Ventilating correctly in winter
Ventilating correctly in winter
Fully open windows instead of tilting, the right humidity and enough heating: this keeps the air fresh in winter and gives mould no chance – following the FOPH recommendations.
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Why ventilating correctly matters especially in winter
Breathing, cooking, showering and washing produce water vapour: according to the FOPH, a four-person household releases about 7.5 litres per day on average, and up to 12 litres with high use. This moisture has to be removed by ventilating – in winter this is particularly easy because cold outdoor air contains little water.
Anyone who ventilates too little risks moisture on cool walls and windows, and therefore mould. Anyone who ventilates incorrectly, for example with permanently tilted windows, loses a lot of heating energy and cools the walls. On this page we summarise the recommendations of the Federal Office of Public Health (FOPH) and EnergieSchweiz.
As of October 2026. The recommendations apply to flats with window ventilation unless stated otherwise.
Hygienic air exchange: why ventilating is necessary
Ventilating removes not only moisture and odours but also substances people give off when breathing and substances from furnishings and building products. According to the FOPH, removing moisture reduces the risk of mites, moisture problems and mould.
Regular ventilation is especially important in new or renovated buildings with an airtight envelope and tight windows: hardly any air exchange happens there by itself. After new construction or renovation, ventilate more during the first weeks.
Your nose does not reliably tell you when to ventilate. The FOPH therefore recommends ventilating on a fixed schedule – or using a CO₂ meter.
Airing with open windows
How often and how long to ventilate in cold weather?
Open the windows fully, if possible with a cross-draught – the air is exchanged quickly without walls and floors cooling down.
| Situation | Recommendation |
|---|---|
| Whole flat | In the morning, at midday and before going to bed, open all windows fully, with a cross-draught whenever possible. If you work during the day: in the morning, when you get home and before going to bed. |
| Duration in the heating season | At least 5 minutes. On very cold days, in wind or with cross-ventilation, at least 3 minutes. The colder it is outside, the shorter you can ventilate. |
| Home office, living room | Ventilate rooms used for a long time or by several people in between as well. |
| Cooking, showering, bathing | Switch on the cooker hood or extractor fan. Without an extractor fan, ventilate during or immediately after showering or cooking and keep the moisture from reaching other rooms. |
| Bedroom at night | Do not leave windows open or tilted at night. Air the whole flat before going to bed; if the bedroom door stays open at least 10 cm, the air quality holds up better until morning. |
| Fogged windows | Always a sign of excessive humidity: ventilate immediately. |
| Flat with a ventilation system | Additional window ventilation is not necessary. If it is too humid in winter, turn the control up one level; if the air is too dry, turn it down one level. |
Why tilted windows do harm in winter
A permanently tilted window exchanges the air only slowly but constantly lets heat escape. According to the FOPH, walls and floors near the window cool down – exactly where the risk of condensation and mould growth rises. The cooled building parts then have to be reheated with extra energy.
The FOPH therefore recommends not leaving windows tilted in the colder season and airing regularly with fully opened windows instead. This also applies to the bedroom at night.
Humidity
What humidity during the heating season?
A simple hygrometer shows the relative humidity. According to the FOPH, the pictograms on some devices are often misleading – go by the numbers.
| Relative humidity | Meaning according to the FOPH |
|---|---|
| 30 to 50 % | FOPH rule of thumb for living spaces. During the heating season, humidity should not exceed 50 %. |
| At most 40 % on very cold days | On very cold winter days, humidity above 40 % can cause moisture problems on cooled building parts in many buildings. |
| Above 50 % | Promotes mites and, at higher humidity levels, mould. In poorly insulated old buildings, even about 50 % can lead to condensation in autumn and winter. |
| Below 30 % for a long time | Not a problem for many people. Sensitive people may suffer from dryness, irritation, conjunctivitis or skin eczema. |
Cold air can hold less water than warm air: air at 30 degrees holds up to 30.3 grams of water per cubic metre, air at 10 degrees only 9.4 grams. When humid room air cools on a cold wall or window, the excess water condenses there.
Humidifiers: Do not humidify in a flat with moisture problems. According to the FOPH, humidifiers should only be used during a cold spell, monitored with a hygrometer and set specifically to a maximum of 40 %.
Heating and ventilating go together
- Start heating in time: If the heating season starts too late, the outer walls cool down and the risk of moisture and mould rises.
- Do not leave any rooms completely unheated: Exceptions are cellars and attics designed as cold rooms.
- Poorly insulated buildings: Do not delay the start of heating and set the room temperature to around 20 °C.
- Well-insulated buildings: Lowering the bedroom temperature to 18 °C is not a problem. Very low temperatures below 18 °C can lead to condensation on cooler walls, especially with the room door open.
- Close doors to cooler rooms: This keeps warm, humid air out of less heated rooms.
- Do not overheat: The FOPH recommends 20 to a maximum of 22 °C during the heating season. Each degree less saves around 6 to 10 % of heating energy.
Rule of thumb: no moisture should condense on windows and cool walls.
Everyday life
Furniture, laundry and other sources of moisture
Small habits make a big difference in winter.
Distance from the outer wall
Leave at least 10 cm between furniture and outer walls (FOPH); EnergieSchweiz recommends 10 to 15 cm. Better not to place large furniture such as wardrobes and wall units against cool outer walls or in outer wall corners.
Drying laundry
Do not dry laundry in living spaces. Use the drying room. Wet laundry releases its moisture into the room air.
Plants and aquariums
Many houseplants and open aquariums release moisture. If it is too humid, removing some plants helps.
Wipe up spills quickly
Wipe up spilled liquids quickly and completely with an absorbent cloth.
Preventing mould: recognising warning signs
According to the FOPH, houses at particular risk are those built before 1980 whose outer walls have not been thermally renovated, older houses with new windows, houses with outer walls less than 35 cm thick and houses right next to larger bodies of water. Here it pays to follow the recommendations especially consistently.
- Fogged windows or wet window frames
- Clearly noticeable mouldy smell – a sign of hidden mould
- Dark spots in outer wall corners, behind furniture or on joints
- Mildew stains or a history of water damage
Mould found
What to do about mould? The FOPH categories
Always identify and eliminate the cause of the moisture first. Tenants should report moisture, mould or mildew stains to the landlord or property management immediately – ideally with a description or photos by registered letter.
| Category | Extent | Recommendation |
|---|---|---|
| Kat. 0 | Superficial traces of mould up to 100 cm² (10 × 10 cm), in one spot, in one room only | You can wipe it off yourself: on smooth surfaces with water and household cleaner. On rough surfaces, after cleaning and drying, disinfect with bleach (Javel water) or 70–80 % ethyl alcohol. Do not use vinegar, wear plastic gloves. |
| Kat. 1 | In one spot, in one room only: up to 0.5 m² (70 × 70 cm) for scattered growth or up to 100 cm² for dense growth | Do-it-yourself removal possible with care, for legal reasons only by owners – not for people with chronic respiratory diseases such as asthma or a weakened immune system. Alternatively, have a tradesperson remove it professionally. |
| Kat. 2 | Larger or denser growth, several spots or signs of greater damage | No do-it-yourself removal: call in specialists in building damage and building physics or a professional remediation. |
Important: According to the FOPH, never switch on dehumidifiers before the mould has been removed. If mould keeps coming back despite airing and heating, a specialist in building damage and building physics can help.
When comfort ventilation makes sense
Mechanical ventilation (comfort ventilation) ensures that rooms are always sufficiently ventilated without anyone having to open the windows. According to the FOPH, all Minergie-certified residential buildings, for example, have it. Windows may still be opened.
- Heat recovery: The heat of the exhaust air is transferred to the supply air; according to the FOPH, the efficiency is usually over 80 %. Cantonal energy regulations require heat recovery for ventilation systems.
- Filters: Fine dust and pollen can be filtered out, and outdoor noise does not disturb you while ventilating.
- Small flats with several people: According to the FOPH, the necessary air exchange may not be achievable with window ventilation here.
- After installing airtight windows: Air exchange drops. If this is not compensated by more frequent airing, mould can form on cool walls – plan the air exchange as part of the renovation concept.
Dehumidifying: In damp cellars, dehumidifiers can help, for example at night in summer with a timer. With mould: remove the mould first, then dehumidify.
Sources
FOPH – Correct ventilation and heating · FOPH – fact sheet «Richtig Lüften» (31 August 2022, PDF) · FOPH – brochure «Vorsicht Schimmel» (August 2023, PDF) · EnergieSchweiz – «Besser wohnen. Tipps und Tricks für mehr Komfort» (03.2022, PDF). As of October 2026, no guarantee. The original sources are authoritative.
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FAQ
Questions about ventilating in winter
How long should I ventilate in winter?
At least 5 minutes during the heating season. On very cold days, in wind or with cross-ventilation, at least 3 minutes are enough – always with windows fully open.
How often should I ventilate in winter?
Air the whole flat with a cross-draught in the morning, at midday and before going to bed, plus after cooking and showering and in rooms used for a long time or by several people.
Why shouldn't I tilt the window in winter?
A tilted window exchanges the air only slowly but uses a lot of heating energy. Walls and floors near the window cool down and the risk of condensation and mould rises.
What humidity is right in winter?
The FOPH recommends 30 to 50 % relative humidity, not above 50 % during the heating season and not above 40 % on very cold days.
How much distance does furniture need from the outer wall?
At least 10 cm (FOPH); EnergieSchweiz recommends 10 to 15 cm. Better not to place large wardrobes against cool outer walls or in outer wall corners.
What should I do if mould appears?
First identify the cause of the moisture. You can remove small traces up to 100 cm² in one spot yourself. Larger or recurring areas should be left to specialists. Only use dehumidifiers after the mould has been removed.
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