Design decisions relating to windows and flooring will be critical in meeting new 7-star thermal performance requirements for housing in Australia, a webinar has heard.

And higher rating wall insulation will only deliver benefits where windows are also double glazed.

During a recent webinar hosted by WoodSolutions, Dr Phillip Christopher, a research fellow in Infrastructure Engineering at the University of Melbourne and a leader in sustainable construction, outlined several strategies that will help builders move from current 6-star requirements to new 7-star mandatory minimums that will commence in October.

These include double glazed windows, suitable window framing materials and insulated floor types such as waffle pods or concrete slabs.

Christopher’s presentation come as new homes and apartments which are approved for construction from October onward will need to achieve 7-star thermal performance ratings under the Nationwide House Energy Rating Scheme (NatHERS).

The new requirements will be adopted across most Australian states (Tasmania will consider 7-star requirements in 2025 whilst the Northern Territory has not indicated whether or not it will introduce 7-star requirements).

They are part of significant changes to energy efficiency and condensation standards which are being introduced under the 2022 update of the National Construction Code (NCC 2022).

The changes follow a 2019 Commonwealth/state agreement to progressively increase NCC energy efficiency requirements under a long-term trajectory which aims to achieve zero energy and carbon ready buildings.

Under that arrangement, energy requirements were set be tightened for commercial and public buildings in NCC 2019 and for residential buildings in NCC 2022.

The changes have since taken on added significance in light of Australian Government commitments to achieve 43 percent reductions in greenhouse gas emissions by 2030 and NetZero by 2050.

All up, buildings (commercial, public and residential) account for 25 percent of Australia’s greenhouse gas emissions. Meanwhile, heating and cooling specifically account for around 42 percent of residential energy efficiency use.

The changes also recognise the importance of regulation in driving better thermal performance in homes.

In Victoria, for example, data on the CSIROs Australian housing data portal shows that 85 percent of detached homes (Class 1 buildings under the NCC) which are approved for construction throughout that state are designed to meet only the minimum six-star rating and nothing more.

This means that for most homes, market forces such as innovation and consumer demand will only deliver designs which exceed minimum requirements in a relatively small percentage of cases.

(Double glazed windows are critical for 7-star ratings)

So How Can 7 Stars be Achieved?

During the webinar, Christopher outlined findings from a study which he was commissioned to undertake by timber industry body Forest & Wood Products Australia. The study aimed to investigate practical measures which can be adopted to deliver upon the new 7-star requirement in a cost-effective manner.

That study involved two phases.

In the first phase, the researchers looked at a specific Melbourne context and considered a baseline HIA Standard (3 bedroom) single-storey home along which achieved the current minimum 6-star rating along with a typical two-storey volume builder home (refer webinar for details).

Speaking of the single-story home, the researchers found that the standard baseline home could be upgraded from six to seven stars using several different combinations of measures.

These included:

• upgrading from single glazed to double-glazed windows (especially if filled with argon)
• using timber or PVC framed windows instead of aluminium window framing, and
• upgrading from a timber sub-floor to either a waffle pod slab foundation (which has polystyrene voids on a concrete slab) or edge insulation on a concrete slab to provide effective floor insulation.

Impacts of other measures such as varying wall types or paint colours were less significant and were not an effective strategy to move from six to seven stars if done in isolation.

Beyond that, the study was expanded to consider a broader national context.

To do this, the standard baseline home used in the first study phase was redesigned to be a ‘high-performance construction’ home. This contained DGAr-Tb-LE windows, brick veneer walls, R4.0 ceiling insulation, a medium discontinuous roof and a waffle pod as a floor type.

The team then moved this home design around different locations and climate zones and varied floor, wall and window options to determine which if any features could potentially be taken away so as to deliver upon the 7-star rating but to do so in a cost-effective manner.

The results were as follows:

• In cool to mild temperate climate zones such as Hobart, Melbourne and Canberra, windows were the greatest factor that influenced the performance of each house. Indeed, the study found that 7-star homes were readily achievable when using double glazing in ‘day spaces’ such as kitchens and living rooms. Compared with single-glazed Low-E windows, simply moving up to double glazing added up to 0.5 stars to the rating achieved. This increases to gains of as much as 1.6 stars when high-performance double glazing was combined with either a timber frame, a PVC frame or a thermally broken aluminium frame.
• Still in these cooler climates, waffle pods delivered the best thermal performance in terms of flooring in Melbourne and Canberra but the timber sub floor was the top performer in Hobart on account of decoupling of the ground and subsequent reduction in heat loss.
• In milder warm temperate climate zones (Sydney and Perth), building 7-star homes was easier. Indeed, without any modifications, the high-performance home referred to above achieved 8.4 stars in Sydney and 8.2 stars in Perth. In terms of options, the main factors which may jeopardise 7-star minimum performance were use of a timber subfloor as opposed to a concrete slab or waffle pod or moving to single rather than double glazing.
• Further north, in the warm to hot climate of Brisbane, the high-performance home referred to above achieved an 8.4 star rating – indicating that 7-stars or above is easier to achieve in warmer climates compared with cooler ones. However, the importance of glazing in both day and night spaces (see below) was particularly accentuated in Brisbane, with single rather than double glazing taking a full 2.0 stars of the Brisbane home rating. Also, the timber subfloor performed poorly in Brisbane as the decoupling the floor from the ground was not ideal with the major conditioning requirements focusing around cooling. Finally, having a 70 ml wall frame was fine in Brisbane. Thick, heavily insulated walls are not essential.

An interesting point to note was the importance of glazing in ‘night’ spaces such as bedrooms as well as ‘day’ spaces such as kitchens and living rooms.

According to the study, failure to insulate night spaces resulted in significant thermal loss in warmer climates such as Brisbane but less so in cooler areas such as Melbourne or Hobart. This is because conditioning is typically needed to cool homes whilst occupants are sleeping in warmer climates but is not required to heat bedrooms (as sleeping occupants use blankets) in cooler climates.

Accordingly, whilst double glazing in bedroom windows is needed in warmer climates to achieve 7-stars, Christopher suggests that cost savings may be realised in cooler climates whilst still delivering 7-stars by applying double glazing in kitchens and living rooms only.

In other study findings:

• Eaves were found to be of greater importance in warmer climates where you have a greater need for cooling. This is especially the case in places like Brisbane for houses with concrete floors. Without eaves, these absorbed the heat gain and made it difficult for the house to cool off overnight.
• Without double glazed windows, moving to higher rating wall insulation delivered only marginal benefit as much of the additional heat retention from the walls is lost out of the single glazed window. Conversely, when double glazing was applied, higher rating wall insulation delivered significant thermal performance improvement.
• With thermal bridging, the performance of both steel and timber frames was affected – steel more so than timber. This was most effectively addressed by use of a thermal break (a sheet of insulation between the steel/timber and the wall). Simply adding more insulation was not effective as a massive amount of insulation would need to be added.
• Narrower lot homes with less solar access have bigger challenges in meeting 7 stars.

(Waffle pod slabs can reduce heat loss through the floor)

Seven Overall Strategies

When designing/building 7-star homes, Christopher encourages consideration of several strategies.

These include:

• Early engagement with NatHERS assessors to enable identification of strategies (glazing, locations, shading, solar access etc.) which may deliver significant thermal performance benefits.
• As per above, using double glazed windows as well as suitable window framing (ideally thermally broken timber or PVC frames).
• Where possible, designing for location to ensure that glazing and shading maximises solar heat gain where needed but also provides shading where required.
• Insulating the whole home with ceiling/wall/floor insulation (in that order) as well as applying insulation between conditioned and non-conditioned spaces such as between the garage and the rest of the home or between bathrooms and living spaces.
• Using zoning which can be delivered through doors or partitions (especially near stairs or hallways) to reduce heat/coolth loss from conditioned spaces to adjacent rooms.
• Using lighter colours for walls/roofs to reflect heat in warmer climates and darker colours to reduce heat in cooler climates.
• Other measures such as using ceiling fans, specifying sealed lighting where possible to avoid ceiling heat loss through unsealed lights and providing opportunities for cross-ventilation through windows and openings on either side of homes.

The session was hosted by Wood Solutions. Sponsors included Forest and Wood Products Australia, Katana Foundations, Utecture, Bondor, Xlam, ARC Camp.H building 4.0 CFC, Bentley Homes and ASH.

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