Company: ARIDON

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.

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.

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 causes 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 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.

The safest way to prevent condensation forming in wall cavities is to keep the wall frame and cavity warm, and ensure the dew point does not sit within the wall fabric (dry zone of the building envelope). The ARIDON® SMART WALL system wraps the building frame in a thermal weatherproof shell – (eliminating thermal bridging) 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!

Check out some independent WUFI modelling demonstrating the superior long term performance of the ARIDON® Smart Wall system versus traditional building systems – you may be a little shocked at the results. It’s also a gentle reminder that design and construction in colder climates should not replicate the successful techniques used in warmer climates.


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