The dew point and how to tackle condensation and mold on
the inside of windows.
Relative humidity is a comparison of the actual vapor density versus the saturation
vapor density at a particular temperature.
Condensation Explained (The Technical Stuff)
When the surface temperature of the window is below the dew point temperature
of the air in your home, condensation, frost or ice appears.
Unfortunately, the reason why the window surface temperature is below the
dew point temperature can become much more complex.
Dew point is defined as saturation vapor density...or put in simpler terms,
when the air reaches 100% relative humidity and can hold no more moisture.
Dew point is the point where the air is no longer able to hold any more moisture.
If the air has reached vapor saturation (100% relative humidity), then the
air will release moisture.
In order to stop condensation from forming on the surface of a window, you
either have to lower the dew point temperature of the air in your home to a
level below the dew point temperature of the window surface, or you have to
warm up the window surface to a temperature above the dew point temperature
of your home, or a combination of both.
There are two ways to lower relative humidity:
1. Increase air temperature
2. Decrease moisture content.
A Very Important Point:
If you increase the air temperature you will lower the relative humidity but
you will not change the dew point - which is based on the amount of water vapor
in the air and is not based on the temperature of the air.
Useful Resource: http://www.answers.com/topic/humidity
The amount of moisture in the air is measured in grams per cubic meter.
For example, imagine that there are two houses. House
A and House B.
House A is at 18 degrees and with a relative humidity reading of 40%. There
are 6.25 grams of water in a cubic meter of air in House A
The dew point temperature of House A is 3.3 degrees. So at 3.3 degrees the
air will be at 100% relative humidity or at saturation vapor density.
House B is at 24 degrees, and it also has 6.25 grams
of water per cubic meter in the air. The relative humidity of House B is
29% - versus A’s 40%.
And the dew point temperature of House B is also 3.3 degrees.
While the relative humidity in House B is much lower than is the relative
humidity in House A; if the surface temperature of the windows in House B is
for example 1.7 degrees it will have condensation on those windows. (Just the
same as House A)
However, if the surface temperature of House A’s
windows are 4.4 degrees (only 2.7 degrees warmer) it will not have condensation
on its windows.
House B’s humidity gauge reads only 29% RH. This
house would have a condensation problem.
While House A’s humidity gauge reads 40% RH. This
house would not have a condensation problem.
If your home hygrometer measures the relative humidity in your home at 60%
while the temperature of your home is 21 degrees, you will have a dew point
temperature of about 10.6 degrees. This means that if the temperature of the
window surface is below 10.6 degrees then you will have condensation.
Storm windows are found in many parts of North America (not so much in the
UK) to serve as a retrofit on existing windows in order to improve their thermal
efficiencies. If you add one on the outside, the surface temperature of the
inside pane of glass jumps up to about 6 degrees, which is a very large improvement.
But these are centre-of-glass readings and not the temperature readings
at the edge of the window where condensation usually forms. A typical clear
glass dual pane window is going to have center-of-glass temperature reading
pretty much the same as a single pane with a storm.
If the double-glazing has a Low E coating and an argon gas infill then the
center-of-glass temperature will be about 14 degrees. This is an 8-degree improvement
over a clear glass double glazing or a single pane with storm window. There
will be a comparable edge of glass improvement as well, particularly if the
IGU (Insulating Glass Unit) was manufactured using a warm edge spacer system.
Also, the double glazing is going to have desiccant between the glass layers.
Desiccant absorbs moisture keeping the inside of the double glazing system
If it gets cold enough outside, the
temperature in the airspace between the panes of glass can get very low. By keeping that space dry, it helps to keep
the dew point temperature very low as well; something not always possible when
using a single pane and storm window.
Although a single pane with a good and tight storm window
can help the interior pane to avoid condensation (when compared with a single
pane and no storm),
the storm window itself will frost up when the temperature is low enough – at
a temperature usually well above the temperature that will cause the dual pane
to ice up. It is unavoidable given the right circumstances
So what does a window temperature of 14 degrees mean? As mentioned earlier
a house kept at 21 degrees with a 60% relative humidity has a dew point temperature
of 10.6 degrees so it is unlikely that there will be condensation problem on
those particular windows despite the relatively high relative humidity in the
But what happens to the dew point if
you keep your home at 21 degrees and you have a 65% relative humidity? Well, for one thing the dew point has jumped
up to 14 degrees, which we have already noted is the same as the window temperature.
Also, anyone with 65% relative humidity in a home at 21 degrees has too much
moisture in their air and they are in serious need of some sort of ventilation
If the relative humidity is lowered because of an increase in temperature
and the increase in temperature does not result in an increase in a corresponding
window glass temperature increase than this may not help to control condensation.
But, lowering the relative humidity by removing water is a different story
because in that case you will also be lowering the dew point as you lower the
relative humidity and that WILL help to control condensation on your windows