A family of four people generates about 9 litres of water vapour every day inside their home, according to calculations based on the CSTB values. This amount is similar to the amount of water in six 1.5-litre bottles, which evaporates into the air and then settles on cold surfaces, often windows, especially when the outside temperature is 7 °C. The calculation shows that nearly half of this vapour comes from breathing, while showers and cooking contribute about a third. The calculation assumes four adults, with each producing 55 grams of vapour per hour when active and 30 grams when asleep. Over 16 hours of wakefulness and 8 hours of sleep, the total vapour produced by four adults is 4.48 kg. Additional vapour comes from daily activities: cooking and dishwashing add about 1.5 kg per day, four showers contribute 1.6 kg, and a washing machine dried indoors adds 1 kg. Including floor cleaning, the total vapour reaches approximately 9 kg, which is equivalent to 9 litres of water. These figures can vary depending on household activities, such as the number of laundry loads or cleaning sessions. Windows tend to fog up first because they cool down more than insulated walls when the outside temperature is 7 °C. In a living room at 20 °C and 60% relative humidity, the dew point is approximately 12 °C, the temperature at which the air becomes saturated. Surfaces colder than this will see condensation. Older or single-glazed windows can drop below this temperature, leading to fogging. A well-insulated wall stays closer to the room temperature, which is why fog appears on the bay window before it reaches the partition. According to the CAUE document on house ventilation, a family of four people produces between 10 and 20 litres of water vapour per day. The calculated 9 litres falls within this range, assuming only one machine is dried indoors and no major cleaning. Over 30 days, 9 litres per day add up to 270 litres, which is enough to fill more than one bathtub. The ADEME guide links condensation on cold surfaces to the appearance of mould and warns that renovated and well-insulated homes need to rethink their ventilation. Insulating a home without addressing this issue risks trapping the same amount of vapour in a house that exchanges less air with the outside. To address condensation, showers and cooking are major sources of vapour, contributing 3.1 kg to the total. These emissions are localized and easier to treat at the source. Opening windows for 5 to 10 minutes after a shower and a meal can help vent some of this vapour before it reaches the windows. Drying laundry indoors emits 1 kg of vapour per machine, and testing the timing of fog appearance on a weekday morning can help assess the effectiveness of ventilation. If fog is delayed or does not return, it indicates successful ventilation; if it persists, the house's ventilation system may need to be checked.