What does “build tight and ventilate right” mean when applied to multi-residential buildings? Sean Maxwell, AM.AIRAH, investigates.
Unless effective ventilation is used concurrently, the more airtight apartments are built, the more risks of condensation, mould, and negative health outcomes increase. To address these risks, the 2022 NCC requires continuous ventilation for airtight apartments. So, how widespread is the problem in Australia?
Data recently published in the CSIRO and ATTMA’s 2024 study “Air Tightness of New Dwellings in Australia” found that 39 per cent of apartments in the study were tighter than 5m³/hr.m² at 50Pa reference pressure, the threshold below which the 2022 NCC requires continuous ventilation. However, only a fraction (most located in ACT and VIC) actually had continuous ventilation. We know from anecdotes and popular media that condensation is a widespread concern in such cases.
The 2022 NCC provisions have good intentions but will have little effect. Ventilation is only required if an apartment is shown to be airtight through testing. By making testing and reporting – and by extension, ventilation – entirely optional, we allow the problem to continue. The ABCB has leaned in the right direction, it but must now act. Continuous ventilation makes so much sense it should always be required, as should airtightness testing.
But what kind of ventilation is adequate? Here are some simple arguments: 1) any mechanical ventilation is better than nothing; 2) distributed, balanced, filtered ventilation with a heat recovery system is better still; and 3) a path should be plotted for more controlled sources of supply air. To support these views, I will share data and some personal experience. We must acknowledge the shortcomings of simple systems and plot a better path forward.
Natural versus mechanical ventilation
We can’t count on air leakage, openable windows or other “natural” ventilation strategies to provide consistent and continuous moisture removal. I can personally attest to their inefficacy. When I moved to Sydney Australia, I lived in a very leaky apartment (>18m³/hr.m² @ 50Pa). Its old “openable” windows were in effect “not closable”. Because the apartment was so cold, it was impossible to keep surfaces and contents above the dewpoint. Mouldy clothes and furniture resulted.
We moved to another apartment also in Sydney, but it was very airtight (<3m³/hr.m² @ 50Pa). It shared a roof-mounted ventilation system with the other 12 apartments in the block and this provided levels of continuous exhaust, about 15 litres per second combined from two bathrooms and a laundry in our unit. The effectiveness of continuous ventilation in interior moisture management was tested when the roof fan was being repaired one winter morning in 2021. Condensation collected on the single-glazed aluminium glass doors in a bedroom, until the roof fan repair was complete. Within one hour, the condensation was gone. Simple, but effective.
This is only one example, but it proves a point: Even a very airtight apartment can greatly reduce condensation with a simple continuous ventilation system. Comparing the first building built in the 1920s to the second built in the 1980s, the thermal performance of the building fabric was similar; both had uninsulated masonry walls and single-pane glazing. Neither building had permanent heating.
Tight buildings with simple ventilation can often avoid many condensation-related issues. While the 2022 NCC provision for ventilating tight apartments will help with condensation, it also helps address particulates and myriad other indoor pollutants that accumulate gradually.
Is simple ventilation adequate?
ASHRAE 62.2 has required continuous ventilation with “outdoor air” for a long time. One way of accomplishing this in a simple building in many Australian climates is to use a continuous exhaust ventilation system. For every litre of air drawn out, another litre is drawn in through the apartment envelope. While the cleanliness of air drawn through building cavities can be debated, my second Sydney apartment example – and a great deal of research – shows that it does provide clear benefits. This positive return for low investment is the reasoning under the language in the 2022 NCC, and an argument to strengthen it.
Still, for anything other than a simple standalone house, more complex airflow dynamics pose challenges. My second apartment illustrates this point. First, because the apartment was on the first of three floors all over a shared garage, effective distribution itself was a challenge. Most likely, apartments closer to the fan on the top floor would be over-ventilated if those farthest away (mine) were adequately served.
A single fan serves 12 apartments, with 36 exhaust registers in total. Expecting to get equal or even adequate flows from each is a questionable assumption. Reasons include poor duct design, poor duct construction quality, and a simple weather-driven stack effect. Years of research by Steven Winter Associates in the US and other experts around the world found that it is very difficult to get central systems consistently right.
Maintenance is always a concern. Individual systems may have an advantage over central systems in simplicity of distribution, but servicing one fan in a common space is very different to servicing 12 fans in occupied units. Though a constant exhaust does not filter supply air, it clearly does remove particulates, as demonstrated by their accumulation on the roof and on registers in the unit. Still, dirty registers draw less air.
Where does makeup air come from?
While exhaust systems remove and dilute pollutants, where does the makeup air come from? Controlling the source is important. ASHRAE 62.2-2022 requires that units are verified as airtight with a blower door, no leakier than 3.6m³/hr.m² @ 50Pa. It also states that measures shall be taken to reduce the transfer of air from undesirable places. Gaskets on doors can reduce the contribution from garages, for example. But even if air comes directly from outdoors through a window crack, is it clean?
My apartment was not far from the M1 highway. Traces of black dust gathered over time along window cracks, showing the debatable contribution of “fresh” outdoor air to our IAQ.
Particulates and other pollutants also gather during bushfires. Many of us remember bushfire season in 2020, when the outdoor air was downright toxic. At the time, I worked at an office in Sydney fitted with a heat recovery ventilation (HRV) system. In these conditions, fresh air straight from outside may not be much better than air that passes through a fibreglass filter (AKA insulation). For a system with ducted supply, regular filter changes are a significant maintenance cost that cannot be ignored.
With all this in mind, asking where replacement air comes from is an innocent question with an extremely complicated answer. The simple system in the newer apartment worked by imparting constant negative pressure, pulling in relatively drier air to dilute interior moisture. But here is where ASHRAE 62.2 saw and addressed a problem. I will use data loggers to illustrate.
In July 2023, I placed data loggers in several places in our apartment while we spent a weekend away. All interior doors were closed and the heating was turned off. For five days, I logged pressures every five seconds on six different channels, collecting more than half a million data points. The goal was to paint a picture of pressure fluctuations and trends in the apartment and the building overall over time.
The findings were straightforward. The apartment was under negative pressure compared to the outdoors and compared to the corridor almost all the time (99 per cent). This means that it was virtually always drawing makeup air either from outside or from the corridor. The corridor was negative compared to outside most of the time (91 per cent), except for two occasional conditions: 1) during the day, when wind gusts play with the building’s pressure, and 2) when the building’s common entrance door or the door to the shared garage was opened. I verified these pressure fluctuations by opening doors myself to observe the effect.
The data is shown in the graph below with the vertical axis representing the pressure of spaces relative to the pressure outdoors. Certainly, pressure in apartments does not fluctuate wildly for an occupant inside, but wind makes pressure outside fluctuate relative to the pressure inside.
Short-term jolts in pressure can be seen and attributed to the opening of the main building doors or the door to the garage. Medium-term shifts can also be seen; they likely result when someone in the building alters the airflow network for some time. Examples may include opening a window or closing doors to the bathrooms or laundry and restricting exhaust flow. All of these decrease building depressurisation and alter the relative influence of weather. Beyond the basic ~5Pa of negative pressure acting on our apartment, there are other clear trends.
The big picture for a bigger building
First, a cascade of pressures presents. The location farthest from the corridor door, the ensuite, is under the greatest negative pressure relative to the outdoors, and air virtually always flows in this direction. The other bathroom is also a pressure sink. Because it is on the way to the ensuite, the large bedroom sees airflow from the hallway and living room areas. This bedroom is the location of the above photo of the window with condensation, while the roof fan was being serviced. Clearly, this zone receives relatively constant ventilation. Some makeup air does come from gaps and cracks in the envelope, including power points, across which I directly measured a pressure drop of nearly 6Pa. But the flow from a single power point is miniscule. It must be coming from somewhere else.
It’s not easy to see the data from bedroom 2 in the chart. That pressure follows the pressure in the apartment so closely that it is virtually eclipsed in the graph. Because that zone shows little pressure drop across its door, it is possible that it receives very little ventilation. Without suction on that zone, there is no draw for makeup air from its envelope leaks. In addition, there is little exterior envelope for that room. Even if pressure existed, there are few pathways for air from outside. Thus, zones like these may be under-served by a simple cascade system like this.
What is most clear is that the great majority of air comes straight from the apartment door to the corridor. In this apartment built in the 1980s, there are no seals on the top, sides, or bottom of the corridor door. Further short-term monitoring showed that most of the air from the corridor comes relatively constantly from the parking garage. These airflow trends are manifest in traces of dust on doors in the building. The side with negative pressure always has dust collection in the same pattern, as particulates are impacted on surfaces they must squeeze through. When ASHRAE 62.2 refers to “outdoor air”, the parking garage was likely not what they had in mind.
Of course, my mini-experiment leaves many questions. What were exhaust rates in other apartments? Would the trends hold in a different season? Exactly how much airflow was actually coming through the door to the corridor? Is the air in the corridor worth breathing? What is the relative humidity in the apartment, or one of many other IAQ markers, in each room, throughout the year? As I said, real-world ventilation function is anything but simple.
Wishful thinking
Gayathri Vijayakumar, a principal at Steven Winter Associates and a former colleague of mine who participated in several meetings for recent revisions of the ASHRAE standard, put it this way in a recent podcast with Renewaire:
“A lot of multifamily builders will pressurise their corridors, meaning they’re just going to dump all the outdoor air into the corridor, and they are going to cross their fingers that that air is going to make it into the apartments. I don’t know anyone who lives in a multifamily building who walks out into the hallway and says, ‘Gosh, it smells really fresh out here. Feels like I’m in the outdoors.’ Right? That outdoor air gets comingled with other air; it’s not going to stay fresh. So anyone who’s looking at ventilation systems for multifamily, if you’re going to deliver air and you want it to go to the apartment, you really should deliver it INTO the apartment.”
It took many years of debate to get to this point. The latest revision of ASHRAE 62.2 simply prohibits the approach of simple exhaust-only ventilation for multi-residential with shared corridors: “4.2 Exhaust-only ventilation systems are not permitted for newly constructed attached dwelling units that open directly to an enclosed corridor.” The reason is that makeup air for these unbalanced systems sooner comes from other building spaces, including quite undesirable ones like garages or neighbouring apartments, than through the building envelope.
Though makeup air through corridor doors is not advisable, it may not be practically possible either. In most buildings, for fire safety reasons, new apartments are frequently provided very airtight corridor doors with effective seals on all four sides. This was confirmed as part of the CSIRO/ATTMA study mentioned above, with one door showing only 4.3m³/hr @ 50Pa of leakage. In modern buildings, very little makeup air may come from corridor doors. We need to rethink sources of fresh air supply.
Writing a new narrative for multi-residential ventilation
The first 1920s apartment example showed that natural stack- and wind-induced ventilation is unreliable and in fact may exacerbate condensation risk, while wasting energy and spoiling liveability. The second 1980s apartment showed that constant exhaust reduces the risk of condensation, but that if efforts are not made to control where makeup air is sourced, it may come from undesirable locations like corridors and parking garages. Apartments built in 2024 may be very airtight, and the provenance of their makeup air is even more mysterious.
The story of ventilation in Australian multi-residential buildings is following a similar arc to narratives worldwide. Let’s begin the next chapter with mandatory continuous mechanical ventilation in our apartments. But let’s also respect the complexities of the issue and seek reliable, truly fresh air to breathe in our homes.
About the author
Sean Maxwell, AM.AIRAH, is ANZ Scheme Manager for the Australian chapter of the Air Tightness Testing and Measurement Association (ATTMA). He has nearly 20 years of experience as a building services professional and is the chair of AIRAH’s Building Physics Special Technical Group.
This article appears in Ecolibrium’s October-November 2024 issue
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