How Stale Does Your Bedroom Air Become Overnight?

What is carbon dioxide (CO₂)?

Carbon dioxide is a colourless, odourless gas. You cannot see it or smell it, which is part of the problem — a room can feel a little stuffy without you ever connecting that feeling to a number. Outdoor air contains about 420 parts per million (ppm) of CO₂, a tiny fraction of the atmosphere.

Every time you breathe out, you release air containing around 40,000 ppm of CO₂ — roughly a hundred times the outdoor level. In an open or well-ventilated space that exhaled air is whisked away and diluted to nothing. In a closed bedroom it has nowhere to go, so it gradually mixes back into the air you are breathing. CO₂ is the easiest indoor pollutant to measure, which makes it a reliable stand-in for “how fresh is this air?”.

Why bedrooms often have the worst air in the house

Bedrooms combine every ingredient for poor air quality. They are occupied for eight or nine hours at a stretch — longer than any other room. The door is usually shut. The window is often closed for warmth, quiet or security. And there are frequently two people breathing in a relatively small volume of air.

Put together, that means a steady source of CO₂ and moisture, a sealed box to collect it in, and a full night for it to accumulate. It is no surprise that the bedroom is where most people first notice the symptoms of an under-ventilated home: waking unrefreshed, a dull morning headache, a stuffy feeling when you walk back in, or condensation on the glass in winter.

Why modern homes need ventilation

Older Australian homes were leaky. Gaps around windows and doors, unsealed floorboards and uninsulated walls let air trickle in and out constantly. It was terrible for energy bills, but it did provide a kind of accidental ventilation that stopped CO₂ and moisture building up too far.

Modern homes are deliberately built airtight, because sealing the building envelope is the foundation of comfort, low energy use and quiet. The catch is that sealing the house also seals in everything the occupants produce. Airtightness is measured with a blower door test, which gives a result in air changes per hour (ACH50). The tighter the home, the lower the number — and the more important planned ventilation becomes.

This is why NCC 2022 now requires homes tested below 5 ACH50 to have designed mechanical ventilation. A genuinely airtight home without ventilation is, in effect, a sealed box — wonderful for keeping the weather out, but it means fresh air has to be brought in on purpose.

What causes condensation on windows?

Warm air holds more moisture than cold air. As the night cools, the air near a cold window can no longer hold all the water vapour it is carrying, so the excess condenses into liquid on the glass. That is the foggy or beaded window you see on a winter morning.

The moisture comes from you. Each sleeping person adds roughly 0.3 litres of water to the room overnight through breathing and perspiration. With nowhere to go, it raises the humidity until the coldest surfaces start to sweat. Persistent condensation, and the mould that often follows, is one of the clearest visible signs a home is not removing the moisture its occupants generate. We cover this in detail in why new-home windows get wet.

Can opening windows solve the problem?

Partly. An open window genuinely does lower CO₂ and humidity, and on a breezy night it can clear a room quickly. But it is uncontrolled: on a still, mild night it moves almost no air, and you have no way of knowing whether it is working. It also undoes much of why you built an airtight home — letting in noise, pollen, dust and insects, creating a security risk, and letting your heating or cooling escape. Opening a window is a useful instinct, but it is a manual workaround for a problem that planned ventilation solves automatically.

What does an MVHR system actually do?

An MVHR system does two jobs at once, continuously and quietly. It supplies fresh, filtered outdoor air to the rooms you live and sleep in, and it extracts stale, humid air from the bathrooms, kitchen and laundry where moisture and smells are produced.

The clever part is in the middle. The incoming and outgoing air streams pass through a heat exchanger, where the outgoing air hands most of its warmth (or in summer, its coolness) to the incoming air without the two ever mixing. So you get a constant supply of fresh air that has already been brought close to room temperature — typically recovering up to around 90% of the energy you would otherwise lose by simply opening a window. There is more detail in what is MVHR?

The result in a bedroom is undramatic, which is exactly the point: the windows stay shut, the room stays quiet, and the air stays fresh all night without anyone having to think about it.

How much fresh air does a bedroom need?

It is tempting to look for a single number, but there is not one that suits every bedroom. How much fresh air a room needs depends on how big it is, how many people sleep in it, and how airtight the home is — a small room with two sleepers in a sealed, modern home needs a great deal more help than a large, leaky room used by one person.

For most homeowners, the useful way to think about it is in terms of outcomes rather than airflow figures. What you actually want is a bedroom that feels fresh when you wake, humidity that stays in a comfortable range, windows that are not running with condensation, and air that is clean and easy to breathe through the night.

Those are the goals worth focusing on. A properly designed ventilation system works backwards from them: a good MVHR design is sized and balanced to achieve those outcomes in every room of the home, so you do not have to think about litres, air changes or open windows at all — you just wake up in a fresh room. Working out the right design for a specific home is exactly what we do at the consultation stage.