NT Part 13.2 Building fabric

Explanatory information: 13.2.3(2)

• The roof space ventilation option applies to a pitched roof with a flat ceiling to ensure that efficient cross ventilation is achieved in the roof space to remove hot air. Roof space ventilation is generally not suitable for most flat, skillion, cathedral ceiling and similar roof types because of the lack of space between the ceiling and roof.
• Care should be taken to ensure that the roof ventilation openings do not allow rain penetration and that they comply with appropriate bushfire provisions.
• Gaps between roof tiles with sarking (or reflective insulation at rafter level) and metal sheet roofing are not acceptable methods of providing roof space ventilation.
• Compliance with the ventilation provisions may result in the ingress of wind driven rain or fine dust, or stimulate the growth of mould or fungus in the roof enclosure. Consideration should therefore be given to the surrounding environmental features prior to adopting this as an alternative to the roof insulation provisions.
• A light coloured roof reduces the flow of heat from solar radiation better than a dark colour roof. A roof with a solar absorptance value of less than 0.55 means the roof is of a light colour such as white, off-white, cream or dull zinc aluminium.

Explanatory information: Tables NT 13.2.3a, NT 13.2.3b and NT 13.2.3c

• Typical construction: The tables above provide examples of various roofs and ceiling, walls and floors. The Total R-Value required is achieved by adding the Total R-Value of the basic element, i.e. roof and ceiling, walls or floors, and the R-Value of any additional insulation incorporated in that element The Total R-Value of the basic roof and ceiling has been determined by adding together the R-Values of the outdoor air film, roof cladding, roof airspace, ceiling sheet lining and internal film.
• The Total R-Value of the roof and ceiling materials may need to be adjusted if other building elements such as sarking are also installed. For example, sarking or sheet insulation under tiles may change a roof space from “ventilated” to “unventilated”.
• Thermal bridging: Irrespective of the framing material used, the minimum added R-Value specified is deemed to include the effect of thermal bridging created by framing members in situations other than described in the following point.
• Thermal break: Because of the high thermal conductance of metal, a thermal break is to be provided where the ceiling lining of a house is fixed directly to the underside of the metal purlins or metal battens of a metal deck roof or where there is no ceiling lining. The purpose of the thermal break is to ensure that the thermal performance of this form of roof construction is comparable to that of a similar roof with timber purlins or timber battens. A thermal break may be provided by materials such as timber, expanded polystyrene strips, plywood or compressed bulk insulation. The material used as a thermal break must separate the metal purlins or metal battens from the metal deck roofing and achieve the specified R-Value. Reflective insulation alone is not suitable for use as a thermal break because it requires an adjoining airspace to achieve the specified R-Value (see last point).
• Location of insulation: The thermal performance of the roof may vary depending on the position of the insulation, the climatic conditions, the design of the house and the way in which it is operated. For example, insulation installed under the roof, rather than on the ceiling, of a conditioned house with a large roof space is less effective because of the additional volume of roof air space that would need to be heated or cooled. Conversely, for an unconditioned house, the use of reflective insulation is more effective when placed directly under the roof.
• Choice of insulation: There are a number of different insulation products that may be used to achieve the minimum added R-Value. However, care should be taken to ensure that the choice made is appropriate for the construction and climatic conditions as the location and relationship other building elements may not be suitable in all circumstances for both practical and technical reasons. For instance, in some climate zones, insulation should be installed with due consideration of condensation and associated interaction with adjoining building materials. Reflective insulation and its adjoining airspace is considered to achieve the following R-Values when used in conjunction with the Total R-Value of a pitched roof and flat ceiling construction. To achieve these values, the reflective insulation must be laid directly under the roof cladding and have a minimum airspace of 15 mm between a reflective side of the reflective insulation and the adjoining lining or roof cladding. The actual R-Value added by reflective insulation and its adjoining airspace should be determined for each product which takes into consideration factors such as the number of adjacent airspaces dimensions of the adjacent airspace, whether the space is ventilated and the presence of an anti-glare coating. When reflective insulation has an anti-glare coating on one side, the emittance value of that side will be greater than the value of the uncoated side. Also, where another emittance value for reflective insulation is used (other than the value used in NT Table 13.2.3d), care should be taken to ensure that the number of airspaces allowed for is consistent with the form of construction and whether the airspace is reflective, partially reflective or non-reflective. Where bulk insulation fills the airspace, the Total R-Value should be reduced to take account of the loss of airspace.

Explanatory information: 13.2.3(5) and Table NT 13.2.3d

When considering the reduction of insulation because of exhaust fans and recessed downlights, 1% of the ceiling area for a 200 m² sized house would permit 2 bathroom heater-light assemblies, a laundry exhaust fan, a kitchen exhaust fan and either approximately 25 recessed downlights with 100 mm clearance to the insulation or approximately 10 recessed downlights with 200 mm clearance to the insulation.

Explanatory information

Walls with a surface density of 220 kg/m² or more are deemed to achieve acceptable levels of thermal performance due to their ability to store heat and therefore slow the heat transfer through the building fabric. These walls are defined by surface density (kg/m²), which is the mass of one vertical square metre of wall, in order to reduce the complexity when measuring the mass of walls with voids.

The following are examples of some typical wall constructions that achieve a surface density of 220 kg/m²:

• Two leaves each of 90 mm thick or greater clay or concrete masonry.
• 140 mm thick or greater dense-weight hollow concrete or clay blocks with—
  • 10 mm plasterboard or render; and
  • at least one concrete grouted horizontal bond beam; and
  • vertical cores filled with concrete grout at centres not exceeding 1000 mm.
• 140 mm thick or greater concrete wall panels and dense-weight hollow concrete or clay blocks with all vertical cores filled with concrete grout.
• 190 mm thick or greater dense-weight hollow concrete or clay blocks with—
  • at least one concrete grouted horizontal bond beam; and
  • vertical cores filled with concrete grout at centres not exceeding 1800 mm.
• Earth-wall construction with a minimum wall thickness of 200 mm.

Explanatory information

The attachment of a Class 10a building, such as a garage, glasshouse, solarium, pool enclosure or the like should not compromise the thermal performance of the Class 1 building. In addition, the Class 10a building may be insulated and so assist the Class 1 building achieve the required thermal performance. Explanatory Figure NT 13.2.6 below depicts examples of a Class 1 building with an attached Class 10a garage.

Insert NT figure (13.2.6 (explanatory)) 13.2.6 as follows:

Figure Notes

In (a), the thermal performance required for the Class 1 building may be achieved by the outside walls and floor of the Class 10a garage.

In (b), the thermal performance required for the Class 1 building may be achieved by the walls and floor of the Class 1 building as if the Class 10a garage is an under floor space with an enclosed perimeter.