How much water does a household actually put into the air each day?

People are consistently surprised by the scale of this. A house is not gaining moisture in traces — a family produces litres of water vapour a day through completely ordinary activity, and all of it has to leave the building or it ends up on a wall. Once you can see the balance, condensation stops being mysterious and becomes arithmetic.

Step by step

1. Count the washing first.A washing machine spins clothes to a defined dampness and the rest of the water evaporates wherever the clothes dry. Dry a load indoors and effectively all of that water goes into your air. It is the largest single source in most homes by a wide margin.
2. Look at the cooking.Pans boiling without lids, a hob used for an hour, a kettle, a slow cooker. Lids on pans, a working extractor hood ducted outside, and the back rings rather than the front all cut it substantially for nothing.
3. Count the bathroom.A shower or a bath releases a lot of water fast. What matters is whether the extract fan removes it before it spreads — a fan with an overrun timer and the door kept shut for a few minutes afterwards handles most of it.
4. Count the people.Breathing and perspiration are continuous and unavoidable, and this is why a bedroom slept in with the door and window shut has streaming windows by morning. The number of occupants is a genuine factor in how much ventilation a house needs.
5. Check the tumble dryer.A vented dryer must be ducted outside; a vented dryer venting into the room is one of the largest moisture sources it is possible to install. A condenser or heat pump dryer collects the water instead, provided the tank is emptied and the filters are clean.
6. Look for the unusual sources.Paraffin and bottled-gas portable heaters produce roughly a litre of water for every litre of fuel, which makes them uniquely bad in a damp room. Large numbers of houseplants, fish tanks and indoor drying racks all add up.
7. Then look at the routes out.Extract fans that work, trickle vents left open, purposeful airing, and background infiltration. A modern draught-proofed house has lost most of the last of those, which is why condensation problems often start after windows and doors are replaced.
8. Balance the two.If the input exceeds what the house can remove, the surplus goes into the fabric. Reducing the input is usually free; increasing the output usually costs something. Do the free part first.

Tips

• If washing must be dried indoors, do it in one room with the door shut, the window open a little or a dehumidifier running, rather than spread across the house.
• A dehumidifier in the room where washing dries turns the largest moisture source into a manageable one, and it recovers some of the heat as it works.
• Emptying a condenser dryer's tank promptly matters — a full tank means the machine stops removing water and the load takes longer, adding to the room instead.

Common mistakes

• Drying washing on radiators — It puts the water into the air at exactly the time of year when the windows are shut, and it also makes the radiator less effective at heating the room, so the surfaces stay colder as well. It is the worst combination available.
• Using a portable gas or paraffin heater in a damp room — Combustion of any hydrocarbon produces water vapour. An unflued heater releases all of it into the room, so it makes the damp measurably worse while appearing to help. There are also combustion product and carbon monoxide considerations.

Questions people ask