Building Air Tightness (Part 1): Are new Green Star targets required to drive building performance?
Whole buildings air tightness is growing in Australia, albeit very slowly. While common and often mandated in Europe, Scandinavia and North America, where buildings are pressurised using a fan and the resulting air flow rate or leakage is measured to ascertain performance, we are a few steps behind in Australia.
With a focus on energy efficiency and thermal comfort, the recognised benefits in colder climates are irrefutable. Tighten your building and you will save money on annual heating and the size of your heating system while improving the health and comfort of occupants.
To avoid a boring convoluted legislative discussion, Section J Deemed-to-Satisfy provisions have little backbone with no testing methodology to illustrate air tightness. So let’s move on.
Dig a little bit deeper, into the realms of Section J – JV3 Alternative Verification, we find requirements for air change per hour (ACH), with a value of 1.0 assumed for a perimeter zone when in operation. As a measure of how many times the air within the perimeter zone is replaced under normal operating conditions over an hour, ACH is a metric from the past that no longer interfaces with modern air tightness discussions.
Fortunately, we are able to convert ACH over to an air leakage rate to be more useful and in line with International comparison and testing. In short, studies show us that ACH is roughly 1/20 of the air leakage rate @ 50 Pa. As such, we can roughly say (rule of thumb warning!) that if an ACH of 1.0 is mandated for JV3 modelling, this is equal to air infiltration rate of 20 m3/hr/m2 @ 50 Pa!
Of course, as very few buildings test for air tightness, the majority of buildings cannot say if they meet this value, this assumption within the JV3 pathway is highly questionable if we are not forced to meet it?
Is Targeting Best Practice Suitable within Australian climates?
Today, the Green Building Council of Australia, with support from proactive Industry Professionals, have provided a useful baseline target of air tightness and what can be achieved.
Enshrined into an Innovation Credit within Green Star Design and As-Built, points are awarded for undertaking testing or meeting ‘Best Practice’ targets.
The question is, if we are to make the assumption that an air infiltration rate of 20 m3/hr/m2 @ 50 Pa is being met within our commercial buildings, what would improving beyond this value render in terms of building performance and other positive outcomes?
Is International Best Practice the best short-term outcome to encourage uptake or would we be better relaxing our short-term ambitions?
Running the Numbers
To answer these questions, naturally, I popped up some thermal models and ran some numbers.
Using a mid-sized office building with typical technologies, profiles and usage for Melbourne and Sydney, the numbers tell an interesting story.
Assuming 0 % is our minimum Section J – JV3 performance baseline, we can clearly see the significant investment and operational benefits of tightening our buildings in temperate (Melbourne) and warm (Sydney) climates.
Focusing on investment, if we build tighter we can guarantee reductions in the capacity or size of the equipment needed to service our buildings. For our Good Practice scenario (grey), it is fair to say that a sizable reduction in heating (8 – 11%) and cooling capacity (6 – 10%) can be reached. Looking at Best Practice (orange), we only realise a small additional benefit in both climates, suggesting that Good Practice is a more suitable target for design.
Looking at operations and annual space heat scenarios, we also see interesting outcomes. Here, an annual savings in heating could be as high as 12 – 16% for Good Practice, with only 1 or 2% further reductions for Best Practice.
One Step at a Time
It is still early days, but it seems that undertaking this challenge is perhaps a stick too heavy and risky for our industry to carry at this point. If we are to encourage the adoption of new building approaches and construction techniques, perhaps we need to aim a little lower to get a little further? Surely meeting Good Practice is still a major improvement to our buildings?
Indeed, given the capacity and operations savings connected to air tightness, it could indeed be argued that this important passive effort to improve buildings is currently undervalued within Green Star Design and As-Built?