When ventilation is decided in a renovation
Ventilation has to be decided the moment you switch to airtight windows or start a full renovation. Later there is nowhere to run ducts: ceilings are closed, wiring is in and the joinery is on order.
An old Barcelona flat used to ventilate itself: through gaps in timber window frames, loose balcony doors and old extract shafts. Nobody designed it, but the air changed. A renovation removes those gaps, and the accidental air exchange goes with them. If nothing replaces it, you get a warm, quiet flat where the bedrooms feel stuffy by morning and condensation appears in the corners by the windows in winter.
What changes after new windows go in
The effect is strongest in bedrooms. In the Eixample many people sleep with the windows shut because of street noise, especially when the bedroom faces the street rather than the inner courtyard. Two people in a closed room noticeably degrade the air overnight, and new windows with good acoustic performance make that feeling stronger.
The second symptom is moisture. Cooking, showering and drying laundry indoors put water into the air every day. Some of it used to escape through the gaps; now it settles on the coldest surfaces: window reveals, the corners of external walls, behind a wardrobe against the façade. If you see black spots behind furniture after a renovation, the cause is almost always air exchange, not the plaster.
The CTE (Spain's building code) has a dedicated section on indoor air quality. How it applies to your renovation is for the project technician to determine, but the logic is simple: fresh air enters the living rooms and bedrooms, and moist, stale air leaves through the kitchen and bathrooms.
Three levels of solution
The first level is extraction from the kitchen and bathrooms plus trickle vents in the windows or walls of the living rooms and bedrooms. It is the cheapest and simplest option, but cold air comes through the vents in winter and street noise in summer.
The second is continuous mechanical extraction: one fan quietly draws air from the wet rooms while fresh air comes in through the vents. It works more reliably than switch-operated extractors that get turned on for five minutes and forgotten.
The third is balanced mechanical ventilation with heat recovery. It supplies filtered air to bedrooms and the living room, extracts from the kitchen and bathrooms, and transfers part of the warmth or coolness of the outgoing air to the incoming air. Windows can stay shut and the filter holds back street dust. It is the most expensive option and the most demanding on space: the unit itself, two duct networks and access for filter changes.
Finding room for the ducts
This is where the decision is usually limited by the flat rather than the budget. Ducts run in a dropped ceiling, typically in the hallway, bathrooms and storeroom, where lowering the ceiling by 20 to 30 cm is no great loss. In Eixample buildings with high ceilings it is easier to find space; in flats from the sixties and seventies with low ceilings the route has to be planned very carefully.
The unit goes where it cannot be heard from the bedrooms and where filters can be reached: above a bathroom, in a storeroom, in a hall cupboard. Then there is the question of where the air exits. A new opening in the façade or an outlet onto the roof affects the building's shared elements, which requires agreement from the comunidad de propietarios, and protected façades may carry restrictions that are confirmed with the city council.
The cooker hood in an airtight flat
The most common mistake is a powerful hood over the island of an open-plan kitchen in a flat with new windows. The hood pushes out a lot of air and has nowhere to draw it from. At best it is noisy and pulls poorly. At worst it draws air through the drain traps or the shared shaft, and the neighbours' smells appear in your flat.
There are two solutions. One is a recirculating hood with a carbon filter: it removes grease and odours but not moisture, which then has to be handled by the general ventilation. The other is ducting outside with a make-up air inlet that opens whenever the hood runs. The choice is made before the kitchen is ordered, because the bulkhead over the island and the duct route in the ceiling depend on it.
An example and the order of decisions
In the 250 m² child psychology centre in the Eixample we installed heat-recovery ventilation: consulting rooms with closed doors, where people sit for hours, do not work without fresh air. In a home of similar scale, say the size of the Urgel flat at 109.80 m² with four bedrooms, the logic is the same: each bedroom with its door shut is a separate room that needs supply air, and there must be a gap or transfer grille under the door so the air can reach the extract.
The sequence is: first choose the level of system, then the duct routes and unit location, then the ceiling and lighting plan, and only then the wiring and joinery. At handover, test each grille with a sheet of paper, or an anemometer if the crew has one, and make sure the filters can be reached without taking the ceiling apart.