Condensation

Definition

Condensation is defined as the physical process by which a gas or vapour changes into a liquid. If the temperature of an object (e.g. grass, metal, glass) 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 internal and external environment, and the glass, that causes condensation to form. Condensation is the point at which water vapour turns to liquid.

How it affect homeowners

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 which 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).

Factor of condensation

The four main factors governing condensation are:

  1. Water vapour content of the air
  2. Inside room temperature
  3. Outside temperature
  4. Variation between inside room temperature, outside temperature and the glazing

The first two factors are normally controllable.

1. 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.

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2. Inside room temperature

This can be controlled to some extent by replacing single glazing with double or triple glazing, thereby maintaining a higher surface temperature of the glass on the room side, and by increasing the air temperature to enable it to hold more water vapour without condensing.

3. Outside temperature

This cannot be controlled, but its effect on the inside room temperature can be countered by the installation of 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.

NOTE: It is often the case that external condensation will appear on some windows but not on others due to variable micro-climates in differing locations. Under average exposure conditions, and provided the room is heated, the room side surface temperature of the inner glass will be higher than would be the case with single glazing. The likelihood of condensation occurring when warm moist air in the room comes into contact with the surface of the glass is thereby reduced.It must be remembered, however, that double or triple glazing is an insulator and not a source of heat; it does not control the amount of water vapour in the air.

When rooms are inadequately heated and there is little heat to retain, double glazing cannot fulfil the purpose for which it was installed.One reason why condensation forms in a room not normally occupied is that many householders, for reasons of economy, do not heat such rooms. Consequently the surface temperature of the inner glass gets very close to the outside temperature. In addition, the windows in such rooms are generally kept closed, but water vapour, generated elsewhere in the house, will find its way in and then not escape. Thus the two conditions necessary to produce condensation – a low glass surface temperature, and high water vapour content in the atmosphere – are present.

How double (and triple!) glazing helps to reduce condensation

Double or triple glazing is an insulator, designed to reduce the loss of heat by conduction from the inside to the outside of a building. Current Building Regulations, (Approved Document L: Conservation of Fuel and Power), specify that all new or replacement windows must meet a minimum performance criteria. This requirement can only be met by the installation of energy efficient windows and doors. Under average exposure conditions, and provided the room is heated, the room side surface temperature of the inner glass
will be higher than would be the case with single glazing.

The likelihood of condensation occurring when warm moist air in the room comes into contact with the surface of the glass is thereby reduced.It must be remembered, however, that double or triple glazing is an insulator and not a source of heat; it does not control the amount of water vapour in the air. When rooms are inadequately heated and there is little heat to retain, double glazing cannot fulfil the purpose for which it was installed.One reason why condensation forms in a room not normally occupied is that many householders, for reasons of economy, do not heat such rooms.

Consequently the surface temperature of the inner glass gets very close to the outside temperature. In addition, the windows in such rooms are generally kept closed, but water vapour, generated elsewhere in the house, will find its way in and then not escape. Thus the two conditions necessary to produce condensation – a low glass surface temperature, and high water vapour content in the atmosphere – are present.

Glass and Glazing Federation
54 Ayres Street
London SE1 1EU

Get in touch

If you have any questions about condensation in your home please do not hesitate to call us on 0800 999 0909 or 01902 661 779. Alternatively you can also send us an Email.

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