Mechanisms of Moisture Ingress

Rajveer Niijer

Rajveer Nijjer

I have been in Building Surveying now for nearly 8 years and, in my experience, one of the leading causes of decay/deterioration in buildings is due to moisture ingress. Moisture can penetrate through the external surface of a building in a number of ways and can gain access through the cladding or at tricky junction details. Whilst water is an essential element for life it can be a nuisance if it finds its way into the wrong environments or onto/into the wrong components leading to potentially catastrophic results. In this blog, I will discuss the various mechanisms of moisture ingress, how they work, and who you should be seeking advice from to avoid these issues.

The following list summarises common mechanisms that can act on a cladding and should be considered in the design of a building:

  • Surface Tension – When water makes contact with a material it forms droplets, this is a normal response as the particles are attracted to the surface (surface tension) in order for it to minimise its surface area. These droplets, can, in certain circumstances, resist gravity through friction and instead of running off a vertical surface, it can stick to and ‘track back’ beneath a horizontal junction. The size of the droplet and the extent and direction of the movement is subject to the normal physical effects, one, in particular, is wind and differences in atmospheric pressure. Water held on a surface by surface tension can be blown into a junction where it can penetrate the building (as shown in the below image).

surface tension

(An example of surface tension to the eaves of a building).


  • Capillary Action – Is initiated by surface tension whereby water bonds to two adjacent surfaces and can be drawn upwards. Wind pressure can also act on the water and drive it upwards even further. This can occur at multiple areas of a building i.e. within small cracks in claddings, joints at windows and junctions between materials. 
  • Absorption and Wicking – This occurs within the body of absorbent materials whereby absorbed water will migrate through a material from a warm to cold area, or from wet to dry. Rapid heating by the sun on surfaces can drive moisture through materials. 
  • Solar Driven Moisture - This is a process that relies on both capillary transfer and vapour diffusion. Rain is first absorbed by capillary transfer into an absorbent cladding, when sunshine follows soon after, some of the moisture is driven through the material deeper into the structure by diffusion.

Solar driven moisture

  • Condensation – Is the process where water vapour becomes a liquid. Air contains water vapour, as the air cools its ability to hold water vapour falls and the vapour is released and condenses as water. For example, steam from a hot shower will condense when it meets a cool exterior window. Condensation can be absorbed by materials and cause damage to them, this also needs to be managed within wall assemblies.     


  • Differential Pressure – This is caused by variation in air pressure. Higher exterior pressure can drive water through unprotected gaps into the lower pressure building interior. These paths that air follows are known as air leakage paths and can become water leakage paths.
  • Momentum - Wind can drive rain against a building and deflect water up behind a cladding. For example, rain can bounce off a horizontal surface and deflect up behind the bottom edge of a cladding. 
  • Gravity – Water may enter by one or a combination of the above mechanisms, with gravity then taking over to aggravate the effect. Gravity can carry water that has penetrated at roof level further down a building, and along the wall assembly. 

As well as the above mechanisms, water entering a building can also be due to a number of other issues such as product failure, incompatibility of materials, poor design and workmanship etc. 

A well-designed joint will need to be able to resist all of the mechanisms of moisture movement described above. One mechanism can act in combination with one or more of the others.

To prevent moisture ingress issues, it is important to ensure that your building has been adequately designed. At the consultant appointment stage, it is crucial to ensure you have an experienced and reliable Architect appointed who understands the mechanisms of moisture ingress and has a good knowledge of the Building Regulations. No building can be entirely watertight, though, so there must be a secondary barrier incorporated into your design. For example, if moisture penetrates or becomes trapped behind the external cladding of a building, a mechanism must be in place that allows it to drain away. It is important that your chosen Architect can assist you in producing a design which can meet the performance requirements of the New Zealand Building Code (NZBC). 

A number of other areas will also need to be looked at during the design stage:

  • Do I need a building consent?
  • Compatibility of materials
  • Will my design comply with the building code
  • Issues with moisture ingress to an existing building are likely to involve the appointment of a timber remediation specialist.

Furthermore, it is vital to choose a reputable contractor who produces a high quality of workmanship to carry out your work and can ensure the design is adequately constructed on-site. 

If you think you need advice in identifying issues with a leaky building or are concerned your property may have moisture ingress problems, please contact our Building Surveying team and we would be happy to help. Or, if you require design advice or guidance then our Architecture team will be able to assist you with your next steps. 

Kaizon can assist with defect diagnosis, watertightness testing and specification of remedial work. Our in-house team can assist with the design, project/cost management, advocating early contractor involvement and can oversee your project from beginning to end. 

A comment from our Senior Registered Architect, Treiz Azzu

Treiz Azzu
Any building must be functional, safe and suit the needs of the people who use them. Architects must consider all of these factors when they design buildings, and they need to be aware of their responsibility to ensure that all plans and specifications will comply with the Building Code.

Our Kaizon Architecture team believes that good and sound design includes ensuring that the building envelope (cladding, insulation, framing, lining) all work together and the building code requirements are met. The most significant codes here to follow are H1 (energy efficiency), E2 (external moisture), E3 (internal moisture), B2 (durability) and G4 (ventilation).