How Much Water Vapour Does a Home Produce?
Nobody pours buckets of water on their living-room floor, yet an ordinary household releases litres of water into its air every single day — invisibly, as vapour. Once you see the tally, half the mysteries of home humidity solve themselves: the fogged windows, the damp bedroom, the flat that “feels humid” for no obvious reason.
The daily tally
Every figure below is a rough order of magnitude — actual numbers vary with habits and household size — but the shape of the tally is robust:
| Source | Roughly per day |
|---|---|
| Breathing and perspiration (per person) | 1–1.5 L |
| Cooking (with lids off, pasta water boiling away) | 0.5–1.5 L |
| Showers and baths | 0.2–0.5 L each |
| Drying a load of laundry indoors | 1–2 L |
| Dishes, plants, cleaning, aquarium | a few 100 mL |
For a family of four, that adds up to somewhere around 5–10 litres of water per day, released into the air as vapour. A bucket or two. Every day.
Why the air alone cannot absorb it
Here is the part most people miss: air holds astonishingly little water. At 20 °C, a cubic metre of air can carry at most about 17 g of water. So the entire air volume of a 20 m² room — call it 50 m³ — maxes out below one litre, and that is at 100% humidity, dripping point.
Your household produces several times that every day. The air is not a reservoir; it is a conveyor belt. If the belt does not move — if the vapour is not carried outside — the water has to go somewhere else, and the “somewhere else” is your home itself.
Where the water goes when it can’t leave
Unvented moisture takes three exits, all bad:
- Cold surfaces. Vapour condenses on anything colder than its dew point: window panes first, then cold exterior walls, window reveals, the wall behind the wardrobe.
- Materials. Plaster, wood, textiles and books absorb vapour and swell with it. This buffering hides the problem for weeks — then gives it back, which is one reason homes turn damp in autumn.
- Mold. Persistently damp surfaces are an open buffet. Mold does not need a flood, just a wall that never quite dries.
Ventilation is the only exit
Since the sources are your life — you will not stop breathing, cooking or showering — the fix is on the exhaust side: regularly swap the humid indoor air for outdoor air that carries less water. Three habits cover most of it:
- Ventilate at the source, immediately: extractor or wide-open window during and after cooking and showers, door shut so vapour cannot roam.
- Do a few short whole-home airings daily — wide open, minutes not hours.
- Handle the big one-off loads deliberately — indoor laundry drying deserves its own room and routine.
One caveat: “outdoor air that carries less water” is measured in absolute terms — grams per cubic metre — not in the relative humidity percentage, which changes meaning with temperature. Cold rainy air at 90% RH is usually still far drier, in real water content, than your kitchen mid-boil.
Let the app watch for you
The daily litres are non-negotiable; the question is when the outside air is actually able to carry them away. Open/Shut compares the real water content of the air inside and outside your home and tells you the moments when opening the windows genuinely drains the bucket.