Date: 22nd May 2018
Author: Jack Devaney
Let’s start with some science. The process of water forming on any surface is in essence
exactly the same as the creation of fog, but being accelerated by the surface temperature.
Firstly let’s explain relative humidity. This is measured as a percentage of when water
vapour turns back into a liquid in the atmosphere. The transformation occurs at 100%
saturation, and then relative humidity is measured as a percentage of the volume of moisture in the
atmosphere of that figure. The difficulty is that this relative humidity changes as the air temperature
changes, the higher the temperature the more water vapour, by volume is needed to
saturate the atmosphere, but the relative humidity is always measured on the same scale. Therefore as
the temperature rises, if no extra water vapour is added to the atmosphere the effect is the
relative humidity will fall and vice versa, if the temperature drops and no water is extracted from the
atmosphere the relative humidity will rise.
Next we need to understand the dew point. The dew point is the actual temperature that the
transformation of the vapour to a liquid will occur, i.e. relative humidity 100%.
There are many references on the internet for a simple calculation to work out what the dew point
is depending on the 2 variables, the atmospheric temperature and the relative humidity, which references
to some work done by Lawrence, Mark 2005:
Dew Point = (Temperature-((100-relative humidity)/5)
E.g. Room temperature 20 degrees Celsius
Relative Humidity 55%
Dew Point = (20-((100-55)/5)
Dew Point = (20-(45/5)
Dew Point = (20 – 9)
Dew Point = 11
In this example, any surface which has an 11 degree surface temperature will accelerate the
transformation of the water vapour to a liquid by cooling the air immediately in contact with
that material to a point that the relative humidity will rise to 100%. This water will adhere to the material
surface in the form of condensation droplets.
Visible water inside the insulated glass unit is the easiest one to explain; The insulated glass unit is made up of multiple panes of glass held apart by a spacer bar which has a small primary seal between the spacer
bar and the glass and a larger thick secondary seal around the perimeter of the insulated glass unit which is
giving structural strength to the insulated glass unit, in most cases the insulated glass unit is now filled with an inert gas.The spacer bar is filled with a material called desiccant, a form of silicone gel granules similar
to the small packers of material found in the box of a new electrical good. This material is
used as it absorbs moisture. Over time the insulated glass unit seals will degrade, moisture will work its way
through the sealants and eventually the desiccant will become saturated, not able to absorb
any more moisture. Eventually the dew point inside the insulated glass unit will be achieved and the vapour
converts back to a liquid on the cold surface. Once this happens the unit has reached the
end of its lifetime and needs replacing.
Next is external condensation. Primarily during the spring and autumn months this is more
common. Water droplets will be witnessed on the exterior of insulated glass units. This has happened
because during the day the relative humidity has built up in the atmosphere. Overnight the temperature
has dropped to a point where the dew point has been reached in the air at the point where it is in
contact with cold surfaces, resulting in the formation of water droplet. This can be seen on
the exterior of the insualted glass units, but also on vehicles and the ground which is also going to be cold
by comparison to the day air temperature.
If the outside air temperature falls enough then this will manifest itself into advection fog
(condensation in free air).
As the temperature rises during the day the condensation on the outside of the insulated glass unit will
evaporate away as the relative humidity effectively drops, the dew point rises and the atmosphere can support
the water vapour.
Lastly internal condensation on the glass (room side). As explained the condensation
forming on the glass is the by-product of the transformation of vapour back to a liquid at dew point.
Modern day fenestration products are far more airtight then products of years ago,
therefore there is very little draught ventilating the property and reducing the relative humidity.
During our normal living activities, people exhaling vapour, boiling a kettle, cooking, baths,
showers, drying clothes on radiators and even house plants are all adding vapour into the
property atmosphere, increasing the relative humidity.
This is not necessarily a problem, however if the heating is on a timer and the property cools
down at night, then it is plausible that the property atmosphere will reach dew point and
condensation will form on the cold surfaces, insulated glass unit first, then mirrors, china and eventually
even the wall corners of the rooms. This is not an issue with any material just our living
There are some practical suggestions that can help with condensation. Ultimately we need
to control the relative humidity, simply changing the air supply in the property or extracting high levels of
vapour when cooking or showering will help to keep the relative humidity under control.
Furthermore, managing the property heat can also help, if there are high levels of relative humidity in the
property, then if the property cools down, the relative humidity will increase and the dew point will be achieved.
Keeping the property a couple of degrees warmer could eliminate the condensation.
Lastly if all else fails a small dehumidifier will reduce the relative humidity by mechanically extracting the
water vapour from the atmosphere.