Relative humidity is a comparison of the actual vapor density versus the saturation
vapor density at a particular temperature.
Another issue that a small number of homeowners
experience is “external
condensation” on the outside of their new double glazed windows and doors.
This is often after the homeowner has installed low emissivity glass (Low E
Glass), which reflects heat back into the room.
This can be quite confusing for the small number of home owners who experience
external condensation as in the main people are expecting reduced or no condensation
once they have fitted double glazing.
Our sister website Ask-Questions.com quite often receives comments on this very phenomenon - Please also see: http://www.ask-questions.com/yabbse/index.php?topic=1104.0
In short the “problem” is
caused by the fact that the Low E Glass is doing its job so well and reflecting
heat back into the
The following is an explanation by Pilkington of the phenomenon of External
Condensation on external glass surfaces
External condensation (dew) can occasionally occur on highly
insulating glass units in temperate climates. Such occurrences will
only happen on cloud-free nights when there is little or no wind and
usually when a warm front follows a dry spell.
The combination of several factors, namely external
air temperature, localised microclimate and the thermal transmittance
of the glazing itself may all contribute to the formation of external
condensation. As a consequence of variable temperatures and
localised conditions, it is possible to experience a situation whereby
both clear and ‘misted’ windows exist at the same time in the same
This phenomenon is influenced by the thermal
insulation of the glazing. Single glazing offers poor thermal insulation
escaping from inside a room readily passes through the glass to the
outside environment. Consequently, the external surface
temperature of single glazing is generally higher than the‘
dew-point’ temperature of the outside air, thus prohibiting the
formation of condensation on that surface.
With conventional double glazing the thermal insulation is
improved, but sufficient heat still escapes through the glass so as
to warm the external surface of the outermost glass, thereby
precluding the formation of condensation in most circumstances.
In common with other low emissivity
glasses, Pilkington K Glass reflects heat back into the room and as such
the quantity of heat
passing through the glazing is reduced. Consequently the external
pane of low emissivity double glazing is not warmed by escaping
heat (which instead is retained within the room) and therefore
presents a colder surface to the outside environment.
In such cases, and in situations where the external
glass surface temperature is lower than the ‘dew-point’ of the
air, (and when weather conditions are comparable to those mentioned previously)
condensation can form on the external glass surface.
However, the combination of these contributing factors is largely
unpredictable and therefore it is not possible to quantify the
number of occasions when external condensation will occur.
Instances of external condensation are relatively rare and in all
cases it will be a transient effect. Upon any one of the
climatalogical variables changing, the condensation on the glazing
will usually dissipate within a short period of time in much the
same way as morning dew.
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