Why Do My New Energy Efficient Windows Mist Up Or Have Condensation Outside?

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.

Energy Rated Windows

External condensation is not unusual

Until recently, condensation has really only been an internal issue however, with the development of much more thermally efficient windows and in certain atmospheric conditions, condensation can form on the external pane of glass.

A consumer’s first response is that the glazing must in some way be faulty, but this is quite the reverse.  As the ambient outside air temperature rises in relation to the cooler external glass pane (typically in the morning), the air in contact with the external glass cools and the water (in vapour) cools and condenses forming external condensation.  A neighbour’s house (under the same conditions) may not have external condensation, but all this really means is that their windows are probably less thermally efficient, which allows the outer pane to be warmed up by the thermal transmittance or heat loss from the adjacent room.

What causes condensation?

There are many other ambient, environmental and geographic factors which influence the formation of external condensation, so it is virtually impossible to predict if this phenomenon will occur.  If there is a tendency for external condensation, then it will only happen at certain times of the year (typically March and October) when the dew point is at an optimum level and when there are greater temperature transitions either side of sunrise or when cool fronts are replaced by warmer air currents.  If the affected window does come into contact with sunlight or even a breeze, then the external condensation will quickly evaporate.

For more information on this subject, why not give us a call or drop into one of our showrooms.

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