COP v3.0:internal-moisture; insulation

10.6 Insulation 

The increase in ceiling insulation standards helps prevent heat leakage into the ceiling space, but it does not affect the passage of water vapour. Ceiling insulation excludes more heat from the ceiling cavity, resulting in colder cavity air temperatures. Warmer air can hold more water vapour than cold air. Therefore, colder temperatures in the ceiling cavity lead to an increased risk of condensation.

10.6.1 Insulation Position 

Insulation must be positioned so that there is a gap of at least 20 mm between insulation and roofing. On flat ceilings, or buildings without eaves, it must be placed so the gap is maintained at the eaves or an eaves insulation barrier should be fitted. To achieve this, an eaves insulation barrier may be required.

10.6.2 Other Insulation 

Reflective foils are not defined as insulating, because they are only somewhat effective against radiative heat loss and have little impact on conductive and convective mechanisms. They also increase the potential for electrocution. As they are non-absorbent, they are not permitted as residential roofing underlay in New Zealand.

When Polyester blanket insulation is used in conjunction with metal roof cladding, special roofing screws should be used to avoid binding.

Composite insulated metal panels are described in 15.5 Warm Roofs.

Insulation must not be laid over the purlins as this compromises the air gap and the efficiency of the insulation where the roofing compresses it.