If windows turn foggy 30. december 2010

In wintertime, a normal heated 20 m² room has 300-400 g of water in the air (see chart – light blue area on the right). At about 7°C the 400 g of water does not fit in the air anymore (light blue area on the left) and starts to condense on the coolest surface. In a house that has been properly built, this surface is the window. If the heater room air contains 500 g of water (dark blue area on the right), the condensation starts already at 11°C. At -10°C, every window, even the most modern one with a high thermal resistance, will have some places in the room below 11°C.

In wintertime, even 150 g of excessive water in the air of a 20 m² room will make any window fog up. This amount of water is easy to accumulate in a badly ventilated room: a human breathes out 250 g of water in one night, the watering can has 500 g and laundry that has been hanged to dry more than one kilogram of water.