The heating comes on in October, and soon the familiar complaints are back: the scratchy throat in the morning, the itchy skin, the eyes that sting by evening. A humidifier seems like the obvious fix, and sometimes it’s the right one. More often though, the dry air is a symptom of how your home is heated and aired and can be fixed without an extra appliance, or your scratchy throat and stinging eyes have causes a humidifier won't address. Here’s how to tell which it is.
Why heated air gets so dry
Cold winter air holds very little water. Warm it up and the amount of water stays the same while the air's capacity rises, so relative humidity falls. That is established physics. A SuisseEnergie fact sheet (the Swiss federal energy program) gives an example: outdoor air at 0 °C, heated to 21 °C with no added moisture, holds about a tenth of the water it could.
How much humidity is right?
The short answer: roughly 30 to 50%, depending on your comfort and health issues. Below 30% and comfort becomes an issue: tight and itchy skin, cracked lips. A 2016 review in the Journal of the European Academy of Dermatology and Venereology concluded that low humidity and low temperatures weaken the skin's barrier and make it more reactive to irritants, and that cold, dry conditions raise the risk of flares for people with eczema. Dry air also favors viruses: a 2024 review in the International Journal of Hygiene and Environmental Health places the lowest risk of influenza and COVID-19 infection between 40% and 60%, and a 2013 laboratory study in PLOS ONE found that flu virus from simulated coughs kept far more of its infectivity at 23% humidity than at 43% (a test room, not a home).
Above 50%, dust mites thrive: in a 2001 study in the Journal of Allergy and Clinical Immunology, homes kept below 51% humidity for 17 months ended up with more than ten times less dust-mite allergen than humid homes. The World Health Organization treats persistent damp and mold indoors as a health risk. Find the level that feels comfortable for you and try to maintain it. A simple hygrometer in two or three rooms, read morning and evening, tells you where you stand.
The right way to heat and air your home in winter
The higher the temperature is set on the thermostat, the drier your air will be. To help keep the humidity level at 30% or more, follow the SuisseEnergie guideline and don’t heat any room above 21 °C. In an older home that doesn’t have a mechanical ventilation system, follow the Federal Office of Public Health's advice: open all the windows wide for at least 5 minutes in the morning, at midday and before bed (3 minutes on very cold or windy days) to bring in fresh air, instead of leaving the windows open a crack all the time. This technique gives you fresh air quickly, instead of losing heat for hours and making your heating work harder.
When a humidifier won't help
Your nose and throat humidify the air you breathe on their own. Irritation blamed on dry air is often coming from something in it: fine particles, formaldehyde and other volatile compounds, or smoke. So, assuming you’re following the heating and airing guidelines above, your first step should be an air quality monitor that measures CO2, VOCs, formaldehyde, PM2.5 and PM10. Based on its readings, air out more frequently, and choose materials and furniture that emit as little as possible.
When a humidifier makes sense
Two situations: when hygrometer readings stay well below 30% even after you have fixed temperature and airing, or when someone in your household has a respiratory or skin condition and their doctor recommends it. Tip: buy a model with a built-in hygrometer that automatically maintains a target humidity level, keep it away from cold windows, and clean it often. Too much moisture breeds microbes in the device itself and on cold walls, and damp can affect health before any stain appears.
Thinking of renovating? What design can do for temperature and humidity
Heat room by room
Heating zoned by room keeps living areas comfortable and lets the bedroom run cooler for sleep. Not too cool, though: the Federal Office of Public Health warns that in older, poorly insulated buildings, bedrooms below 18 °C, especially with the door open, can lead to condensation and mold.
Automated ventilation
When a whole-home renovation is planned, ventilation becomes a design decision: whether to add a system at all, central or room by room, and whether it should recover moisture as well as heat (enthalpy exchangers). That conversation happens with the HVAC engineer early, before ceilings are closed.
Minergie-certified and many newer buildings already have controlled ventilation. Where there’s a system, its settings matter: the SuisseEnergie fact sheet notes that ventilation regulated by CO₂ keeps the humidity people produce in the home, so it rarely falls below 30%, and recommends adjusting systems accordingly.
Let the walls help
A 2022 study in the journal Materials found that clay plaster, rammed earth and unfired brick absorbed and released air moisture faster than concrete, lime plaster or gypsum board, though not every clay product performs equally. Together with the right paint or wall finish, the evidence so far suggests a buffer effect that smooths daily swings, as a complement to ventilation. None of these materials are standard in construction today. They’re design decisions, planned in advance.
Deal with the cold spots
Condensation and mold start where surfaces are cold: old glazing, thermal bridges, the corner behind a wardrobe on an exterior wall. A well-planned renovation identifies these issues and resolves them before furniture or finishes are installed.
Where to start
If your home feels dry every winter, or dry in one room and damp in the next, it’s important to understand why before any renovation decision. Heating, ventilation and finishes are whole-home systems, best designed together. The Longevity Audit measures humidity and temperature alongside light, air, water, noise and EMF, so the conversation starts from your home's own numbers rather than a guess. Or simply get in touch.
Based in Lausanne, I work across Switzerland, in French, English and Mandarin.
