Skillion roofs
For skillion roofs with exposed rafters, install insulation over the ceiling lining between purlins – purlins must be sized to provide sufficient depth for the required thickness of insulation plus a 25 mm minimum air gap between the insulation and the flexible roofing underlay.
For skillion roofs with concealed rafters, install insulation between rafters – the combined rafter/purlin depth is likely to be sufficient to ensure that a 25 mm minimum air gap is achieved.
Note: For both exposed and concealed rafter skillion roofs, an air barrier must be installed over a timber-boarded ceiling to prevent air movement into the roof framing.