This article was written and provided by Boris Iskra, National Codes & Standards Manager of Forest & Wood Products Australia.
The proposed National Construction Code 2022 (NCC) energy efficiency and condensation technical provisions for residential buildings closed for public comment on 20 October 2021. For domestic houses (Class 1 buildings), the proposed Deemed-to-Satisfy (DTS) changes would see:
- a stringency increase in the thermal performance of homes from the current level 6-stars to the equivalent of 7-stars,
- the introduction of “whole-of-home” annual energy use requirements, and
- enhanced condensation management provisions, including additional ventilation and wall vapour permeability requirements.
What could the introduction of these changes mean for timber roof framing systems?
An increase in thermal performance to 7-stars would see a change in insulation levels required in walls and roof systems. For example, in some instances, the required roof insulation R-value could increase to R5.0 or R6.0 in colder climates – taking account of vented roofs, no installed reflective insulation, loss of ceiling insulation area due to downlights. An R5.0 ceiling batt would have a thickness of around 215 – 240 mm (depending on the insulation manufacturer) requiring an additional ventilation airspace above the insulation being not less than 20 mm in height. This is all fine, but consider what happens at roof to external wall locations?
Correctly installed insulation should run to the outer edge of the top plate and not be compressed. For this example, when installing R5.0 insulation, it would require a height up to 260 mm between the top plate and any obstruction (e.g roofing battens, roof covering). To achieve this, the use a raised heel detail (see below), or similar, at the external walls would be required to prevent compressing the insulation, causing a reduction in the thermal performance of the installed insulation, and the ventilation airspace.
(Diagrams – sbcindustry.com/content/1/raised-heels)
From a structural design perspective, other energy efficiency measures to consider in the design of roof framing members, include:
- Increased weight of ceiling insulation?
- Installation of photovoltaic roof solar panels?
- Installation of evaporative cooling system?
- Suspension of internal ductwork within the roof cavity to prevent the compression of ceiling insulation.
As a side note, it is good to see the proposed NCC 2022 changes now recognising the thermal insulating benefits of using timber framing; which is used as the benchmark for roof framing systems. Depending on the minimum R-value required for the climate zone, the use of steel-framed trusses for pitched roofs with flat ceilings would require (1) additional ceiling insulation (an additional R1.0-R2.0 on top of the minimum R-Value required), or (2) the addition of an insulation strip (thermal break) of R0.26 to R0.51 over the ceiling framing or (3) a layer of continuous insulation having a minimum R-Value of R0.25 to R0.51. Clearly demonstrating the known thermal performance benefits of using timber.
What about wall framing?
Again, the proposed NCC 2022 changes now recognise the thermal insulating characteristics of timber which is used as the benchmark for wall framing systems. The use of steel-framed walls would require thermal bridging mitigation which, depending on the climate zone, could be in the form of lining the outer surface of the frame with 10 mm insulation (R0.26 thermal break) through to increasing the wall insulation and lining the outer surface of the frame (thermal break) with additional insulation with an R-Value of at least R0.6.
In relation to the wall framing systems (both timber and steel), there could be an impact on the level of wall insulation and potentially the stud frame sizes required. The proposed changes would require the installation of additional wall insulation (R-value) depending on:- the climate zone in which the dwelling is being built, the external wall height, eave/soffit width, wall colour, the use of lightweight cladding (e.g. weatherboard, fibre cement) and whether the dwelling is of one- or two-storey construction. The effect of these provisions could require an increase wall stud sizes to accommodate the increased insulation; particularly in the colder climates.
(WoodSolutions Publication: R-values for Timber-framed Building Elements)
The proposed changes described above are derived from using the NCC 2022 Deemed-to-Satisfy pathway. Another pathway available is via the use of NatHERS accredited energy rating software. The flexibility of the rating software is that can model the interaction between the various building elements including the building fabric (walls, floors, roofs), window size and thermal performance through to building orientation. This allows a level of “trade-off” between these elements.
At the time of writing, all submitted public comments on the proposed 7-star provisions are now being collated by the Australian Building Codes Board, who produce and maintain the NCC on behalf of the Australian Government and each State and Territory government, and will be considered in due course. It would be timely to consider and plan for the potential changes which may be introduced in the NCC 2022. Time will tell…
Boris Iskra
Forest & Wood Products Australia
National Codes & Standards Manager
Our Principal Partners
