The appearance of moisture condensation on the inside of the windowpane is a frequent and annoying occurrence. It happens mainly during cold autumn and winter seasons. It also occurs more frequently in buildings with old-fashioned box, coupled or single-glazed windows without edge spacers and with single or ‘thermal pane’ glazing with low protection from heat losses.

There is a law of physics that explains why moisture condensation occurs on windowpanes. The principle is, warm air can absorb more moisture than cold air. In the process of cooling, at a certain low temperature point – dew point – the air is 100% saturated with moisture and condensation occurs. The moisture is released in the form of droplets on cold surfaces, which become wet or moist.

The phenomenon can be easily observed in a refrigerator. When we open the fridge door, cold internal surfaces of the refrigerator door and panels quickly gather moisture as the normally moist air in the room in contact with the cold panels rapidly cools to the dew point, causing condensation to occur. In some cases, ice forms on fridge panels.

A similar phenomenon can be observed in case of windows. When we open a window on a cold day, the glass quickly fogs over as the warm air in the room comes in contact with the cold windowpane. Upon closing the window, such condensation quickly disappears.

Misted windows are also familiar to every car driver. When we sit in a car on a cold day, in a very short time the inside of the windscreen fogs up as the warm, moist air exhaled by the driver condensates on the cold glass. By heating the air and the glass, the annoying mist on the windscreen is soon gone.

In the same way, misting occurs on the inside of the windowpane when the window is closed, mainly on cold days and particularly when relative humidity in the room is high. The insides of the window and the glazing are cold, their temperature being at dew point, which causes condensation on the glass to appear. Sometimes, however, condensation also occurs on the surfaces of window frames with poor heat insulation properties. For the same reason, condensation also occurs on the cold peripheral walls of a residential dwelling. Such occurrences are not merely annoying but can also lead to other negative outcomes: damage to the coating or windows, plaster and furniture and the onset of mildew, which is actually detrimental to one’s health.

Where does condensation on the inside of windowpanes occur?

Mainly in rooms with high relative humidity, such as bathrooms, kitchens, laundry rooms and similar. Condensation on windowpanes can also occur in bedrooms when they are not properly heated and well aired. A person exhales about 1 litre of water every eight hours, and a family of four ‘generates’ 10 litres of water in the form of vapour on average every day. If the place is not properly aired, the vapour condensates on cold surfaces, most frequently on glass.

The condensation first appears on the edges of the windowpane, starting with the bottom edge. This is because windowpane edges are always colder than the middle of the glazed surface. This is also characteristic of all insulation glazing and is due to its structure.

What can be done to reduce condensation on windowpanes?

The occurrence of annoying condensation on windowpanes can only be reduced by opting for windows, i.e. glazing, with good thermal insulation properties. The chances of condensation occurring in case of a single-glazed window (U = 5.8 W/m2K) or an ordinary ‘thermal pane’ window (U = 2.8 W/m2K) are significantly higher than with energy efficient insulation glazing (U = app. 0.6 to 1.5 W/m2K).

It is important that the insulation properties of the glazing help to raise the temperature on the inside of the windowpane, which should be above the dew point. The temperature of the inside of the windowpane is conversely proportionate to the U-factor of the window, i.e. glazing.

The optimal U-factor values for windows should be calculated by taking into account the applicable regulations, the location and the architectural design of the building as well as the desired annual consumption of energy required for heating. It is, however, self-evident that all such modern windows feature excellent sealing, which is not the case with old-fashioned windows. Unsealed windows, i.e. windows with no edge spacers, cause ‘draught’ – loss of heat – as well as noise intrusion.

Does energy efficient glazing prevent condensation on windowpanes?

Unfortunately, the answer is no! Even with energy efficient windows featuring low-emissivity glazing (U = app. 1.1 – 1.3 W/m2K), in times of extreme cold, in particular when relative humidity in the room is high (60% – 80% or more) and the room is not properly heated (low morning temperature of the room – cold inside surface of the windowpane), minimal condensation can still occur, manly along the bottom edges of the windowpane.

It should therefore be noted, in particular in winter, that in case of poor thermal insulation of peripheral walls and windows or glazing with high or average U-factors, on very cold days the temperature on the inside of the windowpane can drop below the dew point.

For health reasons, optimal humidity in the living area should be between 40% and 60%, and ambient temperature from 18 °C to 22 °C. The simplest fast solution for eliminating condensation, however, is to raise the temperature in the room as that also warms the cold windowpanes and brings their temperature above the dew point. Unfortunately, that also results in a significantly higher consumption of energy required for heating. It is a fact that by raising the ambient temperature by one degree, the energy consumption increases by 5% – 7%! A long term and economic solution lies in the proper insulation of peripheral walls and installation of energy efficient windows featuring low U-factors while also ensuring optimal humidity in the room.

What can be done to stop condensation on windowpanes?

The only economic solution lies in proper ventilation of the room. In addition, that also prevents stuffiness and the onset of harmful mildew.

Energy efficient ventilation means that airing may be required several times a day, as applicable, e.g. for 10-15 minutes at a time, preferably with the windows wide open or by creating a draught for a few minutes. However, we also need to make sure that the temperature in the room does not fall under 15 °C in the process. That way, the indoor air will retain the capacity to absorb moisture naturally; also, the walls, which accumulate heat, will not cool down. After such ventilation, comfortable ambient temperature can be restored very quickly.

In winter, outdoor fresh and cold air is usually relatively dry. The dry air, which enters the room during ventilation, can reduce relative ambient humidity when the room is being reheated. For that reason, it is recommended to add moisture to the air in the living areas, as air that is too dry is not favourable and may even be harmful to one’s health.

Properly controlled ventilation is particularly important in case of newly constructed buildings, even when still uninhabited. Lingering construction dampness is detrimental to the actual building, furnishings and windows as well as the residents.

Just as important is controlled room ventilation in older buildings where new, modern airtight windows have been installed to replace old-fashioned, unsealed windows. The old windows allowed strong, uncontrolled ventilation through the unsealed gaps between the window and the wall.

The window, however, is not the only ‘device’ for ventilation and prevention of harmful condensation of moisture in the room! The problem of ventilation is much better and more economically solved with the installation of additional ventilation devices, e.g. ventilation ducts featuring grilles or flaps, kitchen range hoods, ventilating fans and other modern controlled ventilation devices. The latter even enable part of the heat energy of the warm, moist exhaust air to be returned to the room. Therefore, it is important to realise that windows are not the only ventilation devices!

When does condensation occur on external surfaces of energy efficient insulation glazing?

Even in windows featuring energy efficient insulation glazing, condensation can occur on the outside of the windowpane. Such condensation is rare and happens only after a very cold night or on very cold mornings.

Energy efficient glazing suppresses radiative heat flow from inside out, i.e. it keeps the heat inside. Consequently, the outside of the windowpane cannot warm up from the heat in the room and cools more during a cold night. The temperature of the exterior windowpane drops below the dew point and the window fogs up. As a rule, windowpane edges remain condensation free since the edges of the insulation glaze let more heat flow outside than is the case in the middle of the glass pane. For that reason, the temperature of windowpane edges remains higher than in the middle, where it is above dew point.

In case of such external ‘misting’ of the windowpane, wiping the glass down does not help. The condensation disappears by itself as the outside temperature rises or when the morning sun has warmed the outside of the windowpane. The type of condensation occurring on the outside of the window is the result of a natural law of physics. It only occurs at very low outdoor temperatures and is actually more common with highly energy efficient glazing.

In such cases, the only solution is to install and use additional external protection such as shutters, which prevent excessive cooling of the outside of the windowpane. Consequently this very rare phenomenon of short-term condensation on the outer surface of energy efficient glazing cannot be the subject of a warranty claim.

Public Relations Services, Inles d.d. Ribnica, 28 July 2004