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Condensation, explained, again!

Cause of condensation in windows

Seems every year about this time, we get interested in condensation or at least the causes of it. The following article will explain what condensation is, how it is formed and possible solutions to avoid it.

Broadly there are three types of condensation which often get talked about when referring to double glazing, replacement windows or doors.

  • Inside.              Water droplets which form between the two glass panes of the IGU.
  • External.          Water droplets on the outside face of the insulated glass unit (IGU),
  • Internal.           Water droplets on the room side face of the IGU and

Let’s start with some science. The process of water droplets forming on any surface, is in essence, exactly the same as fog. Water droplets form a thick dense bank of moisture but is accelerated because of the surface temperature of a wall or glass unit.

I realise some of this will sound techy but please stick with me whilst I explain Relative Humidity (RH). This is the point when water vapour (a gas) turns back into a liquid (water) in the atmosphere. The transformation occurs at 100% saturation, the RH is then measured as a percentage of the volume of moisture in the atmosphere of that figure.

The difficulty is that RH varies as the air temperature changes. Higher temperatures (RH) will hold more water vapour, by volume is needed to saturate the atmosphere, but the RH 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 RH will fall and vice versa, if the temperature drops and no water is extracted from the atmosphere the RH will rise.

Next, we need to understand the Dew Point (DP). The DP is the actual temperature that the transformation of the vapour to a liquid will occur, i.e RH 100%.


What is Dew Point and why is it important?

There are many references on the internet for a simple calculation to work out what the DP is depending on the 2 variables, the atmospheric temperature and the RH, which references to some work done by Lawrence, Mark 2005

Dew Point = (Temperature-((100-RH)/5)

Eg.     Room temperature 20 degrees Celsius

Relative Humidity 55%

DP = (20-((100-55)/5)

DP = (20-(45/5)

DP = (20 – 9)

DP = 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 RH will rise to 100%. This water will adhere to the material surface in the form of condensation droplets.


Condensation inside the sealed unit? Here’s why

The IGU is made up of multiply 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 IGU which is giving structural strength to the IGU, in most cases the IGU 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 IGU 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 DP inside the IGU 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 life and needs replacing. It is possible to drill a hole in the corner of the sealed unit, inject a cleaning fluid, then drain it and plug the hole. This clears the misting up of the sealed units and restores clarity however any gas will have escaped so the unit won’t be as effective as new ones. 


You have condensation on the outside of your windows, this is external condensation

External condensation mainly occurs during spring and autumn, when there are dampness and cold temperatures. 

During the day the RH builds up in the atmosphere, overnight, when the temperature drops, it eventually gets cold enough to reach the due point (DP)

When the DP is reached, the air releases excess water droplets onto the coldest surface, if the coldest surface is your car, the render of your home or surface of the glass, it will form there.

As the temperature rises during the day so any the RH rises and the air can absorb more water vapour and condensation on the outside of an IGU will evaporate away

As the daytime ends, so the outside air temperature falls and advection fog is created (condensation in free air) creating new condensing of water onto cold surfaces.

External condensation shouldn’t be seen as a bad thing, its actual evidence that the external surface is cold (because the unit is stopping heat escaping)

As sealed units are made more and more, energy-efficient, so there’s less heat escaping through the glass, meaning the outside surface isn’t warm, it’s getting colder and thus increasing the likelihood of external condensation.


Lastly internal condensation on the glass (room side)

If you have condensation on the room side face of the glass its internal condensation, most likely caused because of excessive moisture inside your house.

During our normal living activities, we exhale the vapour. When we boil a kettle, run a bath, take a shower, cook or dry clothes on radiators we are adding water vapour into the atmosphere, within the room.

In doing so we raise the temperature slightly and raise the relative humidity.

This is not necessarily a problem and given the room temperature by the day is generally higher, the air can retain this moisture within it.

At night, however, or when the room is colder, the air has to shed its load, the cooling effect condenses the air and water droplet form (like rain!), these droplets find the coldest surface, which is often glass, tiles, or walls.

By adjusting ventilation, you can control the relative humidity (the steam or water vapour) in doing so, it will decrease the risk of condensation forming. That’s why sash windows are so important! By opening the upper and lower sashes, air gets to circulate more efficiently, like changing a room full of air every 15 minutes known as Air changes per Hour (4 ACHprHr) or every 7 minutes (if 8 ACHprHr)

Casements often trap air inside the building and offer far less purge ventilation.

Furthermore, keeping the property a couple of degrees warmer at night, or using thick thermal curtains should help eliminate condensation internally, however the less heat that passes through the sealed units the colder the outside glass will be, and that may increase external condensation!

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