Dehumidifier or VMI: which works better against condensation

Dehumidifier or VMI: which works better against condensation?

It depends on the size of the problem. A dehumidifier removes water from the air in a closed room; it is cheap, needs no building work and goes with you when you move. A VMI (ventilação mecânica por insuflação, positive input ventilation) continuously renews the air in the whole flat; it costs more and needs an opening to the outside, but it doesn't depend on closed doors or emptying tanks.

Rule of thumb: if condensation is concentrated in one or two rooms, start with a dehumidifier. If it appears in almost every room, comes back every winter and the hygrometer reads over 70% at 7 am even with good airing habits, mechanical ventilation is probably the longer-lasting answer.

Neither fixes water ingress or damp rising from the ground. Before spending money, confirm the cause with the free diagnosis and, if it is condensation, read the guide to condensation damp.

How each one takes moisture out of the air

Both tackle the same problem in different ways. A dehumidifier removes water: air passes over a cold surface (compressor) or through a material that absorbs vapour (desiccant), the water collects in a tank and drier air goes back into the room. A VMI replaces the air: it blows in filtered outdoor air and pushes moist air out through gaps and vents.

• Compressor dehumidifier: the most common type, from ~€90 for 10–12 L/day. It loses effectiveness in cold rooms, below ~15 °C.
• Desiccant dehumidifier: works better in unheated bedrooms and basements, but uses more energy per litre.
• VMI: treats the whole flat and renews the air, which also helps with smells and stuffiness. It doesn't remove vapour directly from the bathroom or kitchen.

There is one important difference: a dehumidifier doesn't renew the air. In a home that is always shut, it dries the room but the air stays stale. A VMI, by contrast, guarantees air change, but how far humidity drops depends on the absolute humidity of outdoor air — low in winter, potentially high in summer.

Choosing the right appliance has its own rules: the guide which dehumidifier to buy explains litres per day, humidistat and continuous drainage.

One room or the whole flat?

A dehumidifier only works in the space where it sits, and with the door shut. Put it in the hallway of a three-bedroom flat and it will run hard but dry the bedrooms very little. Most families end up moving it from room to room: bedroom at night, living room or laundry area during the day.

A VMI distributes fresh air through the whole flat from a central point, as long as internal doors have gaps or vents for air to pass. It runs on its own, 24 hours a day, and nobody has to remember anything.

There is also the question of behaviour. A dehumidifier only works if it is switched on, with the tank empty and the door shut. In homes with children, mismatched schedules or frequently changing tenants, that often fails. Mechanical ventilation removes that dependence, which explains part of its cost.

What does each one cost to run? The formula

You don't need made-up tariffs or catalogue promises. Use the power shown on the label or data sheet and this formula:

• Daily consumption (kWh) = power (W) × hours of operation ÷ 1000.
• Daily cost = kWh × the price per kWh on your electricity bill.

Illustrative example: an appliance rated at 200 W that runs 8 hours a day uses 200 × 8 ÷ 1000 = 1.6 kWh per day. Multiply by your price per kWh and by the winter days you use it. With the humidistat set to 50–55%, the dehumidifier switches itself off and the real running hours fall.

The same formula applies to a VMI, but with 24 hours of operation. The fan is usually low-powered; what weighs is the pre-heating element, if the model has one, because it switches on in cold weather. Ask the installer for the fan and heater power separately.

When comparing, think over several winters, not just the first. A dehumidifier is cheap up front but adds electricity and daily effort. A VMI weighs on installation and then costs mainly filters and fan energy. Run the numbers with your own figures.

More price detail in how much a dehumidifier costs and how much a VMI costs.

Noise, space and the landlord's consent

A compressor dehumidifier makes a steady noise similar to an old fridge, louder on maximum. In the bedroom, many people prefer to run it during the day with the door shut, and on quiet mode or off at night. The tank has to be emptied unless you connect it to a drain with a hose.

A VMI sits in a false ceiling, a cupboard or on a wall, ideally away from the sleeping area. Noise depends on the model and airflow; ask for the data-sheet figure at the airflow you will actually use.

For renters, this difference is decisive. A dehumidifier needs nobody's permission. A VMI means drilling the wall or façade and therefore the written consent of the senhorio (landlord) and, where the façade is affected, usually also of the condomínio (owners' association). Our page for tenants summarises who usually pays for what.

When it makes sense to combine both

Ventilation and a dehumidifier are not mutually exclusive. In some situations the combination solves what neither manages alone:

• Laundry drying indoors: the VMI keeps the flat in order, but a loaded airer in a small room produces vapour faster than ventilation removes it. A dehumidifier in that room, door shut, dries the laundry and protects the walls.
• Cold, north-facing bedroom: a very cold wall can stay below the dew point even in a ventilated flat. A dehumidifier in that room lowers local humidity.
• Drying-out period after works: new plaster and paint release a lot of water in the first weeks.
• Rooms outside the VMI's reach: basements, storerooms or annexes where the supplied air doesn't get to.

To estimate the temperature at which your walls get wet, use the dew point calculator. If you are already considering whole-flat ventilation, compare the options in the guide VMI or VMC.

Decision table: what to choose in your case

If you are torn between the two, start with the cheaper, reversible option — the dehumidifier — and measure. Two weeks of 7 am hygrometer readings will tell you whether you need to move on to ventilation.