outdoor air may be no drier
Laundry drying indoors	Both, in that order	Contain the source in one room first, then either extract from that room or dehumidify it with the door shut
One cold room in an otherwise warm flat	Neither, on its own	The problem is a cold surface and a closed door, not the average humidity of the dwelling
Damp coming from the fabric — a leak, rising damp, rain	Neither	Water keeps arriving; a machine only takes away the part of it that reached the air

The cold-room row is the least intuitive of the five, so it is worth expanding. A back bedroom on a patio de luces — the interior light well, pati de llum in Catalan — is colder than the rest of the flat and its air is barely moving. A dehumidifier standing in it will lower the reading while it runs, but the wall stays exactly as cold as it was, and every time the door opens the room takes another delivery of moist air from the flat. What changes that room is heat spread more evenly, a way for its air to move, and furniture off the cold wall.

The empty flat on the coast, where the machine takes over

This is the case that justifies owning one.

The Catalan coastal climate gives mild, humid winters and hot, humid summers. Throw open a flat that has been shut for six weeks in August and you swap still, musty indoor air for fresh outdoor air, which is worth doing on its own terms — but you may not be drying anything, because the air arriving can be carrying as much water as the air leaving. Ventilation removes moisture by dilution, and dilution only works if what comes in is drier than what goes out.

Meanwhile the reasons ventilation is normally indispensable have gone quiet. Nobody is breathing in the flat, showering in it or frying anything. There is no CO₂ being generated and no cooking to extract. The single variable left is water — from the sea air, from a slow ingress nobody has seen, from the ground and the structure — and a dehumidifier removes water regardless of what the weather outside is doing. Set on its humidistat (higrostato) with a hose to a drain instead of a tank to empty, it can hold a closed flat while you are not in it.

Three honest caveats go with that:

• It only works with the power left on and the water going somewhere. A tank that fills and stops the machine is fine for a supervised job and useless for a flat you visit once a month.
• It does not replace airing the place out when you arrive. Nothing else clears the shut-up smell.
• It removes water from the air; it does not tell you where that water came from. A closed building is exactly where a slow leak goes unnoticed for a season, and a dehumidifier quietly emptying a tank makes it easier to keep not noticing.

The flat's own designed openings — façade grilles, the vent to the shared vertical duct, the shunt — are part of the building's ventilation and should be left alone rather than taped over while you are away.

Laundry indoors, where the order of operations is everything

Washing that comes out of the drum still holding water gives all of it back to the air of whatever room the airer is standing in. This is the one household job where the two machines genuinely cooperate, and where doing it in the wrong order wastes both.

The order that works:

1. Contain it. One room, door shut, ideally not a bedroom you will then sleep in with the door closed.
2. Give that room a route out for the moisture: a window open onto the outside, or an extract that actually runs, or the dehumidifier in the room with it.
3. Leave the rest of the flat out of it.

A dehumidifier working on a shut room with a wet airer in it is doing the thing it is best at: a concentrated source in a small, closed volume. The same machine standing in the hall while the airer dries in the bedroom with both doors open is trying to dehumidify the whole dwelling, and losing. If the flat has openings on a single façade, the shut-door version matters more, not less — there is no cross draught to carry the moisture away on its own.

When the tank keeps filling and nothing ever changes

A dehumidifier that has been running for two winters is a measuring instrument nobody is reading.

The tank is telling you something. If it fills fastest on cold nights, after showers and when the flat is full of people, the moisture is coming from what the household does and where it goes is the ventilation route. If it fills at much the same rate whatever the season and whoever is home — or fills hardest in one room, or after every spell of rain or sea wind — then the water is arriving from somewhere other than the indoor air, and the machine is intercepting it after the fact.

Condensation, a leak, rising damp and rain penetration are different mechanisms with different remedies, and they cannot be told apart reliably without an inspection on site. A permanently running dehumidifier flattens the symptom that would otherwise have made the difference visible.

Two costs are worth stating plainly. It runs on electricity for as long as you leave it on, and it returns its own heat to the room — welcome in February, distinctly unwelcome during a Barcelona August. And where dampness has persisted long enough to produce visible mould, the WHO guidelines on dampness and mould support a cautious link between persistent indoor dampness and mould and respiratory symptoms. Holding a number down is not the same as removing the dampness that produced it.

Getting a machine to earn its place

If you already own one, or you have decided the closed-flat case applies to you, a few things decide whether it does anything useful:

• The room, not the flat. It works on the volume it is shut into. Doors open onto a hallway and it is fighting a battle of the wrong size.
• Near the source. Beside the airer, in the room whose walls are wet — not parked wherever there happens to be a socket.
• Tank or drain. Continuous drainage for anything unattended; a tank for a job you will be there to finish.
• The kind of coil. A compressor unit condenses on a cold surface and loses capacity as the room gets colder, which is why desiccant models are the ones usually offered for unheated storerooms.
• Noise. In a bedroom, this decides whether it gets used or unplugged in week two.
• Filters and coil. Both collect dust, and a clogged one moves less air through the machine every month.

And it is not an air purifier, whatever the shop shelf implies by putting them side by side: a purifier filters particles out of the air and, like the dehumidifier, brings none in.

One test settles most arguments. Switch it off for several days in the season it is supposed to be earning its keep, and watch what changes. If nothing does, it was never treating the problem. If the flat is uncomfortable within a day, you have just measured how much work your air renewal is failing to do — and that is the thing to fix, not the reason to buy a second machine.