A home needs fresh air every day, even in winter. Without it, the moisture from cooking, bathing and breathing stays indoors and ends up on cold walls. This guide explains how air enters and leaves a home, what vents and window openings are for, what Portuguese rules require in building work, what changes when you replace windows, and how to air a room without wasting heat.
Updated 4 November 2026. General information, not a substitute for assessment by a technician.
The essentials
- In a dwelling, the 2021 Portuguese rule is to let air in through the main rooms (bedrooms and living room) and take it out through the service rooms (kitchen and bathroom).
- For new buildings, Portaria n.º 138-I/2021 (a ministerial order) gives 0.50 air changes per hour as the minimum. The 2013 order said 0.4.
- When windows are replaced, the same order asks for air inlets in the facade of main rooms: 60 cm² of free area, or self-regulating vents.
- The health authority (DGS) says: never block air inlets and outlets.
- Tighter windows with no ventilation raise the risk of condensation and mould.
How air enters and leaves a home
The order defines natural ventilation as air moving through leaks and openings in the building because of pressure differences, with no fans. Wind and the temperature difference between inside and outside push the air. It uses no energy, but it depends on the weather and on the openings that exist.
The layout the law describes is simple: air comes in through bedrooms and living room and leaves through kitchen and bathroom, where most of the vapour is. For this to work, air must be able to pass from the rooms where it enters to the rooms where it leaves. The sources we read give no measurements for gaps under interior doors.
| Where | What it does |
|---|---|
| Opening in the window frame, or vent in the outside wall | Lets fresh air into bedrooms and living room with the window closed |
| Vent or duct in the bathroom and kitchen | Lets damp air and smells out |
| Electric extractor fan | Removes air faster while bathing or cooking. See bathroom extractor fan |
For naturally ventilated buildings, the 2021 order also asks for means to limit excessive air change in strong wind and a good spread of openings, to avoid pockets of still air. We did not find how each vent achieves this. When choosing, ask for the manufacturer’s data sheet.
What Portuguese law says
New homes and major works. Portaria n.º 138-I/2021 of 1 July (annex II) says ventilation should preferably be natural, with mechanical ventilation added where needed. In dwellings it must serve the whole building, with air admitted through main rooms and extracted through service rooms. Table 10 gives 0.50 air changes per hour for a new building, calculated with the SCE Manual (the energy certification method).
The earlier order. Portaria n.º 349-B/2013, which we read in the Diário da República, asked for an hourly air change rate of 0.4 or more in dwellings. You still find this figure on older web pages.
Replacing windows. For buildings undergoing renovation that installs or replaces the glazed envelope of main rooms, and that is not a major renovation, the 2021 order asks for air inlets in the facade of those rooms. It accepts one of two solutions: a free area of at least 60 cm², or self-regulating vents with a nominal flow of at least 25 m³/h at 2 Pa or 75 m³/h at 20 Pa. A building is exempt if its air change rate, calculated with the SCE Manual, is already 0.50 per hour or more. If the project cannot comply, the designer must adopt alternative solutions that do not put air quality at risk.
We could not confirm whether this alone binds a resident who replaces two windows without a project. Ask the installer whether the frame includes an air inlet, and have it written into the quote.
New windows, a tighter home
A new window closes better and lets less air through the gaps. That saves heating, but it removes ventilation the home had without anyone noticing. Official UK guidance warns that energy efficiency work can reduce ventilation and may need improvements to the ventilation system, to avoid more damp and mould. The Swiss FOPH says that in old buildings with new windows and poor insulation, you should air more often.
The British guidance adds that window air inlets should be controllable by the resident, because permanently open ones tend to get blocked. For people who do not open windows because of security or noise, it suggests the “vent position”: slightly open and locked.
An airing routine
| Situation | What the sources say |
|---|---|
| Every day | FOPH: at least three times a day, with a through draught, opening doors and windows for 5 to 10 minutes |
| After bathing or cooking | FOPH: ventilate or switch on the fans. British guidance: open the window wide and close it once the air has changed |
| Tilted window | FOPH: avoid permanent airing with tilted windows while the heating is on |
| Air humidity | FOPH: 30% to 50% in the heating season, and no more than 40% on very cold days |
| Drying laundry indoors | FOPH: avoid it in rooms with damp problems |
| Furniture | FOPH: 10 cm between furniture and outside walls. See mould in wardrobes |
For how this links to condensation in a bedroom, see bedroom condensation and ventilation.
Safety: do not block vents
The DGS, in a warning reported by the newspaper Diário de Notícias in October 2019, says: never block air inlets and outlets (ventilation grilles, kitchen, bathroom or windows). The same source says never to use a gas hob or oven to heat the home, and that carbon monoxide has no smell. If you suspect poisoning, call 112.
When ventilating is not enough
- Mould in the same corner every winter: see thermal bridges and mould in corners.
- Planning to insulate walls from the inside: see internal insulation and condensation.
- Wall damp from a leak or from the ground: ventilation does not fix the cause. See types of damp in walls.
- Air always feels heavy: a dehumidifier lowers humidity but does not replace fresh air.
What we could not confirm
- Standard NP 1037-1. We did not find the text, so we do not cite it.
- The SCE Manual and the calculation despacho. We did not read the method, so we cannot say how much air change a given vent or window gives.
- Whether the 2013 order was expressly repealed. We read the 2021 text but did not check the repeal.
- Any duty to fit air inlets when windows are replaced without a project, in existing homes.
- Portuguese legal limits for humidity or temperature at home. The figures above are from the Swiss FOPH.
- Prices of vents or of window frames with air inlets.
Sources (read on 4 November 2026)
- Diário da República, series 1, no. 126, 1 July 2021, Portaria n.º 138-I/2021: definitions, annex II point 1 (general requirements, 1.2.1, Table 10) and article 3.
- Diário da República, series 1, no. 232, 29 November 2013, Portaria n.º 349-B/2013 (copy hosted by the University of Lisbon): annex, point 3.
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis, Portuguese version, 2021: airing, air humidity, furniture.
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home, updated 1 April 2026: ventilation, window air inlets, energy efficiency (first 100,000 characters of the page read).
- Diário de Notícias, Cuidados com o monóxido de carbono, o gás que não se sente mas mata, 22 October 2019: DGS recommendations as quoted by the paper; the primary DGS text was not read directly.
- Portaria n.º 353-A/2013, at informador.pt: we saw it covers commercial and service buildings, so we do not use it.