Coming to terms with condensation

This Issue This is a part of the Water woes feature

By - , Build 144

Current BRANZ research is looking at the roles of vapour barriers and vapour retarders in walls. As an introduction, we look at what various water condensation terms actually mean.

Figure 1: Vapour pressure and the effects of temperature.

BRANZ’S VAPOUR CONTROL in New Zealand walls project, see Build 118, aims to define the condensation limit for typical New Zealand walls and to clear up confusion in the building industry over the role of vapour barriers and vapour retarders.

Explaining technical terms

The project is currently in the main experimental phase and the results will follow. Here, we introduce some of the technical terms related to water vapour. Some generalisations and simplifications have been made to avoid the text being too complicated.

In future articles, some of the traditional methods for analysing the condensation risk in walls and some of their limitations will be discussed.

Water vapour

Water vapour is the gaseous form of water. In its liquid form, individual water molecules tend to cling together because of a phenomenon known as hydrogen bonding. This means that liquid water behaves more like H10O5, rather than simply H2O.

If enough energy – in the form of heat – is provided to the water molecules, then these intermolecular bonds are broken and the individual molecules can escape into their gaseous form.

The difference in size between the groups of molecules in liquid water and the individual molecules in water vapour is why some materials are resistant to liquid water but let water vapour breathe – the small individual H2O molecules can pass through the material, but the larger groups of molecules in liquid water cannot.

Water molecules are also smaller than the main constituents of air – oxygen (O2) and nitrogen (N2). Using the same rationale, we can understand why some materials can act as an air barrier but not a vapour barrier – although often other physical processes are at play.

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Condensation

The process where water vapour changes to liquid water.

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Evaporation and boiling

This is the process where liquid water changes to water vapour. Evaporation can occur at any temperature, but boiling occurs at 100°C at standard atmospheric pressure.

In the case of evaporation, water molecules at the surface of the liquid have enough energy to break the intermolecular bonds and change into water vapour.

In the case of boiling, the change occurs below the surface of the water and bubbles of water vapour form and rise to the surface. This is because the saturation vapour pressure (see Figure 1) is greater than the atmospheric pressure.

Figure 1: Vapour pressure and the effects of temperature.

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Vapour pressure

Pressure is simply a measure of force per unit area. When dealing with gases such as water vapour, this force comes from the thousands of tiny molecules in the gas whizzing around and colliding with a surface.

The gas pressure can be increased by:

● having more molecules (hence more collisions with the surface)

● increasing the temperature – each molecule then moves faster, so again there are more collisions with the surface, but additionally each collision has more force.

When dealing with a mixture of gases, the pressure of each component is called the partial pressure and all of the partial pressures add up to the total pressure of the gas. For example, air at sea level, and saturated with water vapour at 20°C, has partial pressures of about 2,330 Pa of water, 78,000 Pa of nitrogen, 21,000 Pa of oxygen and 900 Pa of argon.

In building science, we are normally concerned with the partial pressure of water vapour in air, but more often than not, this is simply referred to as the vapour pressure.

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Saturated vapour pressure

There is a maximum water vapour pressure possible for any given temperature. As a hypothetical example, consider a jar with a closed lid that is half full with liquid water. For the purposes of this discussion there is no other matter in the jar, such as air. Liquid water will begin to evaporate into water vapour and the pressure of the gas – the vapour pressure – above the liquid water will increase.

Eventually, the pressure will level off at what is called the equilibrium vapour pressure and there will be an equal number of molecules moving to and from the gaseous and liquid states. At this point, the vapour is said to be saturated.

If the temperature of the whole system was increased, the vapour pressure would increase again until it levelled off at a new equilibrium vapour pressure associated with the new temperature.

In effect, the equilibrium vapour pressure is a measure of the evaporation rate of the substance, if you were to fill the jar with petrol – which is more volatile than water – the vapour pressure in the jar would be much higher than with water.

Air is said to be saturated with water when the partial pressure of the water vapour is equal to the equilibrium vapour pressure for water at the specified temperature.

As discussed, the equilibrium vapour pressure, and hence the saturated vapour pressure, increases with temperature and it can be calculated using various empirical relationships. The relationship between vapour pressure and temperature can be summarised in a psychrometric chart (see Figure 1).

Figure 1 shows that air at 17.5°C has a saturation vapour pressure of 2,000 Pa – go up from the green dot until you reach the solid line corresponding to 100% relative humidity.

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Relative humidity

Relative humidity is a measure of how much water vapour is in the air relative to saturated air at that temperature.

For example, using the data shown, the vapour pressure of saturated air at 17.5°C is 2,000 Pa. At a relative humidity of 50% at the same temperature, the vapour pressure would be 50% of 2,000 Pa, so 1,000 Pa (see green dot on Figure 1). The pressure is lower because there are less molecules of water than in the saturated case.

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Absolute humidity

This is the mass of water vapour per unit mass of air. Also known as the humidity ratio.

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Dewpoint

The dewpoint is the temperature at which the air becomes saturated with water vapour – a relative humidity of 100%.

If a parcel of air is cooled below its dew point, condensation occurs. The dewpoint is not a physical location within a wall or other building element, that is, it is not a point in space.

Again, taking the example in the psychrometric chart, if you were to cool air at 50% relative humidity and 17.5°C it would become saturated at 7°C – shown by the black arrows in Figure 1.

Stay tuned

In subsequent articles we will use these terms to see how water vapour can move through a simple wall system and how potential problems can arise or be avoided.

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Articles are correct at the time of publication but may have since become outdated.

Figure 1: Vapour pressure and the effects of temperature.

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