Ventilation and damp: airing, PIV, MVHR or a dehumidifier
A family of four puts about 10 litres of water a day into the air through breathing, cooking, showers and drying clothes. If that vapour does not leave, it ends up on the coldest surfaces: glass, corners, outside walls. Ventilation is the most direct way to get it out of the home, and in many Portuguese homes with new, well-sealed windows it no longer happens on its own.
These guides are for anyone with condensation choosing between changing routines, buying a dehumidifier or installing mechanical ventilation. They compare PIV (positive input ventilation, VMI in Portuguese, which pushes filtered air into the home) and mechanical ventilation, VMC in Portuguese (which extracts air and, in balanced heat-recovery versions known as MVHR, also supplies it), with costs and limits.
The key idea: start with what is free and measure before investing. Airing properly and using extractor fans can bring relative humidity into the 40–60% range; if that is not enough, compare what a PIV unit costs with mechanical ventilation. And confirm it is condensation first: ventilation does not fix water ingress or rising damp.
Start here
How to air your home in winter without losing heat
Airing for 5–10 minutes with a through-draught removes moisture without cooling walls and furniture. When to open up, why a window left ajar is worse, and a simple routine.
Full guide →VMI or VMC: which to choose to stop condensation in a flat
How VMI (positive input ventilation) and VMC (mechanical extract or heat recovery) work, when each fits a rented flat or a renovation, noise, filters and prices.
Full guide →Dehumidifier or VMI: which works better against condensation
A dehumidifier dries one room; a VMI renews the air in the whole flat. Compare reach, running costs, noise, landlord consent and when to combine both.
Full guide →Frequently asked questions
As a rule, it is better to open the windows wide for a few minutes, ideally on opposite sides of the home to create a through-draught, than to leave them ajar for hours. The air is replaced quickly and the walls do not cool down as much. Do it when you get up, after showering and after cooking, which is when there is most vapour in the air.
Usually not. Cold winter air holds little water in absolute terms, even at 90–100% relative humidity. Once it comes in and warms up, its relative humidity drops considerably. That is why airing still works on damp January days in Lisbon. What matters is the comparison with the indoor air, not the figure shown in the weather forecast.
It usually needs no structural work: the unit sits in a service space such as a false ceiling or a cupboard and blows air through one or a few vents. But the air needs a way out, through extractor fans or vents. A PIV unit in a flat costs an estimated €800–2,500. Ask for an assessment of the installation before deciding.