Condensation and the Causes of Condensation

Learn about condensation, why and when it occurs and which of its causes you can control to reduce its likelihood of happening in your home.

What is condensation?

Condensation in a household setting is when airborne water vapour condenses into a liquid and is deposited on interior (or exterior) surfaces.

If the temperature of an object (e.g. grass, metal, the glass of a windowpane) falls below what is known as the dew point temperature for a given relative humidity of the surrounding air, water vapour from the atmosphere condenses into water droplets on its surface.

This dew point varies according to the amount of water in the atmosphere and air temperature (known as relative humidity). In humid conditions, condensation occurs at higher temperatures. In cold conditions, condensation occurs despite relatively low humidity.

With regard to windows and doors, it is the difference in temperature between the environment, be it internal or external, and the glass, that causes condensation to form.

Why does it occur in homes?

The air surrounding us in our homes always contains water vapour, which is invisible. A typical example is the steam cloud from a kettle, which rapidly becomes invisible – it has in fact been absorbed into the atmosphere.

The warmer the air, the more water vapour it can hold – but there is a limit to the amount it can hold for a given temperature. When that limit is reached, the air is said to be ‘saturated’.

When saturated air comes into contact with a surface that is at a lower temperature than itself, the air is chilled at the point of contact and sheds its surplus water vapour on that surface – initially in the form of a mist and, if excessive, eventually in the form of droplets of moisture.

An example of this is when a person breathes onto a mirror: condensation occurs because the exhaled air is saturated and its temperature is higher than that of the mirror (which is at room temperature).

The factors governing condensation

1. The water vapour content of the air

This is produced by normal living activities such as washing, cooking, bathing, etc., and can be controlled by the use of extractor fans, cowlings, and ventilation at appropriate places.

2. Inside room temperature

This can be controlled to some extent by replacing single glazing with energy-efficient double or triple glazing, thereby maintaining a higher surface temperature of the glass on the room side.

This will help to retain the room’s air temperature, which, along with adequate ventilation, will enable the room to hold more water vapour without condensing.

3. Outside temperature

This cannot be controlled, but its effect on the inside room temperature can be reduced by the installation of energy efficient double or triple glazing.

4. Internal and external temperature variation

This cannot be controlled as the main variant is the outside temperature. However, this variation may also be affected by building orientation, localised atmospheric conditions, shelter from nearby trees or buildings, air currents, wind speeds and nearby vegetation.

Some things to note:

– It is often the case that external condensation will appear on some windows but not on others due to variable microclimates in differing locations.

– Condensation may occur on the exterior of a window or door when the surface temperature of the outer pane is below the dew point. This can be the result of the reduction in the transfer of heat from inside to out and is visible evidence of the energy efficiency of the window or door.

– Following the installation of replacement windows and doors, it is important that adequate ventilation is included to remove the airborne vapour. Failure to do this may result in this vapour condensing on the coldest surface which would no longer be the window but could be an outside wall.