When replacing windows, do not seal away the ventilation your home needs: plan controlled air inlets in bedrooms and living rooms, internal air transfer and extraction in wet rooms. More airtight frames reduce unwanted drafts, noise and energy loss, yet they can also expose condensation, stale smells and mould that old gaps used to conceal. In Lisbon, where hot days, humid nights and rainy spells alternate, this decision belongs in the renovation design before the frames are ordered.
Contents
- Why does replacing windows change the way a home breathes?
- What air route should you plan for an apartment?
- Window inlets, wall inlets or mechanical ventilation: how do you choose?
- How can you avoid drafts, noise and overheating?
- What mistakes tend to appear after the work is finished?
- What should be agreed before ordering the windows?
Why does replacing windows change the way a home breathes?
An old window may let air through without control. That is uncomfortable in winter, admits noise and dust, and does not ensure that fresh air reaches the rooms where it is needed. A well-fitted new window stops that accidental flow. This is an improvement, provided ventilation is treated as a route through the home rather than as a by-product of gaps.
Portugal’s energy regulation states that, in homes, air should enter through main rooms such as bedrooms and living rooms, with extraction in service spaces such as kitchens, bathrooms and laundries. It also calls for opening layouts that prevent stagnant areas. Read the Diário da República regulation on building ventilation before finalising the design.
The aim is not to leave every window open. It is to renew indoor air while windows are shut, with fewer abrupt temperature changes and less noise intrusion than a permanent crack in a sash.
What air route should you plan for an apartment?
Think of one clear line: outdoor air enters through the living room and bedrooms, crosses dry areas, and is removed through the kitchen, bathrooms and laundry. For that to work, each internal door cannot act as an airtight lid.
- Mark main rooms and service rooms on the plan.
- Show where outdoor air will enter, avoiding a location directly over a bed, sofa or desk.
- Confirm how air transfers beneath or through internal doors.
- Check that existing ducts, grilles or extractor fans genuinely discharge outdoors and remain accessible for cleaning.
In a typical two-bedroom flat, air may enter through the living room and both bedrooms, move through the hall, then be extracted in the kitchen and bathrooms. An open-plan kitchen does not remove the need to control this route: cooking moisture and bathroom air should not travel back into bedrooms.
In older buildings, this logic must work with the existing structure, vertical service runs and condominium rules. If the work affects shared ducts or changes the façade, check the scope first and see Obra Lisboa’s guide to building and planning records before renovating a Lisbon apartment.
Window inlets, wall inlets or mechanical ventilation: how do you choose?
There is no single answer. The right option depends on wind exposure, street noise, the possibility of forming ducts, the condition of existing extraction and how the home is used. A self-regulating inlet built into the frame can be discreet and appropriate where wet-room extraction works properly. A wall inlet needs closer coordination with insulation, weatherproofing and finishes. Mechanical ventilation can suit a deeper refurbishment, particularly when suspended ceilings and technical routes are already being created.
The technical reference document on residential ventilation recommends controlling the home’s overall air permeability and sets out intake, outlet and transfer openings. It also notes that frames and doors should be selected according to exposure. See the diagrams and guidance in the Diário da República document on ventilation and air-permeability requirements.
Do not ask an installer only for “very airtight windows”. Ask for a proposal that identifies the intake system, its location, adjustment, rain and insect protection, and maintenance method. Airtightness without an air route is an incomplete specification.
| Option | May suit a home where | Key point to check |
|---|---|---|
| Window-frame inlet | Wet-room extraction works and there is little room for building work | Street noise, cleaning and position relative to occupants |
| Wall inlet | The façade and internal finishes are already being refurbished | Insulation, rainproofing and façade permissions or rules |
| Mechanical ventilation | The refurbishment includes technical ceilings and duct routes | Design, filter access and equipment noise |
How can you avoid drafts, noise and overheating?
Position matters as much as airflow. Do not put an inlet directly behind a headboard, over a work desk or next to the usual resting spot. In a living room, assess the wall, solar orientation, curtain position and proximity to the sofa. In a bedroom, ask for the solution to be considered with the furnished layout, rather than only on a window drawing.
On a traffic-heavy street, a basic opening can bring in the noise that new windows were intended to reduce. Look for acoustic performance that matches the façade and confirm it in the technical data. Do not solve the issue by blocking the inlet after installation: the planned route is lost and condensation and stale odours can return.
During hot periods, ventilation needs a measured routine. IPMA defines a heatwave as at least six consecutive days with maximum temperatures above a local climatological threshold and reports more frequent summer extremes over recent decades. Follow IPMA’s heatwave information and adapt the routine: shade rooms during the day, ventilate when outdoor air cools, and use effective extraction after cooking or showering.
An air inlet does not replace shutters, external shading, suitable glazing or a plan for an overheated room. It is one part of the system. For façade solar protection, also read what to decide before adding external shading to an apartment.
What mistakes tend to appear after the work is finished?
The most common mistake is replacing windows, sealing doors and keeping the same routine, expecting the home to behave as before. Another is treating the kitchen hood as the answer for the whole home: a recirculating hood does not remove moisture outdoors, and a cooker hood does not solve bathroom ventilation.
- Grilles blocked by curtains or furniture: intake falls and the planned solution stops working.
- Internal doors with no air transfer: bedrooms are isolated and the route to extraction is interrupted.
- Fans connected to no duct or to a blocked duct: they make noise without renewing air.
- Inlets with no acoustic provision on a noisy street: residents end up keeping them closed.
- Condensation treated only with anti-mould paint: this may hide a stain for a while but does not remove vapour, leaks or thermal bridges.
If condensation always appears in the same corner, marks emerge after rain, an extractor does not discharge outside, or shared ducts need to change, stop before closing ceilings and finishes. The cause may involve the façade, roof, vertical services or electrics and needs technical diagnosis.
What should be agreed before ordering the windows?
Take this list to the survey visit. First, note the orientation of every façade and identify the bedrooms in use. Then record street noise, existing shutters, visible condensation, present ducts and grilles. Finally, ask the proposal to describe the entire air path rather than only the profile, glass and hardware.
- Set out fresh-air inlets and how they will be adjusted.
- Confirm internal transfer paths between rooms.
- Test or inspect extraction before work starts.
- Coordinate frames, shutters, curtains, insulation and internal finishes.
- Allow future access to grilles, filters and equipment.
A well-chosen window can greatly improve comfort; a window integrated into a ventilation plan can also improve indoor air and reduce the likelihood of hidden problems. This coordination is especially useful where work includes internal insulation, new bathrooms or changes to room layouts.
Key takeaways
- Map the air route from living rooms and bedrooms to the kitchen, bathroom and laundry before choosing frames.
- Specify controlled inlets and internal air transfer; do not rely on gaps in old windows.
- Check existing extraction before closing ceilings, walls or built-in furniture.
- Do not block grilles because of noise or drafts: revise the underlying solution instead.
If you are replacing windows as part of a renovation, include ventilation, shutters and finishes in the same construction decision. To coordinate this work in a Lisbon apartment or house, see Obra Lisboa’s construction and renovation service and request a review of the planned scope.
Sources and further reading
- Diário da República regulation on building ventilation
- Diário da República document on ventilation and air-permeability requirements
- IPMA’s heatwave information
AI-generated editorial image; it does not depict an Obra Lisboa project.
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