J4D4 covers roofs, including their ceilings, and any ceiling that is part of an intermediate floor being part of the building’s envelope, or where there is no ceiling.

J4D4(1) details the insulation properties and minimum Total R-Value required of a roof or ceiling.

Part or all of this may be provided by the roof construction itself and any inherent insulating property of the roof and airspaces reduces the amount of insulation needed.

Where the ceiling space below the roof is used as the return air plenum, it is considered part of the conditioned space. In this instance, the envelope boundary for the roof and ceiling construction is located at the roof.

The direction of heat flow stated should not be taken as the only direction in which any insulating properties operate but it is a statement of the prominent direction for that particular climate zone. It is assumed that materials, be they construction materials or insulating materials, will also have insulating properties in the other direction. For a residential building, the night time direction is important as the building is most likely to be occupied at that time and the outside temperature likely to be the lowest of the day.

The Total R-Value in J4D4(1) is dependent on the climate zone.

As with walls, the effect of thermal bridging must be taken into account when determining if the minimum R-Value of a roof has been achieved. In some cases, thermal breaks will be necessary to achieve compliance.

A thermal break may be provided by materials such as timber or expanded polystyrene strips, plywood or bulk insulation. Reflective insulation alone is not suitable for use as a thermal break because it requires an adjoining airspace to achieve the specified R-Value.

The weight of roof or ceiling insulation needs to be considered in the selection of plasterboard, plasterboard fixings and building framing.

There may be instances where there is a loss of ceiling insulation because of downlights, fans and other penetrations. In these circumstances it is the responsibility of designers to determine how they will achieve the required Total R-Value given the construction of the roof and the penetrations.

Details of the Total R-Values of typical constructions are provided in Figures J4D4a to J4D4g and Tables J4D4a to J4D4g.

The Total R-Value of the basic roof and ceiling has been determined by adding together the material R-Values of the outdoor air film, roof cladding, roof airspace, ceiling sheet lining and internal air film.

The Total R-Value of the roof and ceiling materials may need to be adjusted if other building elements, such as sarking, are to also be installed.

Note that it should not be assumed that these figures are representative of all construction scenarios. For example the spacing of framing members, the presence of roof lights or the specific type of frame could all affect the actual Total R- Value by creating thermal bridging between elements or by compressing insulation. If following a Deemed-to-Satisfy compliance pathway, the code requires in J4D3(5) that AS/NZ 4859.2 be used to calculate the Total R-Value of a building’s envelope.

Insulation can be installed in the roof, the ceiling, or a combination of both, provided the required thermal performance is achieved and other aspects of the building’s integrity are not compromised. It should be noted that the thermal performance of the roof may vary depending on the position of the insulation, the climatic conditions, the design of the building and the way in which it is operated. For example, althoughnot recognised in the values,insulation installed underthe roof, rather than on the ceiling,in a building with a large roof space in a cold climate, or when a room is being air-conditioned, may be less effective because of the additional volume of roof airspace that would need to be heated or cooled.

For a material that is not listed as an item in the figure below, other than air, the R-Value may be determined by dividing the thickness of the item in metres by the thermal conductivity in W/m.K (typical values are described in Specification 36).

There are a number of different insulation products that may be used to achieve the minimum added R-Value. Care should be taken to ensure that the choice made is appropriate for the construction and climate conditions. For instance, in some climate zones, an impermeable insulation sheet needs to be installed with due consideration of condensation and associated interaction with adjoining building materials.

1. The R-Value of an item, other than an airspace, air film or air cavity, may be increased in proportion to the increased thickness of the item.
2.  The Total R-Value of a form of construction may be increased by the amount that the R-Value of an individual item is increased, except where the item is thermally bridged.
3.  Where an airspace is filled, the R-Value listed for the airspace must be deducted from the Total R-Value of the roof construction.
4. For information on a roof space that is considered to be ventilated, see S36C2(4).

Figure J4D4a: Roof 15° to 45° pitch—horizontal ceiling—metal cladding

Figure J4D4b: Roof 15° to 45° pitch—horizontal ceiling—clay tiles 19 mm

Figure J4D4c: Roof 15° to 45° pitch—cathedral ceiling—10 mm plaster on top of rafters—metal external cladding

Figure J4D4d: Roof 15° to 45° pitch—cathedral ceiling—10 mm plaster on top of rafters—tiles external cladding

Figure J4D4e:    Skillion roof less than 5° pitch—10 mm plaster below rafters—metal external cladding

Figure J4D4f: Skillion roof 5° to 15° pitch—10 mm plaster on top of rafters—metal external cladding 

Figure J4D4g: 100 mm solid concrete roof to 5° pitch—10 mm plaster suspended ceiling—applied ex- ternal waterproof membrane