Vapour / Condensation Control

“Vapor diffusion issues tend to be greatest in cold climates, where even small amounts of internally generated moisture will condense inside cold wall cavities during winter months”


The safest way to prevent condensation forming in wall cavities is to keep the wall cavity warm, and ensure the dew point does not sit inside within the wall fabric (dry zone of the building envelope). The ARIDON® SMART WALL system wraps the building frame in a thermal weatherproof shell, which maintains the wall cavity at a temperature very similar to that of the room itself. The theoretical dew point of the SMART WALL system is near the exterior face (wet zone side) of the panel – exactly where it should be!


WUFI Bio Modelling assesses the risk of mould growth, based on measured or computed local climate conditions*. Because we are so confident our system performs consistently across a range of locations, temperatures and Relative Humidity ranges– we are presenting only the 80% Relative Humidity initial material moisture content results, i.e. the materials contain 80% of the moisture they can hold at that particular temperature.

*WUFI (Wärme und Feuchte instationär) is state of the art hygrothermal analysis software developed in Germany which allows realistic calculation of the transient heat and moisture transport in building constructions exposed to real weather conditions.

Modelling Parameters: Location Christchurch and Queenstown. Orientation to south-west for driving rain; external cavity ventilated 1 ACH; Cedar weatherboard cladding, no adsorption of rain; standard gypsum plasterboard internal - airtight (no air infiltration into the construction from internal); Initial condition at 80% relative humidity and 15 years simulation period. Internal medium moisture load, 50%RH with 10% amplitudes and 21 ºC with 1ºC amplitudes. The green traffic light indicates a wall construction with no mould growth.

Refer Base of Page for Downloadable WUFI Reports for Christchurch & Queenstown

Interestingly - traditional NZ building systems (i.e. 90mm framing + Building wrap + 2.8 Glasswool Cavity Infill) do not perform adequately in terms of vapour diffusion and combatting condensation in these colder climates even at an average relative humidity (60%). A gentle reminder that design and construction in colder climates should not replicate the successful techniques used in warmer climates.

What is water vapour?

Air almost always contains some water vapour and makes our lives more comfortable, but there is a limit on how much vapour can be contained in the air. Air at higher temperatures can hold more water vapour than air at lower temperatures.
Water vapour can be present due to atmospheric conditions, but significant quantities are also generated inside the house.

Table 1: Causes of Moisture in the Home¹: Activities such as cooking, washing, drying clothes and even breathing create additional moisture in the home. In addition using an unflued gas heater dramatically increases moisture levels. The following table provides an estimate of the amount of moisture generated from these activities.

An average room can transport around 8-10 litres of water during a 12 hour period.


How does water vapour get into our walls?

Water vapour gets into our walls typically one of three ways:

  1. Moisture present in building materials during construction and /or leaks where the water evaporates.
  2. From air leaks in the building envelope – a substantial volume of water vapour is carried by moist air entering the wall cavity either from inside or outside the building.
  3. From vapour diffusion through building materials: Warm air vapour diffuses through vapour permeable building materials such as plasterboard interior wall linings.

How does water vapour in the wall turn into condensation?

Water vapour in the wall frame is not a problem in itself. Most walls are constructed from organic materials which are best kept in conditions which are healthy for humans. The problem occurs when water vapour meets cold surfaces (thermal bridges) or cold air and condensation (dew) forms. The term used to define when this occurs is the dew point.

Water vapour in the air is absorbed by surrounding materials, including the materials that make up the building fabric. However as vapour passes through the fabric of the building and where the temperature is dropping inside of the fabric, the vapour will reach a Dew Point and start to condense at a location within the wall fabric (Interstitial Condensation).

Figure 1: Section through an external wall illustrating the Dew Point. The drops show where the mould might grow within your wall. Source:

This condensed water can then travel down through the wall assembly – often ending up in areas far removed from the original source. This moisture then causes rot, mildew and dampness to form within the wall and is much more difficult to remove than the original water vapour.
“Interstitial Condensation reduces insulation performance and cause fabric deterioration. If the relative humidity levels in the building exceed 70% for prolonged periods, there is a high probability that the condensation occurring on cold surfaces will lead to mould growth. This can seriously affect the quality of the air for the occupants and mould spores can have a detrimental effect on human respiratory system”.

INTERESTING FACT: The colder the air, the less water vapour it can hold. New Zealand buildings in the lower North Island and Canterbury South have a high risk of interstitial condensation – causing mould growth within the building fabric structure where it is invisible; hence, cannot be cleaned off, creating an unhealthy living environment for the occupants. Mould spores have a detrimental effect on human respiratory system and with 1 in every 6 adults and 1 in every 4 children in NZ suffering from asthma this emphasises the importance of managing condensation in building fabric.


How can we Stop condensation forming in our walls?

The safest way to prevent condensation forming in wall cavities is to keep the wall cavity warm, to ensure the dew point does not sit inside wall cavity (dry zone of the building envelope).

The ARIDON® SMART WALL system wraps the building frame in a thermal weatherproof shell, which maintains the wall cavity at a temperature very similar to that of the room itself. The theoretical dew point of the ARIDON® SMART WALL system is near the exterior face (wet zone side) of the panel – exactly where it should be!

Therefore if condensation is not forming on your furniture, it won’t be forming within the wall.

By removing the risk of condensation forming, water vapour can easily pass in and out of the wall cavity via plasterboard linings and maintain equilibrium with the internal environment – combatting interstitial condensation.

The ARIDON® SMART WALL system provides a long term robust solution to condensation control by keeping the wall framing and cavity near room temperature for the life of the building.

How important is managing Condensation – No one talks about this?

The three main consequences of condensation trapped in the building envelope are:

  1. Air with higher levels of moisture will take more energy to heat to comfort levels, which will increase your energy bills or reduce your thermal comfort.
  2. The structure of the house could deteriorate.
  3. Respiratory health problems associated with uncontrolled mould growth.

Leaky homes are commonly known to leak from the outside in – the disaster that hit our building trade in the 90’s was largely addressed with the introduction of a Risk Matrix assessment of the building envelope and use of claddings with a drainage cavity in higher risk scenarios. However – the second wave of leaky homes is just coming to the surface – leaky homes caused by interstitial condensation - the same phenomena experienced in Canada. This 2nd wave of leaky homes emerging in colder NZ climates could be even more detrimental than the first wave of leaky homes, as it can only be seen once the mould and mildew has become established and the air contaminated.

Canada has taken proactive steps to understanding the movement of moisture through the building envelope and changed their building code requirements/guidelines to ensure enhanced envelope performance can be achieved through insulating on the outside of the building frame, greater envelope airtightness and improved water management capabilities, amongst other factors.

We need to learn that you can’t design and build a new building in a cold climate the same way you build in a warm climate, as it will fail the functional requirements of NZ Building Code E3.2 (Internal Moisture) clause: Buildings must be constructed to avoid the likelihood of a)Fungal growth or the accumulation of contaminants on linings and other building elements.

Water vapor will always try and migrate from a warm humid environment to a colder (drier) environment and so there is a natural outwards drive of water vapour in winter conditions in buildings with standard insulation and construction. Attempting to stop water vapour from entering the wall cavity is also extremely difficult given the complexity of modern construction and the uncertainty around any future minor alterations that may be made to the building envelope.

The simplest solution to combat interstitial condensation is to keep the frame warm and dry through an exterior insulation and vapour control layer – such as the all-in-one SMART WALL system.


Can I control condensation with a ventilation system alone?

Good ventilation is essential in any house to control the amount of water vapour in the internal space and to ensure the air is healthy for the inhabitants. This can be achieved via a combination of opening windows and mechanical ventilation systems and the requirements are set out in the New Zealand Building Code documents E3 (Internal Moisture) and G4 (Ventilation). E3 requires minimum levels of both ventilation and insulation to control the effects of internal moisture.

There are several different types of ventilation systems available and the cheapest and most common type is Direct Ventilation. Direct ventilation systems generally seek to create positive air pressure by forcing dry air into the internal space which then forces moisture-laden air out through gaps in the building envelope. The problem with this approach is that the moist air which can cause condensation in the wall cavity is actually forced into the wall cavity due to the positive pressure being created in the interior. Therefore insulation on the outside of the wall cavity is an essential part of condensation control in cold climates.

What is Relative Humidity (RH)?

Relative Humidity (RH) the amount of water vapour present in air expressed as a percentage of the amount needed for saturation at the same temperature and pressure.

What is a dew point?

The dew point is the temperature at which the water vapor in air (at a given barometric pressure) condenses into liquid water at the same rate at which it evaporates. At temperatures below the dew point, water will leave the air. The condensed water is called dew when it forms on a solid surface.

E.g. Take a cold can out of the fridge and it meets a warm air environment, the water vapour turns to liquid on the exterior of the can. This is what can happen in your walls if you allow the framing to remain cold.