The bathroom makes more water vapour than any other room: a hot shower fills the air with water in a few minutes. If the vapour does not leave, it stays on the tiles, the ceiling and the joints, and mould follows. This guide sets out what the housing regulation, the Swiss public health office, the UK health guidance and one fan manual say about windows, extractor fans, safety around gas appliances and upkeep.
Updated 4 November 2026. General information, not a substitute for an assessment by a qualified specialist.
The essentials
- Portaria no. 138-I/2021 says home ventilation should be natural where possible, topped up with mechanical ventilation when needed. In homes, air comes in through the main rooms and leaves through the service rooms, such as the bathroom.
- The FOPH (Switzerland) says to ventilate or switch on the fans when bathing, cooking and ironing, to repair broken fans quickly and to clean extractors from time to time.
- SNS 24, the Portuguese health line, advises not keeping gas installations in the bathroom and checking the water heater’s flue regularly.
- A fan only removes air if fresh air can get in. One manufacturer’s manual requires enough replacement air for the fan and for any fuel-burning appliance.
- A fan does not repair a cracked sealant joint or a leak. Cause first, paint second: see mould on walls.
What the regulation says
Annex II of Portaria no. 138-I/2021 (1 July) sets requirements for new or renovated buildings. It is not a rule for someone living in an existing home. What we read:
| Point | What it says |
|---|---|
| 1.1 a) | Ventilation preferably natural, supplemented by mechanical where needed |
| 1.1 d) | In housing, ventilation covers the whole building: air in through main rooms, extraction through service rooms |
| 1.2.1 and Table 10 | Minimum air change rate of 0.50 per hour in a new building, a major renovation or a renovation of the ventilation system |
| 1.2.1 b) | If only the windows of main rooms are replaced: a façade air inlet with 60 cm² of free area, or self-regulating grilles with a nominal flow of at least 25 m³/h at 2 Pa or 75 m³/h at 20 Pa |
Table 13 of the Portaria gives extraction flows for toilets and bathrooms, but it sits under point 1.2.2, commercial and service buildings. We did not apply it to a home. For housing, the text we read gives only the overall rate of 0.50 per hour and the rule of extracting through service rooms.
Window or extractor fan
| Window | Extractor fan | |
|---|---|---|
| Advantage | Free, changes a lot of air quickly | Works with the window shut, or with no window |
| Limit | Depends on you and the weather | Needs an air inlet and upkeep |
| What sources say | FOPH: tilt windows and very small windows can cause poor ventilation | DHSC: the fan should be powerful enough to clear shower moisture; one with an overrun timer keeps running after the light goes off |
The FOPH adds that mould does not always come from a lack of ventilation: poorly designed air outlets, dirty extractors and small windows also count. DECO PROteste (21 March 2026) advises cutting the vapour, for example with a less hot bath.
With no window, the fan is the only way to remove vapour. With a window, the FOPH asks for airing with a through-draught for 5 to 10 minutes, opening windows and doors. Closing the bathroom door during the shower, so vapour does not drift into other rooms, is our own advice, not from a source we read.
Safety with gas
This is where an extractor fan can be dangerous.
- SNS 24 (page undated) advises regular checks of the water heater’s flue system and not having gas installations in the bathroom.
- A manufacturer’s manual (Manrose, Conceal Silent 100 mm fans, reference 077645 03/16) warns that back-flow of gases into the room from the open flue of a gas or other open-fire appliance must be avoided when the fan is fitted in an outside window or wall. It adds that in a room with a fuel-burning appliance the installer must make sure replacement air is enough for both.
- The newspaper Diário de Notícias (22 October 2019) reported that the Fundão case, with three deaths, was attributed to a faulty extractor and its exhaust system. We cite the news report, not an official finding.
Our own advice, not from any of these sources: if you have a water heater, boiler or other combustion appliance in the bathroom, the kitchen or the same air space, do not fit or swap an extractor without a gas technician’s opinion. We did not find the applicable Portuguese rule.
Fitting and cleaning
The Manrose manual is British, and local electrical rules may differ. It shows what one maker asks for:
- installation supervised by a qualified electrician;
- not inside a shower cubicle or anywhere that can be splashed;
- on an outside wall, always fit the external grille;
- on a duct carrying humid air, fit a condensation trap in vertical runs, slope horizontal runs away from the fan and insulate ducts that cross cold voids;
- the timer is set at the factory to about 15 minutes and can be adjusted between 5 and 30;
- disconnect from the power supply before any maintenance.
The manual gives no cleaning steps. The FOPH only says extractors should be cleaned periodically. It also notes that sealant around the bath and shower tray loses elasticity, shrinks and lets water in: renew it in good time.
Step by step
- Open the window and/or run the fan during the shower and keep it going afterwards (DHSC: it should continue after the light goes off).
- Close the bathroom door during the shower (our advice).
- Check that the fan really moves air (a sheet of paper held at the grille sticks to it) and clean the grille and impeller with the fan disconnected from the power. This test and cleaning method are ours, not from a source.
- Make sure air can get in: a window, a grille or a gap under the door.
- Replace old sealant on the bath and shower tray.
- If mould appears, clean it and find the cause before painting. For paint, see which paint for each room and painting tiles and ceramic.
- If the problem is in a bedroom or living room, see bedroom condensation. If the mould is in corners, see thermal bridges and mould in corners.
What we could not confirm
- A minimum extraction flow for the bathroom of an existing home. The Table 13 we read is for commercial and service buildings.
- The current Portuguese rule on extractor fans and gas appliances in the same room. We read only general warnings.
- The maker’s cleaning steps: the manual we read has none.
- The cost of fitting or replacing a fan.
- How often to clean, in months: no source we read gives it.
- Airflow figures of fans sold in Portugal.
Sources (read on 4 November 2026)
- Diário da República, series 1, no. 126, 1 July 2021, Portaria no. 138-I/2021, Annex II, points 1.1, 1.2.1 and 1.2.2, Tables 10 and 13 (PDF read).
- FOPH (Switzerland), Bolor em espaços habitáveis, Portuguese version, February 2021 (sections on ventilating and repairs).
- Department of Health and Social Care (UK), Understanding and addressing the health risks of damp and mould in the home, updated 1 April 2026 (first 100,000 characters; the rest not read).
- SNS 24, Como evitar acidentes por intoxicação, undated page.
- Diário de Notícias, Cuidados com o monóxido de carbono, 22 October 2019 (news report, secondary source; it cites the DGS).
- Manrose, Conceal Silent Fans, 100 mm Standard and Timer, Installation Instructions, reference 077645 03/16, PDF created 26 August 2024 (copy hosted by a retailer).
- DECO PROteste, Que tipo de humidade tenho em casa?, 21 March 2026.