Some causes, some advice.

The Issue

This page explains why condensation occurs on internal and external surfaces and offers some advice about control.

Internal condensation

Condensation on windows and in conservatories, and the damage it can do to paintwork, curtains, wall coverings and window fittings, are problems sometimes encountered in all types of building.

Modern aids to home comfort have created rooms which are warmer but which often have less ventilation and fewer air changes. The result is that the water vapour produced by normal living activities is no longer able to escape up the chimney or through door jambs, window joints and other outlets.

In certain circumstances, all these aids to comfort combine to create ideal conditions for the formation of condensation, which could form on the coldest surface within the room. This may not necessarily be on the glazing.

The question of how to reduce condensation without sacrificing the benefit of increased comfort is covered within this page.

External condensation

Due to recent innovations in the efficiency of double and triple glazing, along with updated requirements of building regulations and the lowering of carbon emissions, certain weather conditions may allow the formation of external condensation on energy efficient windows and doors.

This is a natural phenomenon and a clear indication that the window or door is preventing heat loss from your house. Further explanation can be found within this page.

What is condensation?

Condensation is the point at which water vapour turns to liquid.

Technical definition of condensation

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.

What this means to the Householder

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).

A single glazed window cannot retain the heat within the room and the lower temperature of the glass allows the moisture in the air to condense on the cold surface. This is often more evident in rooms in which there is a lack of ventilation.

Although a double glazed window is capable of retaining far more internal heat, the less efficient types allow a certain amount to pass through the air space and thus warm up the outer pane. This would not therefore allow condensation to form on either pane. This assumes the room is heated and ventilated.

Some examples of where the water vapour comes from internally:

Breathing: two sleeping adults produce approximately 1 litre of moisture in 8 hours, which is absorbed as water vapour into the atmosphere.

Cooking: steam clouds can be seen near saucepans and kettles, and then seem to disappear. The clouds have been absorbed into the atmosphere. The heat source itself may be a source of water vapour; e.g. an average gas cooker could produce approximately 1 litre of moisture per hour.

Heaters: a flueless gas heater can produce up to 350cc of moisture per hour. Paraffin heaters produce 4 litres of moisture for every 3.5 litres of fuel burned.

New property: the bricks, timber, concrete and other materials in an average 3-bedroomed house absorb about 7000 litres of water during construction. Much of this is dissipated into the indoor atmosphere during the drying out period.

Washing up: the vapour clouds given off by the hot water are rapidly absorbed into the atmosphere.

Indoor plants: a frequently unrecognised but nevertheless significant source of water vapour.

Bathing, laundry, and wet outer clothing: these are often major sources of water vapour in the home.

Where the water vapour comes from externally:

It is always present and the levels are dependent upon atmospheric conditions (temperature and humidity).

A typical example is the formation of condensation on the entire surface of a car, including the glazing, when left in an exposed area. This condensation would typically be removed using both the wipers blades and a squeegee.

The factors governing 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.

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.


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