Glazing and Reducing Condensation

Condensation can form on different parts of a window unit – find out how to reduce its occurrence based on where it appears in your home.

condensation - windows


Window condensation

Condensation occurs when moisture-saturated air comes into contact with a cooler surface. For windows, this could be the inside of the inner pane, the outside of the outer pane, the cavity in between double or triple glazed windows, and even the window frame.

Wherever condensation is forming on a window, it is usually due to one (or a combination) of the following factors:

  • Inadequate ventilation
  • Inadequate heating
  • A fault in the construction/assembly of the window unit
  • Energy efficient properties of a window preventing heat from passing through it

Location: Room side surface of the inner pane

diagram showing condensation forming on the inside pane of a window

Why condensation occurs here

Condensation on the room side surface of the inner glass means that the temperature of the glass surface is lower than the ambient temperature of the atmosphere in the room.

This is most likely to occur on the surface of single glazed windows but can happen on double or triple glazed windows if the room isn’t heated.

What action to take

  • Provide natural ventilation through an opening section of the window, through a proprietary ventilating unit, or through an airbrick.
  • Where there is no open fire, or where existing flues have been blocked off (and cannot be unblocked), ensure that wall vents are fitted and kept clear.
  • Open at least one window in each room for some part of the day to permit a change of air.
  • Ensure permanent ventilation of all rooms where gas and oil heaters are used. (Note: This is a statutory requirement which will be monitored by the heating engineer.)
  • Fix hoods over cookers and other equipment producing steam, and ventilate them to the outside air.
  • Ensure that bathrooms and kitchens are ventilated in accordance with national standards.
  • Draught proof internal doors and keep them closed, to prevent transfer of air with high water vapour content from the main moisture producing rooms – kitchens, bathrooms, and drying rooms. Keep in mind that water vapour does not remain in the room where it is first generated, but tends to migrate all over the house because:

a. The air pressure in the original room may be higher than elsewhere, and so the moist air will be forced out into rooms with a lower pressure, and

b. Air movement will carry it through the house.

  • Increase slightly the air temperature within the house.

Location: Within the cavity of an Insulated Glass Unit

Why condensation occurs here

Condensation will not form on the inside of a correctly functioning Insulating Glass Unit (IGU) such as a double or triple glazed window unit.

Condensation within the cavity of a hermetically sealed IGU – known as interstitial condensation – denotes a failure of the seal.

What action to take

Interstitial condensation may be the result of a fail IGU seal. Although it may be possible to repair, it is recommended that the unit be replaced.

Location: Within the cavity created by secondary glazing

Why condensation occurs here

Where the double glazing is achieved by the installation of secondary glazing, condensation on the cavity surface of the outer glass generally (but not invariably) indicates leakage of moist air from the room into the cavity.

It is not possible to hermetically seal secondary windows, so some migration of air from the room into the cavity is to be expected.

Condensation can occur occasionally on the cavity surface of the inner glass when the sun is shining on the window. This means that something in the air space itself, such as an unsealed wooden separator or desiccant, contains moisture. This source can also be responsible for condensation on the cavity surface of the outside window’s IGU.

What action to take

Make the seal of the secondary frame, and the sealing of the secondary glass to this frame, as near airtight as possible. Particular attention should be paid to all joints.

Location: Exterior side of the outer pane

Diagram showing condensation forming outside a window

Why condensation occurs here

Condensation forms on the outside surface of glass when its temperature drops below the outdoor dew point temperature.  

Windows manufactured with a double or triple glazed unit containing energy efficient low-emissivity glass have enhanced thermal insulation properties thanks to a high performance transparent coating that reflects heat from radiators or fires back into the room.

As a result the outer pane of glass does not get warmed by heat escaping from inside the building through the glass and remains cooler in comparison to less thermally efficient windows.

What action to take

As this is caused by external atmospheric conditions, little can be done to prevent this condition at certain times of the year. In many cases the condensation is not present for long periods and the sun often warms the outer glass enough to evaporate the moisture.

Should you require its removal sooner than would naturally occur, the use of a squeegee is recommended.

Note: The presence of external condensation is an indication that the glazing is thermally efficient and should not be considered detrimental. The more thermally efficient the glazing, the higher the likelihood of condensation.

Location: On the window frame

Why condensation occurs here

There are circumstances which will allow condensation to form on the inner surface of the window frame. This is more common on steel or aluminium frames.

What action to take

There are aluminium frames which can combat this by having a thermal break, however this is only effective if the window is fitted correctly into the opening.

For more advice and information about reducing condensation in your home, get in touch with a local GGF Member company.

Related topics

What is condensation and why does it occur on windows?
– Window condensation – where the water vapour comes from
How double or triple glazing helps reduce window condensation
Top tips on reducing condensation room by room
A guide to home ventilation