Close the door, shut the windows and turn up the heating, and you have all the ingredients for a sick bedroom in your home – and you spend about eight hours in there every night.
Kitchens and bathrooms usually take the blame for stale air and damp. But researchers spent a winter monitoring airtight, naturally ventilated homes across Scotland. In more than four out of five homes the carbon dioxide (CO₂) in the air remained above the level considered a safe threshold for the entire night. Despite this, more than nine in ten of the occupants thought the air in their bedroom was good.
The reason is not the room. It is you. Awake in the living room, you are a thermostat with hands. When the air turns stuffy, you crack a window. When you feel cold, you close it. This correction stops in bedrooms once you fall asleep, and two things build up: the CO₂ you breathe out and moisture with it.
High levels of CO₂ act as a proxy for stale air. CO₂ levels are measured in parts per million (ppm), a unit of measurement used to describe very small concentrations of a substance.
A study examining 40 Danish bedrooms revealed a troubling threshold: when CO₂ levels reach 1,800ppm, participants’ cognitive function the following day suffered. And sleep quality starts to suffer well below that.
Recognising this risk, strict European standards allow living spaces to reach 1,350ppm above outdoor levels, while restricting bedrooms to just 950ppm above outdoor levels. For reference, CO₂ levels in fresh outdoor air are typically between 420 and 450ppm.
Between 2022 and 2025, I continuously monitored temperature, humidity and CO₂ data using sensors from around 200 Irish rooms in highly energy-efficient but airtight Irish social housing. The bedrooms spent roughly two hours every night in winter above unhealthy levels, while the living rooms next door were fine. The same bedrooms were fine again during summer. The likely reason is that people keep their windows shut in winter to stay warm and open in summer.
Serious health consequences
Two sleeping adults breathe out about a pint of water a night. I run a dehumidifier at home over the winter – it holds two litres, and from October onwards I empty it every other morning.
Moisture, when trapped indoors, settles on the coldest surface – usually, the window or the wall behind a wardrobe. A cold, damp surface in a warm, moist room is an ideal condition for mould to grow.
This is not just a cosmetic problem. Prolonged exposure to mould in a flat in Rochdale, a town in Greater Manchester, killed two-year-old Awaab Ishak in 2020. Consequently, Awaab’s law now legally compels social landlords in England to address damp and mould within strict, fixed deadlines.
Small children are the most exposed of any of us, their lungs and immune systems are still developing, and they sleep longer than adults.
Airing a room by opening the window clears the stale air and the moisture at once, but it lets the heat out. That is the trade-off between fresh or warm air.
The trickle vents (small slots built into window frames for background airflow) are often found shut after energy upgrades, and a 2024 Irish study of retrofitted homes found bedroom CO₂ higher after the work than before, with around half showing visible mould.
People often shut trickle vents. s.borisevich/Shutterstock.com
This feels like deja vu. After the oil shocks of the 1970s, offices were sealed and ventilation cut to save fuel, and the occupants got headaches, itching eyes and a fog that lifted only when they went home. It was called sick building syndrome. It’s the same – except nobody gets to go home from their bedroom.
Energy-efficient but sick buildings often cost society far more than they save. Renters sit in the middle of it, unable to carry out expensive building changes.
What to do
If an open window loses heat, at least keep trickle vents open. Leave the bedroom door ajar, which gives the moisture somewhere to go.
A generational practice and German daily ritual of Lüften is now popularised on TikTok as “house burping”. By throwing the windows wide open for ten to 15 minutes immediately after getting up, you rapidly purge the stale air and moisture while saving heat loss.
When it is extremely cold, even for Lüften, people often reach for a dehumidifier. These devices cost roughly between £90 and £300, and a typical dehumidifier draws around 250 watts and costs around 6p to 9p an hour to run.
Most dehumidifiers have a humidistat (a small electronic device used to measure and control the humidity) that you can set to a minimum target. A sensible target for a bedroom is 60% – the machine runs until the room reaches it, shuts itself off, and wakes again when the humidity creeps back up. Others run for a specified period, set as desired, or continuously.
Dehumidifiers are a genuine help for a cold, damp bedroom when there are few other options.
While none of this replaces a proper ventilation system, it changes what you breathe at night. For definitive proof of bad air quality in your bedroom, use an indoor air quality monitor. Seeing the hard data is often the best prompt to open a window, clear a vent, or switch on a dehumidifier.
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