The largest changes in NCC 2019 can be found in J1.5, now a one-stop-shop for glazing and wall performance requirements for Class 2 (multiple dwelling unit) common areas, a Class 5 (Office), 6 (retail or restaurant), 7 (car park or warehouse), 8 (laboratory or manufacturing facility) or 9b (school) or 9a (health-care) and Class 3 (hotel or student accommodation), 9c (aged-care) or a Class 9a (health-care) ward areas.
Introducing new metrics and concepts to the Australian construction industry, J1.5 establishes both the minimum performance requirements for wall and glazing components of a wall-glazing construction and the methods needed to illustrate compliance against them.
By definition, a wall-glazing construction combines wall and glazing components of the envelope, excluding display glazing which is treated separately and opaque non-glazed openings. So, in other words, it combines the performance for 1) glazing and 2) walls to get single values for a ‘wall-glazing construction system’.
With various levels of stringency set as a function of when the building is typically occupied, wall - glazing performance requirements are broken up by Climate Zone and Building Class. Total System U-Value and Total R-value are the metrics of choice, creating an unnecessary complication for the industry as one is the inverse of the other!
In all Climate Zones, for a Class 2 common area, a Class 5, 6, 7, 8, 9a or 9b building other than a ward area, we now need to have a Total System U-Value ≤ U2.0 for a wall-glazing construction.
Expanding on the existing definition of Total System U-Value, a revised 2019 definition now states that thermal transmittance allows for the effect of thermal bridging. While it was already implied in 2016 but not categorically stated, when combined with the definition for glazing and that ‘supporting frames located in the envelope’ are to be included, this lays the foundation for an important change to the way we approach the thermal bridging associated to frames.
For Class 3, 9c or a Class 9a ward area building, performance requirements vary by Climate Zone, from the most stringent requirements in alpine areas (Total System U-Value ≤ U0.9), less stringent requirements in warm temperate climates such as Sydney, Brisbane and Perth (Total System U-Value ≤ U2.0) to somewhere in between for all other climates (Total System U-Value ≤ U1.1). The Total System U-Value of display glazing falls outside the wall-glazing construction definition, is unambitious and set to be not greater than a Total System U-Value U5.8.
As for the wall components of the wall-glazing construction, thermal backstops to the performance of wall systems have now been introduced, whether they are spandrels within a curtain wall system or a more typical precast, timber or masonry wall build-up.
- Where the wall is less than 80% of the area of the wall-glazing construction, we get a Total R-value performance requirement in all Climate Zones and Building Classes of R1.0. Given that most spandrels will not meet this value without being thermally broken, this is a game-changer!
- More than 80% wall area, then we are again subject to variable requirements as set by Climate Zone and Building Class. Again, the 2019 definition now includes thermal bridging. Do you see the trend? Thermal bridging folks!
In all climates except sticky ol’ Darwin, for a Class 2 common area, a Class 5, 6, 7, 8, 9b or 9a building other than a ward area, we now need to have a Total R-value ≤ U1.4 while the constant heat of Top End gets a significant jump up to a Total R-value ≤ U2.4.
As for Class 3, 9c or a Class 9a ward area, we get more variable Total R-value performance requirements that broadly align to that set by the Total System U-Value performance requirement. So, alpine regions along with Darwin remain the most stringent (Total R-Value ≥ U3.8 and U3.3), Sydney, Brisbane and Perth are the most lenient (Total R-Value ≥ U1.4), while Melbourne, Canberra, Hobart are somewhere in between (Total R-Value ≥ U2.8).
Putting aside the accuracy or potential impact of the performance requirements set out above, Climate Zones and Building Classes that are deemed to be conditioned and occupied during the day have the most stringent wall-glazing construction performance requirements.
By recreating the approach to wall-glazing construction discussed above, it was also required to take a similar approach for solar control for vision and wall areas. As such, solar admittance or the fraction of incident irradiance on a wall-glazing construction has been added as a new metric to account for this total heat load.
As we have seen above, Climate Zones and Building Classes set the performance requirements with various levels of stringency set as a function of when the building is typically occupied. Speeding this up, in all Climate Zones and Building Classes, different performance requirements are set for only north, east, south and west aspects with a view to simplifying the impact of a more thorough stipulation of all cardinal directions.
Incredibly, in most cases, the performance requirement for each aspect is deemed to be the same, so the southern side of the building would ‘benefit’ from shading and reduced SHGC. While an allowance for southern and western aspects is provided in Class 2 common area, a Class 5, 6, 7, 8 or 9b or 9a for alpine regions, it does not carry over to Class 3, 9c or a Class 9a ward areas. Through such simplification is a fundamentally flawed approach and difficult to understand as to how this has sneaked into NCC 2019.
Specifications J1.5a and J1.5b set out the methods for compliance for wall-glazing construction and determine the performance of spandrel panels, respectively. For Specifications J1.5a, we are provided two methods for U-value and solar admittance compliance, Method 1 (based on a single aspect) and Method 2 (allowing for trade-offs between multiple aspects). With clear boundaries set by AS/NZS 4859.2 for accounting for thermal bridging and clear formulas to follow, Specifications J1.5a is unsurprising in its methodology or intent.
Specifications J1.5b, however, begs more questions, as its methodology is less clear. Responding to the unequivocal fact that poor spandrel performance has been ignored for years, spandrel panels are now deemed to have either default Total System R-Value if they match the description of a very specific configuration (Method 1), or Total System U-Values are calculated by an equation (Method 2). Yes, that is two different units that fail to make this process as simple as it should be!
For example, of the four generic default configurations provided, configuration 2 consists of a thermally unbroken (bridged) frame and a centre of spandrel panel consisting of a double-glazed opaque face with a 50mm air gap and a back pan with no performance specifications provided. So, if the stars are aligned, the gods are smiling, and you can manage to describe your spandrel as per configuration 2’s generic labels, you can then pull a performance value from a table based on the proposed R-value of the insulation. In this case, if we assume an insulation R-value of 1.5, we can then nominate a Total R-Value of 0.44! The limitations of this approach are abundantly clear and render it as a largely pointless exercise as we simply don’t have enough information provided to ever truly match up to a configuration.
Of course, we can always take Method 2, where we calculate our way to a more accurate representation of what will be installed. For those not familiar with the Total System U-Value of spandrel systems, assume a value of 1.5 is typical for a standard spandrel system, but this, of course, is subject to its dimensions. Convert this back to a Total R-Value renders a value of approximately 0.65 and a failure to meet the minimum backstop. The only way to truly deal with the poor performance of spandrels is to ensure that the frame is insulated from the interior. Thermally breaking the spandrel is, of course, an advantage but ensuring a thermal line that is not made of aluminium has its benefits!