What is condensation?

Condensation is the process of a substance changing from a gas to a liquid and is most commonly used to describe the appearance of water on surfaces. It is normally thought of as occurring when warm moist air comes into contact with cold surfaces, but this can be misleading when trying to understand its cause and in determining actions to prevent it occurring.

The air around us is a mixture of several gases. One of these is water vapour, which is water in a
gas state.

Some description

The amount of water vapour that can be held in the air is dependent upon its temperature. Cold air can hold less than warm air. The amount of water vapour in the air is measured as a proportion of the maximum amount that could possibly be held at that particular temperature. This is called relative humidity. The importance of this feature is that, for any given relative humidity and air temperature there is another temperature known as dew point.

The dew point is the temperature at which the air can no longer hold the water as water vapour and it starts to appear as liquid water - condensation. It is important to remember that the factors
influencing the formation of condensation are the relative humidity of the air and the air temperature. These two things determine the surface temperature needed for condensation to form.

For example, the surface temperature required for condensation to occur when the air is warm and
very humid is much higher than that needed when the air is cold and very dry.

Condensation and Windows/Doors

There are three areas of our products where it is
possible for condensation to occur.

  • The surface of the product which faces onto the building
  • The surface of the product which faces the external environment.
  • The surfaces within the sealed units

Of these three, only the last is a product fault.

Condensation on the windows on the surface inside the house

Condensation can form on parts of the product that face into the building - the surface which you
can touch when standing inside the room - when they are at or below the dew point of the air inside the building in the vicinity of the product.

The temperature of the internal face of the product is dependent upon both the inside and outside
temperatures and is therefore within the control of the householder to some extent. We have already determined that the condensation depends upon the relative humidity and the air temperature, neither of which are product related but are within the control of the occupier of the building.

The control of this type of condensation is therefore the responsibility of the householder and its presence is not a product fault and should not lead to a replacement of a sealed unit. As it is not caused by a faulty product, a replacement unit subjected to the same conditions will result in the same type of condensation. 

This type of condensation can be controlled by such actions as reducing the humidity through
ventilation or ensuring surfaces do not get too cold by increasing room heating.

Some description

For more detailed information, refer to the GGF booklet "Condensation some causes, some advice".
It is not uncommon for people to describe this kind of condensation incorrectly as "internal" as they
interpret internal to mean inside the house rather than inside the sealed unit.

It IS possible that condensation can form on the surface of the product which faces the external environment - the surface which you can touch when you are standing outside the building.

Some description

High performance glazing can result in the outside surface becoming quite cold. This is because (as
intended) the heat in the house is being kept inside and is not getting through to heat the outside
surface of the glazing.

At night, the outside surface radiates heat to the environment, trying to become the same temperature. If the heat from the house is not being transmitted you can see that the outside surface can get quite cold.

If it is a cold, clear night, the outside surface of the glass may become much colder than the air and drop below the dew point of the air. In this circumstance, dew can form on the glass just as it does on the glass of a car (fig 84 - labelled C).

The formation of this condensation is actually an indication that the product is doing what you expect of it - keeping the heat in your house and sharing as little as possible with the outside.

Condensation and visual quality of sealed units

This formation is very variable and it is also very common to observe it on one pane and not on
others. This is because the dew point of the air can vary. Movement of the air will effect it, as can the presence of nearby vegetation.

Additionally, what is happening inside also has an effect as rooms may be kept at different
temperatures resulting in the outside surfaces of different windows being at different temperatures.

The formation of condensation in this way is not a product fault.

Condensation inside the sealed unit

This is between the two panes of the sealed Unit, and unlike the other two examples of condensation above, you would be unable to wipe it off with a cloth as you do not have access to the surface where the condensation has formed.

The formation of condensation on a surface within a sealed unit, usually upon the Inside face of the
external pane, is likely to have been caused by sufficient water penetrating the seal and using
up the capacity of the absorbent materials In the sealed unit construction.

This results in a rise in humidity within the space between the two panes and when the temperature of the glass falls below the dew. Point, condensation occurs. As this condensation is within the sealed unit it cannot be removed.

If this can be observed and the sealed unit is within warranty. Call your installer to arrange an inspection.

Visual quality of sealed glass units because of the nature of the glass production process, perfect optical quality and surfaces free of any marks cannot be guaranteed. Some blemishes are to be expected.

The following extracts are based upon recognised European and industry standards. This is supported by the Glass & Glazing Federation document "Visual quality of double glazing - after
Installation." which forms our basic standard of supply Viewing sealed units for scratches on the outer faces of the panes must be carried out as early as reasonably practicable following installation.

How to check:

  • Stand no less than 2 metres away from the panes. 3m for toughened, laminated or coated glass. Where it is not possible to stand the right distance then stand as far away as possible
  • Look through the glass not at it
  • Check in natural light
  • No moisture on the glass surface
  • Exclude from the check the 50mm wide band around the edge of the glass.

What to expect when viewed as described. The-sealed unit is acceptable if the following are neither obtrusive nor bunched.

  • Bubbles or BIisters
  • Hairlines or blobs
  • Fine scratches not more than 25mm long
  • Minute particles

If you have any queries regarding the visual quality of your glass, please contact us.

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Weathertight Windows & Doors Ltd
Unit 5, Strathallan Estate,
PO Box 2455

0800 455 501