An air-conditioning unit can cool or heat a room without renewing the air trapped inside it. In a renovation, the sound decision is to plan the fresh-air route first — supply to bedrooms and living areas, transfer through the home, extraction from the kitchen and bathrooms — and choose climate-control equipment afterwards. This avoids a sealed-up home with fogged windows, lingering smells and mouldy corners even when the temperature feels comfortable.
In Lisbon, where long hot spells are becoming more relevant, closing everything while the air conditioning runs is understandable. Thermal comfort and indoor-air quality are separate jobs, and both need to be addressed in the same project.
Contents
- Why air conditioning is not enough
- Draw the air route before closing ceilings and walls
- When natural ventilation works — and when it does not
- Which approach suits a smaller renovation?
- Balancing ventilation, noise, heat and security
- Mistakes that become expensive after painting
Why air conditioning is not enough
A typical wall-mounted unit mostly recirculates the air already in the room: it removes or adds heat, filters some dust and may reduce some moisture while cooling. That does not mean it brings in enough outdoor air or removes bathroom vapour, cooking odours, carbon dioxide and pollutants from people, furniture or household products in a controlled way.
The Portuguese health authority explains the link between ventilation, humidity, temperature and indoor-air quality, and notes that CO₂ is a useful indicator of ventilation and occupancy. A CO₂ meter is not an automatic instruction to throw every window open; use it to notice patterns, timing and moisture sources.
The basic principle is straightforward: every habitable room needs fresh air; every wet or pollutant-producing zone needs extraction. The transfer route between them cannot be cut off by overly tight doors, blocked grilles or cabinets pushed against a service wall.
Draw the air route before closing ceilings and walls
In an apartment, draw one continuous arrow on the plan: air enters bedrooms and the living room, moves through the hall, and leaves through the kitchen, bathroom and laundry area. This prevents extraction from pulling air only through random cracks, service shafts or shared parts of the building.
- Identify existing air inlets in façades and window frames.
- Locate ducts, ventilation stacks and discharge points that can actually be used.
- Confirm how air moves beneath or through internal doors.
- Keep access for cleaning grilles, fans and ducts.
Where works amount to a major renovation or renew the ventilation system, Portugal’s SCE framework requires homes to achieve an air-change rate of at least 0.5 changes per hour, subject to specific verification rules. Read the published SCE Manual methodology before allowing an improvised idea to become a construction specification.
When natural ventilation works — and when it does not
Natural ventilation can work well in a home with two aspects, openable windows and a clear route between inlet and outlet. It can fall short in a single-façade one-bedroom flat, an internal bedroom, an enclosed balcony or an apartment where street noise makes it unrealistic to keep windows open at useful times.
Portuguese rules accept natural ventilation where it can provide the required minimum fresh-air flow for at least 90% of relevant occupied hours; having a window in a room is not enough. Check the criteria in Ministerial Order No. 138-I/2021. In practical terms, ask the designer to explain the winter scenario, when windows are opened less, and the summer scenario, when night cooling is sought.
Do not confuse occasional cross-ventilation with a permanent system. Opening two windows can be excellent on a mild morning; it does not, by itself, solve a windowless bathroom or a bedroom used with the door shut overnight.
Which approach suits a smaller renovation?
There is no single correct device. Properly sized mechanical extraction in wet rooms can complement planned air inlets in dry rooms. In apartments with genuine façade, noise or layout constraints, decentralised mechanical ventilation with room-by-room units may deserve consideration, while a fuller ducted system can work where there is ceiling void and service space.
Compare proposals using decisions that can be checked: where air enters, where it leaves, the route it follows, how flow is controlled, the noise reaching the bedroom, where maintenance happens and what the proposal changes on the façade or in common parts. Do not accept a specification that merely says “mechanical ventilation”.
If you want to reduce window draughts, window weatherstripping can help, but it must never eliminate a designed air inlet. Sealing an uncontrolled leak is different from sealing the whole home.
Balancing ventilation, noise, heat and security
Ventilation does not mean leaving windows open all day. Portugal’s health authority recommends airing the home daily and during activities that release moisture or pollutants, including cooking, cleaning, bathing, washing and drying clothes. Read the general indoor-air recommendations as an everyday-use routine, not as a substitute for a poorly planned system.
During severe heat, close shutters or blinds during peak sun, keep air moving indoors, and air the home at dusk when outdoor air is cooler. The health authority gives this advice for hot weather; the IPMA classified summer 2026 as Portugal’s third warmest since 1931. Still, assess noise, security and outdoor-air quality before keeping openings open overnight.
Well-fitted blackout curtains help control light and solar gains. During a renovation, coordinate them early with a recessed curtain track and with grilles or equipment that must remain accessible.
Mistakes that become expensive after painting
Some clues show that the air route was forgotten: a bathroom door that shakes when the extractor starts, recurring condensation in one bedroom, cooking smells settling in wardrobes, an inaccessible grille behind fixed furniture, or an outdoor unit selected without a condensate-drainage plan.
- Do not cover grilles because they “let cold in”; first establish whether they are supply, transfer or extract points.
- Do not discharge extraction into a loft void, shutter box or suspended ceiling without a designed route outdoors.
- Do not treat a recirculating cooker hood as if it removes water vapour outdoors.
- Do not close the project without testing controls, noise, airflow and cleaning access.
A smell that remains after renovation can also be made worse by poor air renewal. Before adding more air freshener, revisit the strategy for ventilating after painting. For new ducts, façade penetrations, alterations to common parts or doubts about airflow, bring in a qualified professional and secure any required approvals before work starts.
Key takeaways
- Treat climate control and ventilation as different systems that must be coordinated in one design.
- Plan air to enter dry rooms, travel through the home and leave through kitchens and bathrooms.
- Do not block inlets, grilles or transfer gaps without understanding their purpose.
- Require every ventilation proposal to show air routes, maintenance access and expected noise.
If you are renovating and want ventilation, suspended ceilings, windows and climate control coordinated before walls are closed, ask Obra Lisboa for a renovation proposal with the service routes defined within the project scope.
Sources and further reading
- general indoor-air recommendations
- Portuguese health authority explains the link between ventilation, humidity, temperature and indoor-air quality
- Ministerial Order No. 138-I/2021
- published SCE Manual methodology
- IPMA classified summer 2026 as Portugal’s third warmest since 1931
Editorial image; it does not depict an Obra Lisboa project.
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