NCC 2019: Bridging the Wall System Performance Gap
Coffee fuelled or simply sharpening up your geekiness, if you made it through my last post or have delved into guts of NCC Section J 2019, it’s hard to disagree that J1.5 Walls and Glazing is certainly an industry game changer.
Whether you see the challenges as cost driven, aesthetics, procurement limitations, climate applicability or other, understanding our minimum energy efficiency performance requirements, changing our marketing material, reviewing product strengths and weaknesses or rethinking our building designs are all realities as we approach May 2020.
While it might seem like a distant challenge and one for the long finger, some academic institutions are already aspiring to take on the changes. While not fully understood, they clearly represent the direction we need to go to future proof our commercial building stock.
As part of my last review of NCC 2019: J1.5 Walls and Glazing, I only dipped the proverbial toe into the challenges and impacts of one element of what comes next for façade design and construction, Total R-values and their impact on wall systems.
Staying away from heat sucking, poor performing spandrels this time, which have had enough attention already, what about more traditional wall build-ups and the new requirement to include thermal bridging for framing? Additional administration, construction costs increases or a value to our built environment and time for a reset?
What are the new performance requirements for wall systems?
Sticking with the Deemed-to-Satisfy provisions of Section J 1.5 (d), let’s revisit those performance requirements to set the scene.
As a part of a wall-glazing construction, a minimum performance Total R-value ‘backstop’ has been provided for wall systems….so we are talking precast, timber, masonry wall build-ups or similar that form the thermal line of the building envelope.
Specifically, the adoption of the 2019 Total R-values as opposed to the 2016 R-value sets forth the priority of including thermal bridging, or parts of the building envelope where thermal transmittance and the direction and density of heat flow change significantly. In many cases, this is due to the penetration of the thermal line with highly conductive framing materials thus increasing the thermal transmittance and ultimately the heat losses and gains!
As you can imagine, thermal bridges can be structural components or purely geometric bridges, where complex junctions as a result of the shape of the building lead to additional thermal losses. It is the structural thermal bridge that the ABCB have focused on in this NCC introduction to the subject, and the repeating thermal bridge of frame studs and noggins within walls that are currently not accounted for.
As per J1.5 (d), where a wall is ≤ 80% of the area of the wall-glazing construction, a scenario that seems very likely for most walls where heat loads, daylight or ventilation are also provided, we get a Total R-value performance requirement in all Climate Zones and Building Classes of just R1.0.
Given this is typically designed as per NCC 2016 to between R2.5 – R3.2, Climate Zone and Building Class dependent, this is one hell of a drop to account for repeat thermal bridging! Suitable as a backstop for spandrels made of highly conductive aluminium, but a very low minimum performance for a wall construction.
Walls greater than 80% of the area of the wall-glazing construction? Then we are again subject to variable requirements as set by Climate Zone and Building Class, as per the table below and ranging between highs of Total R3.3 and as low as Total R1.4. In reality, many wall constructions are unlikely to see this requirement unless it is a party wall or similar without the need for glazing but the performance requirements are significantly higher.
So, in simple measures, we can state the while metric of choice has changed, from an R-value to a Total R-value to account for repeating frame bridging, the likely backstop of R1.0 will become the typical design team meeting value of discussion for those hitting minimum wall performance requirements (that’s the vast majority of buildings!).
Bridging the Performance Gap!
So let’s contextualise those numbers and work out what they really mean. Today, based on the 2016 code, we are designing to somewhere between R2.5 – R3.2 (let’s call it R2.8 average), Climate Zone and Building Class dependent and not including thermal bridging.
In our likely scenario, where a wall is ≤ 80% of the area of the wall-glazing construction, where we get a Total R-value performance requirement of just R1.0, a very real performance gap exists (indicated by the red dotted lines below) as a result of thermal bridging. In other words, in most scenarios, the numbers we use today are simply wrong and this change in approach attends to this over simplification to get us back in line with more realistic performance values.
How would standard wall systems stack up?
In order to bridge this performance gap, we need to account for more accurate resistance values, which is where the studs and noggins come into the equation. But what would typical wall build-ups look like as per the calculation requirements of NZS 4214-2006? Let’s have a look at some frame thermal bridges to find out.
In terms of conventional construction practise, we are only really looking at 2 main alternatives for most framing applications, timber (pine @ 90mm X 35mm) and steel (1.2mm thickness X 33mm) studs and noggins. Timber is more variable in terms of its ability to conduct heat as it is subject to moisture swings with the seasons while steel has a much higher ability to conduct and transfer heat. Let’s assume studs at 600mm centers and noggins every 1350mm @ a wall height of 2700mm and stuff the wall with 90mm of mid performance glass wool.
Ahh, another pretty graph defaulting to generic Excel formatting, but what does it tell us? Well, the horizontal dotted lines give us our performance targets (Climate Zone and Building Class dependent), with the Total R-value of 1.0 (blue dotted line) the most likely of interest.
Using a standard brick veneer construction, we can immediately see the impact of adding timber framing (orange bar) to an NCC 2016 (blue bar) construction with no thermal bridging accounted for. In this case, we get a drop from an R-value of 2.9 to a Total R-value of 2.5. Add steel in the mix, replacing the timber of course, and you drop below the magic Total R-value ‘backstop’ of 1.0 to 0.9. Yes folks, while steel is preferable for other reasons, it does not belong in the thermal envelope of your building! Unless, of course, you thermally break it. Ok, it’s still steel and it’s never going to be great, but it is back up at a Total R-value of 1.9.
What does it all mean?
Well, first up. I would have to assume that the impact on the market is relatively low(ish) and this should be embraced as a quick readjustment to reflect a greater focus not just on design, but on the operations of a buildings. It would seem that the proposed Total R-value of 1.0 where the wall is < 80% of the wall-glazing construction represents something achievable for both spandrel and traditional wall systems, although the performance requirement remains concerning low particularly for cold and hot climates!
This does raise some interesting questions about the way we have been generally approaching the design of mechanical systems to date. If indeed, the 'real' value for wall (and spandrel) resistance is significantly lower than which we use as a result of NCC 2016 requirements, surely it brings into question the accuracy of mechanical system sizing and a clear disconnection to facade performance?
Unless you are dependent on the construction efficiency of steel, cost premiums seem unlikely but let’s not forget, this is only a backstop to the minimum performance requirements and part of the overall Total U-value of the wall-glazing construction. It will be the combination of these two façade elements that tells the true performance story and the costs that go with it.
On the supply side, given that performance is now more clearly aligned to the total wall systems, how will suppliers state that products meet the thermal performance requirements by R-value alone, as it is no longer just an insulation requirement? I am sure we will see some creative approaches to market, but let’s not make them more confusing for the consumer and get those thermal bridging disclaimers right upfront!
All up, thermal bridging needs an introduction to the Australian market, and this is one of the simplest ways to do it. We will learn along the way, with no major cost uplifts (besides from steel construction) and this is the type of reset required to bridge a performance gap that will ultimately be a stepping stone to improving our built environment. Next step, will be pushing those performance requirements back up, once we have got our head around the changes