VMI or VMC: which to choose to stop condensation in a flat

VMI or VMC: which one fixes condensation in your flat?

If your windows are wet every morning, there is mould in the ceiling corners and your hygrometer reads 70% or more at 7 am, the problem is not enough air change. Both VMI and VMC fix this continuously, without relying on you remembering to open windows. The difference is how they move the air, how much building work they need and the price.

As a rule, VMI (ventilação mecânica por insuflação, positive input ventilation) is the simpler option for a lived-in flat, including rented ones. VMC (ventilação mecânica controlada, mechanical extract or balanced heat-recovery ventilation) makes more sense during a renovation, when ceilings are being opened and ducts can be run.

Before choosing either, make sure it really is condensation. If the stain gets worse after rain, or creeps up from the skirting on a ground floor, ventilation will not fix it. The free diagnosis separates condensation, rising damp and water ingress in a few minutes.

How does a VMI (positive input ventilation) work?

A VMI unit has a fan that draws air from outside, filters it and blows it into the flat, usually at a central point such as the hallway. The flat ends up at slight positive pressure: the moister indoor air is pushed out through window gaps, vents and extractor fans.

In winter, outdoor air holds little water in absolute terms, even when outdoor relative humidity is 90%. Once it comes in and warms up, its relative humidity drops. This steady exchange is what brings the hygrometer reading down and keeps wall surfaces away from the dew point.

• Installation: in a flat the unit goes in a false ceiling, a cupboard or on a wall, with one opening to the outside. It is usually a one- or two-day job, with no ducts through the whole flat.
• Pre-heating: many models have a heating element that warms incoming air when it is cold outside. It avoids the feeling of a draught but increases electricity use.
• Limitation: a VMI does not extract directly from the bathroom or kitchen. Extractor fans in those rooms are still needed.

The airflow is adjustable and should be set for the home. A two-bedroom flat with two people does not need the same airflow as a three-bedroom flat with four people and laundry drying on an enclosed balcony. Too much airflow cools the flat and makes noise; too little will not bring humidity down.

How does a VMC work, and how is it different?

A VMC does the opposite: it extracts air from the wet rooms — kitchen, bathrooms, utility room — through ducts connected to a central fan. Fresh air comes in through the dry rooms, bedrooms and living room, via trickle vents in the window frames or in the wall. The flat sits at slight negative pressure and the vapour leaves where it is produced.

There is also balanced (double-flow) VMC, which both supplies and extracts through ducts and passes both airflows through a heat exchanger. The outgoing air warms the incoming air without mixing. It is the most complete solution, but also the one that needs the most space and work.

In practice a VMC needs ducts to every wet room, hidden in false ceilings or bulkheads. In a 1960s–80s block with plaster ceilings and no false ceiling, that means real building work. In a full renovation the extra cost is much smaller, because the ceilings are being opened anyway.

Rented flat or renovation: when to choose each

If you rent, any opening in a wall or the façade needs the consent of the senhorio (landlord) and, where the façade is affected, usually also of the condomínio (owners' association). Get it in writing before asking for quotes. Our page for tenants explains who usually pays for what.

To compare ventilation with a portable appliance, see the guide dehumidifier or VMI.

Noise, filters and maintenance: day-to-day differences

Both systems run 24 hours a day. Noise depends on the model, the airflow setting and where the unit sits: next to a bedroom, an undersized VMI or one running flat out can be a nuisance. Ask the installer for the noise level from the data sheet at the airflow you will actually use, not at the minimum.

• Filters: a VMI filters the incoming air, and so does balanced VMC. Dirty filters cut airflow and increase noise. Clean or replace them as the manufacturer says, and ask how much replacement filters cost and where to buy them.
• Extract valves: with VMC, clean the valves in bathrooms and kitchen; grease and dust reduce extraction.
• Ducts: they should remain accessible for inspection. Ask for their route to be documented.
• Don't switch it off: the most common mistake is turning the system off in winter because of cold or noise. Condensation comes back within days.

How much do VMI and VMC cost in a flat?

The figures observed in October 2026 are estimates and vary with flat size, access and model:

• VMI in a flat: around €800–2,500 installed (installer estimates). Details in how much a VMI costs.
• VMC: around €2,500 or more installed, rising with the number of rooms and with balanced heat recovery. See how much a VMC costs.
• No-works alternatives: better airing, a dehumidifier and small fixes typically cost €20–300.

When comparing quotes, check whether they include the wall opening, the external grille, the electrical connection, any false ceiling or bulkhead, and making good. A cheap quote without these can end up dearer. If in doubt, send the proposal for a free second opinion.

The public E-Lar and Vale Eficiência schemes have been closed since July 2026 and no new ones have been announced, so budget for the full cost.

What the installer should measure before proposing anything

A serious proposal starts with numbers, not a catalogue. Before accepting, check that the installer has done or asked for the following:

1. Relative humidity and temperature at 7 am in several rooms over a few days — a ~€10 hygrometer is enough.
2. Temperature of cold walls and corners, compared with the dew point; you can check it with the dew point calculator.
3. Floor area, number of occupants and habits: drying laundry indoors, showers, cooking.
4. Existing extractor fans in kitchen and bathrooms, and whether they really vent outside.
5. Window type and whether trickle vents already exist.
6. Proposed airflow and the location of the unit, air inlets and outlets.

If the proposal has no readings and no airflow figure, ask for them in writing. To understand the mechanism better, read the guide to condensation damp.