Being narrow, mountainous, islands lying in the path of strong prevailing winds, New Zealand is subject to high rainfall and high humidity. Compared to much of the world — where 70% relative humidity is considered the threshold of corrosion and health problems — New Zealand has very high humidity, particularly in northern regions, where the mean annual humidity levels are often around 80% or more. This means the dew point (temperature at which condensation begins to form) is also higher than in colder but drier climates.
The design requirements to deal with this environment are specific to New Zealand, which is reflected in our building practices. It is not advisable to use design or installation practices from countries with different environmental conditions without a comprehensive assessment of the management of internal moisture under NZ conditions.
Relative humidity (RH, given in per cent [%]), is the most widely known method: It gives the content of water vapour in the air relative to the maximum amount of water this parcel of air can hold at its present temperature.
Other measures are absolute humidity and water vapour pressure.