COP v3.0:roof-ventilation; external-environment

10.2 The External Environment 

The external environment that affects roof ventilation includes:

  • The Climate.
  • Water

10.2.1 Climate 

Being narrow, mountainous islands lying in the path of the prevailing wind, New Zealand is subject to high rainfall and high humidity.
Compared to much of the world where 70% humidity is considered to be the threshold of corrosion and health problems, New Zealand has very high humidity, particularly around the northern regions, where the relative humidity is often more than 90%.
Temperature variations, on the other hand, are quite low; nearly all the inhabited areas of NZ being in what is defined as a Temperate environment.
The design requirements to deal with this environment are specific to NZ’s climate and building practices. It is not advisable to use design or installation practices from countries with different environmental conditions, without comprehensive assessment.


10.2.2 Water 

Water exists in 3 states: solid (ice), liquid (water), and gas (water vapour). Water in liquid form is relatively dense, and ice slightly less so. This is largely due to attraction the molecules have to each other. The molecular attracton makies water form droplets on a surface.


10.2.2A Hydrogen Bonding

Water molecules in liquid form bonds which create a dense material.

In vapour form, water molecules do not have this bonding. Water Vapour is very light compared to other gasses in the atmosphere, which is 78% nitrogen.  


The kinetic energy of water molecules in gas form creates space between the molecules.

10.2.2C Shower Condensation

The high humidity created while showering causes condensation on even relatively warm surfaces because of the high concentration of vapour.


10.2.2D Condensation on a Cold Glass

In warm conditions, condensation will form on a cold surface, even when the concentration of water vapour in the atmosphere is low.