Dryfix News

Condensation the basics

October 27th, 2013

All posts, Condensation, Energy Efficiency / Insulation Improvements, Mould Remediation, Surveying

Condensation Explained

Upstanding the basics between Relative Humidity, Specific humidity, Vapour pressure, Thirst, Boundary layers and Dew point.

Before we begin explaining the above where does water vapour come from?

Simple daily life produces water vapour through cooking, washing, ironing, drying clothes and even breathing, therefore it is extremely unlikely that an empty property will suffer with issues of high humidity, condensation and mould unless another source of water is available such as a leak. Water vapour in most occupied houses is nearly always the result of lifestyle activity, this does not however mean the cause of a condensation and mould issue is solely due to lifestyle activity.  Poor heating, thermal performance of the building, construction, layout, orientation, and levels of insulation and ventilation all increase the risk of an issue occurring, therefore evaluation of all these aspects must be considered during a damp/ condensation survey.

Water vapour released into the air travels throughout the property to the regions of lowest air pressure. When the warm moisture laden air settles in cool air environments or adjacent cold surfaces the air is cooled increasing the humidity of the atmosphere and as the air cools it is no longer capable of holding the same amount of water vapour it did at a warmer temperature.  The volume of water vapour in the air is usually expressed as Relative Humidity (RH), which is the volume of water vapour in the atmosphere at any given temperature. Relative Humidity is expressed as a percentage (%) of the maximum amount of water vapour that can be held in the air overall at that temperature.  An example below  using a bucket analogy clarifies this further.

Relative Humidity

Below the bucket represents the air at a given temperature and the amount of water vapour in the air is expressed as a percentage Relative Humidity (RH). Relative Humidity is not the actual amount of water in the air or water in the bucket, but the percentage of space taken up by vapour in the air at that temperature.  As can be seen below in example 1; The air is 20 degrees and is 60% saturated with water vapour (just over half full).  As the temperature of the air reduces so too does the capability of the air to hold water (i.e. the size of the bucket reduces). The humidity level increases as the same volume of water occupies a greater volume in the air or bucket. This happens even though the actual amount of water in the air stays the same. As the air temperature reduces to 15 degrees with the same amount of water in it, the Relative Humidity increases to 83% because the air has a further lower capacity to hold water causing the humidity to rise.

Eventually when the air reaches a temperature where it is no longer capable of retaining vapour which in this example is 12 degrees the air becomes fully saturated and condensation occurs as water vapour is expelled from the air as a liquid condensate. This temperature is known as due point and varies dependent upon atmospheric conditions.

Bucket analogy of Relative Humidity

Specific Humidity

Specific humidity is the actual quantity of water vapour in the air. This usually expressed in Grams per kilogram (GPK) or Grains Per Pound (GPP). I’ll explain further, consider you drive and visit the petrol station to fill your car with fuel, you place the nozzle in your petrol tank and pull the tiger to allow fuel into the tank. When you next look at your petrol gauge the tank now says 1/2 full but it doesn’t tell you how many litres of fuel you have in your tank, it simply means of the maximum capacity the tank can hold it is half full, this is what Relative Humidity is to the atmosphere. The fuel pump however or clever little car computer if you have one tells you exactly how many litres of petrol you have pumped into the fuel tank, for example 100 litres, this is the Specific Humidity, the actual amount of water in the air. For example if Specific Humidity is 80 GPP, the air is holding 80 grains of moisture in each pound of air.

Basic understnading of Specific Humidity

Vapour Pressure

Molecules of water vapour in the air exert a pressure on the surrounding environment which is usually measured in either Inches of Mercury (inHg) or kilopascals (kPa). By understanding the pressure exerted on the air by the water vapour we can understand air movements, as high pressure air will naturally migrate to areas of lower pressure highlighting at risk areas.

For example, if the atmospheric conditions within the first floor bathroom are 20 degrees at 60% RH using a vapour pressure chart we can work out the pressure exerted on the air which in this example is 1.4 kPa (Vapour Pressure). The room adjacent has a temperature of 18 degrees at 50% RH and has a vapour pressure of 1.15 kPa, therefore we can understand that naturally the air within the room of greater pressure will move towards the room of lower pressure. Understanding vapour pressures and temperature allows us to work out why certain walls or rooms within properties are more at risk of condensation and mould than others.


Thirst is the maximum volume of available space remaining space in the atmosphere before saturation occurs. Therefore if the Relative Humidity of the air is 60% saturated, the thirst of the atmosphere is the remainder before saturation occurs which is 40%.

Boundary layers

The boundary layer is the layer of air which lies directly adjacent a surface. Atmospheric conditions within the boundary layer are likely to be different from that of a rooms atmosphere mainly due to the surface temperature of the wall. Calculating the conditions within the boundary layer is essential during a damp / condensation survey as this will reveal further what is happening to the air directly adjacent the walls or floors.

Dew point

As previously explained as air is cooled it becomes relatively more saturated with water vapour until it reaches 100% Relative Humidity. When air reaches 100% RH it can no longer hold its water vapour as it is saturated, at this stage the atmosphere will expel moisture vapour from the air in a process known as condensation. Condensation is a physical change of water vapour to a liquid. The point at which the air becomes fully saturated is a temperature known as Dew Point which varies dependent upon atmospheric conditions, although most hygrometers will provide you with the dew point of the atmosphere. As can been seen from the bucket analogy above the atmospheric conditions in that scenario had a dew point temperature of 12 degrees when air became fully saturated.

So what does all the above information do for us and why is it useful. Here is a little example of how understanding the basics can enable us to work out why certain areas of a building become at risk of mould, condensation problems?

The problem

For example, we place a hygrometer centrally within a room and record the atmospheric conditions at; 21 degrees, 50 % RH (Relative Humidity), Vapour Pressure of 1.25 kPa, Specific Humidity of 7.8 GPK with a dew point temperature of 10 degrees. The south facing wall within the property is suffering with mould although no visual evidence of condensation can be seen. The temperature of this particular south facing wall is however cooler than the rooms atmospheric temperature, the wall is in fact 12 degrees therefore as explained above the conditions within the air directly adjacent the wall (the Boundary Layer) will change due to the cooling effect of the walls surface.

Using a Psychometric chart we can recalculate the conditions of the micro climate within the boundary layer which work using the same quantity of water vapour (Specific Humidity) and following a reduction in temperature from 21 degrees to 12 degrees. In this scenario the conditions within the boundary layer work out at; 12 degrees, 90 % RH (Relative Humidity), Vapour Pressure of 1.25 kPa, Specific Humidity of 7.8 GPK with a dew point temperature of 10 degrees.

As can be seen the conditions within the boundary layer are considerably different to those conditions recorded centrally within the room. The Quantity of water vapour (Specific Humidity) in the air hasn’t changed, neither has the dew point temperature yet the Relative Humidity has increased dramatically from 50% RH – 90% RH due to a reduction in temperature of the air adjacent the wall (ie; the holding capacity of the air has reduced). As yet the air is not fully saturated but it is close placing this particularly wall at risk of condensation issues. Moulds however do not require physical condensation to germinate they simply require enough atmospheric moisture to be present which usually starts around 80% RH dependent upon species and occurs within 12 – 24 hours with visible mould growth within 48 – 72 hours.

Now from understanding the basic principles of psychometery we can use this knowledge to work out how air travels throughout a building, what happens to the air with changes in temperature and what particular rooms, walls or surfaces within a building can be at risk.

I hope the above information helps aid your understanding of the importance of evaluating not only the atmospheric conditions within the air but the change of conditions adjacent surfaces.

For advice and assistance contact us on York (01904) 791388, info@dryfix.net or visit our website http://www.dryfix.net/

Suspect a Problem?

If you suspect a problem and require a survey, or need advice please Contact us.

Many thanks

Russell Rafton

C.S.R.T / I.S.S.E

Dryfix Preservation Surveyor















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