A relatively recent phenomenon occurring in high performance windows is that of exterior condensation on insulating glass units. While occasionally a source of concern for the customer, this condition is normal given certain combinations of glazing systems and environmental conditions.
Exterior condensation is usually observed under the following conditions:
- high outside relative humidity
- high performance glazing systems ( eg. Low”E”/Argon units)
- early morning
- unobstructed exposure to a clear sky to the North, West or South
- no wind
The cause of condensation in general is due to the temperature of a surface being lower than the dew point of the surrounding area. The dew point is the temperature at which a given volume of air with a given water-vapor content is completely saturated. Since air can hold an increased amount of water-vapor with increased temperatures, and vice-versa, lowering the temperature of air below the dew point will cause condensation to occur. This can happen when the air comes into contact with a cooler surface (eg. the side of a cool beverage glass), or simply through cooling of the air itself, which is how rain and snow form.
A common example of this process in the window industry is condensation on the inside of windows. This occurs when the indoor humidity in a house is such that the dew point temperature is higher than the temperature of the glass at the bottom of the window. When this temperature happens to be below 0 degrees Celsius, then frost, rather than condensation forms on the glass.
But how can an exterior object become cooler than the surrounding air? This is what must occur in order for condensation to form on an exterior surface. The answer is that all objects naturally lose heat in the form of radiation. Everyday objects radiate energy at a rate proportional to the temperature they are at. In other words, a hot object will radiate more than a cool object. When an object is surrounded by other objects which are of a similar temperature to it (eg. a person or objects in a home) there is no net change in the objects temperature, since the surrounding objects are radiating towards it at the same rate it is radiating towards them. But when an object is surrounded by other objects which are at a lower temperature than it is, it will cool down as it loses its heat through radiation.
As a result of developments in insulating glass technology over the last number of years, interior condensation occurs somewhat less frequently. Improvements in the insulating value of glazing systems have resulted in higher inside glass temperatures when it is cold outside. The onset of incidences of exterior condensation in recent years suggests that there may be a link between these two developments. There is.
As explained, when an object is surrounded by other objects which are cooler than itself, it will lose heat. When the exterior glass surface of a window is exposed to a clear sky, it will radiate towards it (as well as towards other surrounding objects). Since the sky is at a very cold temperature (close to absolute zero), it will not radiate back towards the window to any significant extent. As a result the exterior glass surface cools down. As this happening, the trees, the grass and the surrounding buildings also lose heat and the air typically cools down as well. However, when the sun rises, the surrounding objects are typically warmed. If particular object, such as a window surface is not warmed by the sun during that period it may remain cooler than the surrounding air. If the surrounding air temperature is just above the dew point and the glass surface temperature remains just below, condensation will form on the glass surface.
The phenomenon does not occur on windy days because air movement past the outer pane of glass will warm it to the outside temperature quite quickly. It does not occur on eastern exposures because the sun will warm the window pane as the sun rises. It does not occur on “dry” days because the dew point temperature is significantly lower than the outside air temperature. It does not occur on windows which do not have a clear exposure to the sky because whatever obstructs the exposure prevents the outer pane from cooling down.
It should be noted that the condition can occur in winter when frost, rather than condensation may occur.
Exterior condensation has only recently become an issue. Why has this not been seen until now? With less insulative glazing systems, heat flows from the inside of the house, through the interior pane, to the exterior pane of glass, raising its temperature to a point somewhat above that of the outside air. This process ensures that condensation will not occur as the exterior glass temperature is almost always above the dew point. It is the high insulation value of the recently available glazing systems, preventing heat from escaping to the outer glass surface which allows exterior condensation to form.
In fact, exterior condensation is seldom seen, even on very high performance glazing systems. This is because the above conditions do not coincide very frequently in most locations. When it is seen, it is evidence that a high performance glazing system has been installed and is working.