COP v3.0:internal-moisture; additional-information

10.12 Additional Information 

10.12.1 Cold Roofs 

With cold roof construction, the under-surface temperature of the metal roofing will at times be quite low, so the primary tool of managing condensation is controlling the concentration of water vapour in the attic space. Some condensation is inevitable, and it must be managed to ensure the wetness is not excessive in either degree or duration — allowing moisture to accumulate.

In typical cold roof construction, the insulation is at ceiling level and there is an air gap between the insulation and the roof surface.

10.12.2 Warm Roofs 

With Warm Roofs, the insulation is in direct continuous contact with the underside of the roof.  The most common form of Warm Roof in New Zealand is pre-formed insulation panel.  Other proprietary systems may consist of several layers with a vapour control layer on the underside.

Warm roofs do not inherently have the same natural ventilation as a cold roof, so the internal environment may require management to prevent condensation problems.

10.12.3 Night Sky Radiation 

Roof cladding absorbs radiation from the sun and the attic space becomes warmer; some of this heat is radiated into a clear sky at night.

Because all objects radiate heat to cooler objects, night sky radiation will occur when there are no clouds in the sky. The radiation rate depends on the emittance of the roof cladding.

Radiation to the sky can cause the cladding temperature to drop as much as 5˚C below that of the surrounding air; that causes dew when the surface temperature reaches dew point or frost if the temperature falls below zero.

10.12.4 The Mechanics of Condensation 

Water exists in 3 states: solid (ice), liquid (water), and gas (water vapour).


10.12.4A Hydrogen Bonding

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

In ice and liquid water, individual H2O molecules bond together in a special way, called ‘hydrogen bonding’.

In gas form, the kinetic energy of the molecules has overcome these hydrogen bonds, and so the individual water molecules are free to move. The water molecule itself is light compared with other gases in the atmosphere, so it tends to migrate upwards, ie, into the roof space.

Water vapour may condense into liquid form when the concentration rises or the temperature drops. The temperature at which air can hold no more water is called the 'Dew Point'. The water vapour capacity of air is relative to temperature.


10.12.4B Shower Condensation

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


10.12.4C Condensation on a Cold Surface

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



10.12.5 Underlay Standards 

Permeable underlays must comply with NZS 2295, Amendment 1:2017, as shown in Properties of Roofing Underlay, or have an appropriate Product certification such as a Codemark certificate.

Reflective foil underlays must comply with AS/NZS 4200.1:2017

10.12.5A Minimum Requirements for Underlays for Metal Roof Cladding

Classification R1R3R2R4
Grade HeavyweightHeavyweightSelf support >/td>Self-support
Type KraftSyntheticKraft >/td>Synthetic
Application  Residential or light commercial buildings>/td> 
PropertyUnit  >/td> 
Absorbencyg/m²≥ 150≥ 150≥ 150 >/td>≥ 150
Water Vapour ResistanceMN s/g≤ 7≤ 0.5≤ 7 >/td>≤ 0.5
Water resistancemm head≥ 100≥ 100≥ 100 >/td>≥ 100
Tensile Strength MDKN/m≥ 9≥ 3≥ 11 >/td>≥ 3
Tensile Strength CDKN/m≥ 4.5≥ 2≥ 6 >/td>≥ 2.5
Edge Tear Resistance MDN≥ 40≥ 100≥ 70 >/td>≥ 150
Edge Tear Resistance CDN≥ 35≥ 80≥ 55 ≥ 130

Based on Table B1 of NZS 2295 Amendment 1:2017.

  • Self-supporting (S/S) is defined as strong enough to support its own weight up to a 1200 mm span.
  • pH between 5.5 and 80.
  • Kraft based underlays shall have shrinkage less than 0.5% and maximum run-length of 10 m.
  • Synthetic underlays may have any run length.
  • Any underlay is regarded as fire-retardant if it has a Flammability Index (FI) of 5 or less when tested to AS/NZS 1530 Part 2.


10.12.6 Relative Humidity 

RH is strongly dependent on temperature. For instance, a parcel of air at 15°C and 50% RH is cooled down to 10°C. Now, the relative humidity of this parcel of air will be close to 70%, without the actual amount of water having increased.  Relative Humidity expresses how close the air is to being saturated with water vapour. Warm air can hold more moisture in absolute terms, cold air less. If the air becomes saturated (RH 100%) water vapour will condense as mist in the air or as water on adjacent cold surfaces. 

Relative humidity is a suitable measure when the risk of condensation on surfaces or mould growth is to be evaluated. 

10.12.7 Absolute Humidity 

Absolute Humidity is measured in grams of water per volume of air (grams per cubic metre [g/m3]). It is not temperature dependent and in the example above the absolute humidity would remain unchanged at around 6.4 g/m3, regardless of the temperature change.

Absolute humidity is a suitable measure if one is looking for sources or sinks of water in an environment where temperature is changing. If the absolute humidity is cycling during the day, eg, increases in a roof cavity as the temperature rises during the day. It could indicate that moisture is released by the building materials during the day and absorbed or condenses during the cooler nights. 

10.12.8 Water Vapour Pressure 

Water Vapour Pressure is more based on the fundamental physics and expresses the contribution (ie, the partial pressure) of water vapour to the total pressure of an air mix. For example, at a pressure of 1000 Pa (1kPa), the partial pressure of nitrogen may be 700 Pa, the partial pressure of Oxygen 200 Pa and the partial pressure of water vapour 99 Pa, and other gasses 1 Pa.  

This parameter is useful to evaluate moisture migration from one point in the building to another.