If windows turn foggy 30. december 2010

In cold weather even very good windows can fog up if the excessive humidity does not get out of the room in any other way.

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.

In the below chart it is shown how much water there is in the air of a 20 m2 room.

Table header translation: [Room temperature /Grams of water in the air of a 20 m2 room at relative humidity]