What are the Effects of leakage on energy use and even safety?

Ductless risers and service risers are large rectangular cavities within the building envelope that can significantly contribute to stack effect—turning tall buildings into giant chimneys that unintentionally channel airflow.

Poor sealing is common across all types of ductwork and builder’s risers. In general, the larger the riser or duct, the more leakage problems we encounter. Our testing shows that duct systems typically leak between 8% and 18%, with extreme cases reaching over 35%. It’s important to note these figures are from random samples gathered during energy efficiency retrofit feasibility studies. When contractors are given advance notice about which duct sections will be tested, performance often improves due to extra care taken during sealing.

Leaky ductwork and risers can interfere with stair pressurisation system commissioning. These issues are time-consuming and dangerous during emergencies. On windy days, poor pressurisation could fail to protect occupants.

In buildings with underground car parks—particularly in cold climates—risers connected to the car park can draw carbon monoxide into the conditioned spaces above, posing serious health risks.

Energy Consumption Impact of Leaky Ducts and Risers

Despite the view expressed in AS 4254:2002, a few simple calculations suggest that there is reason for concern about the impact of duct leakage.  Consider a typical air conditioning system in which the designer follows AIRAH DA09 [4] and assumes a supply duct leakage rate of 5%. To deliver the design air quantities to the spaces served, the fan must handle 1/0.95 times the sum of the room air quantities or 105.3% of the nominal air flow. Applying fan laws gives an increase in fan power of 117%, so the widely accepted leakage rate of 5% has added 17% to supply fan energy, for every hour the plant operates. At 10% leakage the extra fan energy is 37%.

But that’s just the beginning—duct leakage also affects the energy used by heating and cooling systems. The impact depends on where the ducts are located:

• If the ducts are within the conditioned space, low leakage might not be a major issue because some lost air still contributes to heating or cooling.

• If the ducts are in a return air plenum or unconditioned space, the leaked air offers no useful benefit. It simply circulates inefficiently, wasting fan energy and reducing return air temperature.

• If the supply ducts are outside the conditioned space, like in a ventilated roof cavity, leakage results in direct energy loss. In this case, the 17% fan energy increase is compounded by additional losses in heating and cooling—and increased greenhouse gas emissions.

The analysis for return air ducts also depends on where the return air duct is located. If the duct is in the conditioned space, leakage has little or no effect since the air leaking into the duct is the air that would have been returned anyway. If the return air duct is outside the conditioned space, the effect is more pronounced.

For example, let’s say the system uses 15% outside air. If there’s a 5% leakage rate in the return ducts (from outside), this effectively increases outside air intake to 19.3%—a 28% rise in outside air load. For a typical Sydney cooling system, this could add 5% to peak cooling demand. It will have less effect on a VAV system with an economy cycle but more on a constant volume system with a lower percentage of outside air.

In summary a 5% leakage rate can contribute to the following:

• +17% fan energy use (supply side)
• +5% heating/cooling loss (supply ducts in unconditioned space)
• +5% extra load from return duct leakage pulling in outside air
  → Combined, this can easily add 10–15% to HVAC energy use and greenhouse gas emissions, depending on system type.

We do not have published data for the effect of duct leakage in Australian systems but there have been of a number of overseas studies dealing with the issue. One [5] estimated the heating energy wasted by duct leakage in Belgium at 15 GW.h (0.054 PJ) per annum and 0.75 TW.h ((2.7 PJ) per annum for the rest of Europe. Another study of VAV systems in large commercial buildings in California [6] calculated that, compared to “tight” duct systems (2.5% leakage), systems with 10% leakage had annual HVAC system operating costs 9 to 18% higher, while those with 5% leakage used 2 to 5% more energy.

Additional wastage occurs if supply and return air risers are collocated in the same structural riser shaft. This can cause short-circuiting, direct energy wastage and danger to HVAC control due to the miss matching of room temperature and return air temperature. The room will continue to call for cooling, but the return air sensor will be telling the chiller to reduce output.

What are the best methods to solve air leakage?

The conventional method of combining foam seals in transverse joints and mastics can effectively seal the ducts. However, the inspection and verification of seal being applied can cause issues. Ductless risers can be quite challenging to troubleshoot but Efficiency Matrix has mining grade cameras for detailed visual inspections of builder’s risers. This ensures there are no large holes, but to also allows us to audit the application of sealants to joins and inspect bracing for high pressure ductless riser systems.

Visual inspection can only do so much, especially when space is limited or when the seal is covered by other materials such as insulted ducts or attenuators. Pressure testing ducts can reveal the issues and pinpoint the leaks with the help of tracer smoke. In some cases, traditional method cannot be applied, such as sealed masonry risers. A more advanced solution is automated aerosol-based sealing, which internally pressurizes the duct and seals leaks from the inside.

Some builders are concerned about making home too airtight Some arguments they give are:

1. “Humans need to breathe, and so does a building.”
2. “We don’t want occupants suffocating—fresh air has to come from somewhere.”
3. “If the building is too tight, we’ll need to install an HRV or ERV system.”
4. “If we’re relying on natural ventilation, why worry about air leakage?”
5. “We don’t want mould growing in our buildings.”

To begin with, these concerns stem from of lack of ventilation, not airtightness.  Air leakage is NOT ventilation. So lets get back to basics:

What is Ventilation?

Ventilation is the intentional introduction of outdoor air into a building, either through natural or mechanical means. In apartments or enclosed buildings, it’s also crucial to consult with a fire engineer and implement a pressure relief strategy for fire safety.

• Natural ventilation relies on open windows, vents, and other planned openings, driven by wind and temperature differences.

• Mechanical ventilation uses fans to deliberately supply or exhaust air.

For ventilation to be effective we need to consider the quantity, the quality, and the distribution of air into an occupied space. Can air leakage satisfy these three factors? Here’s why it falls short:

• Air from leaks is not filtered. It may come from a mouldy, dusty, or vermin-infested building cavity.
• There is no way to easily temper the air coming in, so it brings the temperature, humidity or dryness of the outside with it.
• Air leakage leaves you at the mercy of the weather. On windy, hot, or cold days, there is lots of air exchange. But on calm days there is almost none.
• Because you can’t locate or control all air leaks, it’s very hard to make sure each room gets the right amount of fresh air.

There is no way to compare air leakage with ventilation! Not only does infiltration fail to meet ventilation needs, but it also has additional harmful effects to your building and the health of its occupants.

Condensation and Mould Risk

When infiltration brings air from outside, it can create localised cold spots in walls, floors, and ceilings. These have the potential to create condensation, which can lead to mould growth or stains on finishes. Unchecked, condensation can lead to rotting or corrosion of materials, causing structural damage. It also makes a more inviting habitat for a termite infestation. Mould hidden inside walls can spread spores throughout the home and is nearly impossible to clean.

For buildings that are designed for natural ventilation, the openings should be fitted with condensate drains and be constructed using water-resistant materials. The location of these openings should be accessible for cleaning and maintenance. The most common example is mould that forms on leaky windows, which can be cleaned easily.

Reduced Insulation Performance

When insulation materials are rated in the laboratory, they are under controlled conditions with no air moving through the test sample. In the real world, air leaks can pass air through insulation, which substantially reduces insulation performance in two ways:

• Direct bypass – air leakage can pass right around both rigid and fibrous insulation
• Wind washing – air can pass through bulk insulation material itself, disturbing air pockets trapped by the fibres and stealing the heat it is meant to retain

Unwanted distribution of pollutants throughout the building

In leaky buildings, pollutants can distribute through a building via unintentional gaps and cracks. This can create problems ranging from minor nuisances such as odours to more serious issues such as carbon monoxide from a garage or car park. Most dangerously, they are a potential path of smoke in the case of a fire.

Other issues from air leakage

Air leakage in larger buildings can also contribute to noise infiltration and noise production from whistling under windy conditions. Lastly, any hole to the outside, garage, underground car parks or other unconditioned spaces serve as easy paths pollutants or unwanted pests. Sealing up these paths is the cheapest and first method of pest control.

To address concerns for airtightness in homes:

When someone says a house “needs to breathe,” ask them to breathe through their eyes. Sounds ridiculous, right? Humans breathe through their mouth or nose—designed for that purpose. Homes should do the same: breathe through designed ventilation systems.

Key points:

• You can’t make a home too airtight if you provide proper ventilation.

• For typical homes, mechanical ventilation can be simple and low-cost. A continuously running, remote-mounted exhaust fan is quiet and energy-efficient.

• Air should never come through walls—true fresh air should be filtered, controlled, and delivered where needed.

• To prevent mould and condensation, combine mechanical ventilation with thermal bridge control. Even uninsulated outdoor areas like carport soffits can grow mould, showing that moisture control is more complex than ventilation alone.

Efficiency Matrix tests to BSRIA BG65-2016

Underfloor Air Distribution (UFAD) systems have become increasingly popular in new commercial buildings worldwide. Efficiency Matrix is equipped with some of the most advanced technology globally for testing, diagnosing, and documenting issues in UFAD systems.

Major Projects completed:

• Nishi (Canberra): 27,411 sqm floor area
• 150 Collins Street: 20,000 sqm floor area
• Deakin University Burwood (Corporate Centre)
• 60 Cremorne Street: 10,000 sqm floor area
• Latrobe City Council Library

… plus many other smaller projects.

We deal with ducted and non-ducted underfloor air distribution systems.

Underfloor plenums can be quite difficult to cost-effectively get airtight.  We have the experience in:

• Testing and software processes,
• Expert troubleshooting techniques
•  A comprehensive understanding of plenum performance to support cost-effective and reliable installations

We deploy a broad range of diagnostic tools and methods tailored to UFAD systems using our specialised equipment.

For particularly complex air leakage issues, we also offer a unique sealing process that goes beyond conventional methods—designed to identify and resolve leaks quickly and effectively.

Why Air Barrier Integrity Auditing Matters

Air barrier integrity auditing is essential for ensuring that airtightness targets are met efficiently. The key benefits of this process include:

• Precisely locating air leakage issues.
• Verifying the correct air barrier materials are being used for long-term performance.
• Providing timely reports, allowing remediation before finishes are applied.
• Capturing photographic evidence of remediation work.
• Identifying discontinuities in the air barrier at building junctions and façades using ultrasonic technology.
• Recommending compliant remediation solutions in line with the Australian Building Code.

If a QA process is not conducted regularly at key intervals during construction, airtightness targets can become a very expensive exercise. Remediation works that happen after construction can have the potential to not last the test of time. Or they may not even be possible due to safety.

Location, Location, Location

Commercial Buildings can be large and complicated, so it is important that plans are marked up with problematic details. If trades cannot find issues, they just won’t fix them.  If issues cannot be relocated to verify remediation, then the whole process of air barrier integrity auditing is rendered ineffective and pointless.

Verification of building materials

In some circumstances, building materials specified for the air barrier may be inadvertently costed out, or supplied with an inappropriate product.  Certain tapes or sealants may not be compatible together or may react undesirably when exposed to environments during construction.  It’s important to catch these issues during construction so that they can be repaired in a timely manner before finishes.

Report turnover

Projects can move extremely fast, and it is not uncommon for the fit-out to begin before service risers are complete.  Once an inspection is conducted, a report must be issued with all the location details marked off an up-to-date floor plan. Additionally, it should include recommended fixes and dimensions of the issue, with the direction of how the air barrier should be continued.

Photo evidence

Photos of issues should be marked up to clearly articulate to trades the whole that must be closed in.  Airtightness is not always obvious.

Troubleshooting during construction

Troubleshooting and confirming air leakage through the air barrier when the building is under construction is difficult.  There are technologies available to help confirm non-obvious gaps in building elements.  Ultrasonic sound auditing can add significant value.An array of ultrasonic transmitters which generate ultrasonic waves are set up on the outside.  This makes it possible to find gaps in curtain wall facades, leaks through building elements, door frames/window frames and window/door seals, even ducts and plenums.

The ultrasonic detector picks up ultrasound via a microphone and converts the inaudible ultrasound into an audible frequency and in some circumstances overlays the frequency emission location on top of a visual image with a representative highlighted colour.  A visual representation is bundled together using a directional array of receivers capable of picking up ultrasonic sound waves while suppressing other noise and outputting onto an image. When auditing ultrasonic sound gets louder the closer you get to the leak and is quickly muffled as it moves through the air.  The ultrasonic leak detection can pinpoint even the smallest leaks in a partly constructed building, which is the most economic time to organise a solution.

Recommended Remediation

During construction, drawings sometimes miss air barrier continuity.  Different materials that make up the air barrier have different surfaces. Therefore, different products need to be used depending on the circumstances to ensure the longevity of the air barrier over the building’s lifetime.

Air barrier integrity auditing is a critical part of building commissioning. An engaged airtightness coordinator coupled with good building design and detailing, makes airtightness targets like PassiveHouse achievable and economical.

Not all wall wraps are created equal. Different users have different requirements and expectations for what they want their wrap to do.
In most cases we should, and can expect modern nonwoven textile wraps to control:

1. Water – Prevent rainwater from entering the building during construction before cladding is installed. This protects insulation, internal services, and finishes that aren’t designed for exposure, reducing risks to both occupant health and structural integrity.
2. Air – After construction, uncontrolled air movement can worsen moisture problems, enable water ingress, introduce unwanted noise, increase heat loss/gain, and allow the spread of fire and smoke.
3. Water Vapour – Condensation and high humidity can cause mould, rot, and long-term damage to structural elements.
4. Fire Safety – Wraps should not contribute significantly to combustibility in the building envelope. Some glass/aluminium laminates are classified as non-combustible under the NCC, while vapour-permeable and foil laminate wraps are generally of low flammability, not non-combustible.

Sometimes we may be forced to compromise on some of these control layers, but we also need to pay attention to the minimum requirements in the National Construction Code (NCC).

NCC 2019: Improved Clarity for Combustibility Compliance

The 2019 update to the NCC clarified which materials can be used in areas requiring non-combustible construction. Key excerpts from Clause C1.9 include:

• (a) In Type A or B buildings, external walls (including all components like facade, framing, and insulation), lift pit floors, and certain internal fire-rated walls must be non-combustible.

• (b) Non-loadbearing shafts must be non-combustible if they span more than two storeys in relevant building classes.

• (c) A loadbearing internal wall and a loadbearing fire wall, including those that are part of a loadbearing shaft, must
  comply with Specification C1.1.

• (d) Exemptions include gaskets, caulking, sealants, termite barriers, laminated glass, glazing thermal breaks, and damp-proof courses.

• (e) Permitted materials where non-combustibility is required include plasterboard, fibre-cement sheeting, bonded laminated materials (if each layer is non-combustible and adhesives are thin), and sarking-type materials ≤1mm thick with a Flammability Index ≤5.

The NCC 2019 update resolved long-standing conflicts where non-combustibility requirements clashed with the need for air, water, and vapour control—especially in external wall assemblies.

The Challenge of Wrap Performance: Vapour vs Airtightness

There never has been a wrap product available that meets the NCC requirement for non-combustibility, while at the same time as being an effective air and water barrier, combined with sufficient vapour permeability for use in cool and temperate climates.

Current products on the market today that are deemed non-combustible are made from pure aluminium foil laminated, an adhesive layer and a woven glass cloth.  Aluminium Foil when reinforced can be very airtight, but these products are also extremely vapor tight (Incapable of allowing moisture vapour to escape/permeate to the outside).

To improve vapour permeability, some manufacturers perforate the foil—creating a trade-off. These perforations compromise water and air barrier performance. After all, nobody sells umbrellas with holes in them.

On the other side of the equation, with aluminium foil type wraps, if the holes are so small and infrequent, these types of products still struggle as a vapour permeable underlay. Like single glazed glass windows, aluminium foil is a non-porous material, and it is prone to condensation forming on its surface. Over the past year, some manufacturers have published questionable vapour permeance values, with figures shifting from plausible to implausible and back again.

Polyolefin Textiles: A Better Option

Well established polyolefin textile air and watertight vapour permeable wraps can provide a far superior vapour permeability performance without compromising other important features.

If following a deemed to satisfy path using NCC 2016, it will require some compromise of water tightness, air tightness or vapour permeability.  The only realistic option to date has been to insulate externally the non-combustible sarking, use a non-combustible vapour permeable sheathing board, or follow a performance solution / fire engineered pathway.

Water Barrier Classification: AS 4200.1

Looking at the requirements of a water barrier under the AS4200.1: 1994 code for sarking you can see that almost all perforated aluminium-based materials are unhelpfully classified as:

“Water Barrier – Unclassified”

6.4 Water barrier – The water barrier classification shall be neither high nor unclassified. The resistance to water penetration shall be determined, as required, by the method described in AS/NZS 4201.4. as follows:

(a) High The material shall only be classified as high if it passes the test,

(b) Unclassified When the material is not classified as high, it shall be unclassified.

In terms of AS4200.1 : 1994 classification, if a datasheet is reporting a product to be “unclassified”, it means it has not passed the water barrier test which is a simple 10cm standing head of water test.

The current version of AS4200.1: 2018 is more helpful to the user and the product will now be defined as:

“Water Barrier – Non-water barrier”

5.3.5 Water control classification

The water control classifications shall be determined as follows;

(a) Water barrier – if the membrane passes the test specified as AS/NZS4201.4.

(b) Non-water barrier – if the membrane fails the test specified in or has not been tested to AS/NZS 4201.4.

How then have things changed for classification of vapor permeance?

Vapour Control Requirements

In the 1994 version of the code there was no classification of vapour permeance. Products were either a low, medium or high classification as a vapour barrier.  Just because a material wasn’t a particularly fantastic vapour barrier does not imply that it is sufficiently vapour permeable.

The 2019 NCC Condensation Management DTS requirements for climate zones 6,7 & 8 now requires a vapour permeable membrane, In the updated 2017 version of A4200.1 vapour barrier and vapour permeable membranes using table 4.  See below.  (Southern Australia and Alpine areas)

We should only be seriously using class 4 products with a vapour permeance of >1.14µg/N.s  Class 3 seems like a bit of a holdover from the old building code to cover the old “perforated” foil wraps Perforated sarking).

When considering that a typical paper faced plasterboard has a permeance of around 2.0µg/N.s  then it is a good rule of thumb to ensure the external wrap is at least as permeable (>2.0µg/N.s) as the interior plasterboard lining. In order to control moisture successfully inside a wall system the vapour permeability of materials is increasingly vapour permeable toward the outside of the wall system.

TABLE 4

VAPOUR CONTROL MEMBRANE (VCM) CLASSIFICATION

Vapour permeance (see Note)

Class	VCM Category	Min. (μg/N.s)	Max. (μg/N.s)
Class 1	Vapour Barrier	0.0000	0.0022
Class 2		0.0022	0.1429
Class 3	Vapour Permeable	0.1429	1.1403
Class 4		1.1403	No max.
	ASTM-E96 Method B Wet Cup - 28 C 50%RH

NOTE: Vapour permeance is the inverse of vapour resistance. It shall be calculated as follows:

Vapour permeance μg/N.s = 1/(Vapour resistance MN.s/g)

Almost all building wraps are technically combustible, but the caloric value (the heat produced by the complete combustion) of wraps is quite small compared to the volume of materials that are non-combustible in a wall system.

According to NCC2019 building wraps/membranes that are less than 1mm thickness with a flammability index of no greater than 5 can be used wherever a non-combustible material is required.  This means you can now have your cake and eat it.  For applications using NCC 2016, in most cases, your only solution is to discuss with your fire engineer about a performance solution.

Using Vapour Permeable Wraps at the Bottom of wall detailed with air tightness.

Getting an airtight barrier at the bottom of a brick veneer wall system using damp-course as your air barrier, with an overlap from a vapour permeable wrap. If you have any questions, please contact us.

1.  What capability of duct air tightness testing can Efficiency Matrix do?

Testing capabilities for builder’s risers (speedpanel/precast/blockwork/riser liner), plenums or sheet metal ducts.

250Pa – 3500l/s leakage

500Pa – 2500l/s leakage

1000Pa – 750 l/s leakage

2000Pa – 280l/s leakage

3000Pa – 100l/s leakage

Check out a video we did on Blockwork air tightness

2.  Why is ventilation compliance not being met across the industry?

The core issue isn’t always compliance—it’s performance. The better question is: “Why are systems not operating as designed?”

Historically, compliance has been based on “deemed-to-satisfy” approaches, where manufacturers and installers follow prescribed methods, not necessarily outcome-based performance testing. In the past, only special buildings may prescribe pressure testing of ductwork by the mechanical consultants.

National Construction Code (NCC) 2019 Vol 1 J5.6 states:

Ductwork Sealing

Ductwork in an air-conditioning system with a capacity of 3000 L/s or greater, not located within the only or last room served by the system, must be sealed against air loss in accordance with the duct sealing requirements of AS 4254.1 and 4254.2 for the static pressure in the system.(i) apply to ductwork located within the only or last room served by the system; and

It is unclear if this clause includes the air leakage testing requirements of AS 4254.2 for all HVAC ducts. The NCC 2019 Guide to Building Code of Australia (BCA) Vol 1 come to the rescue.

The guide explicitly said the leakage test clause 2.2.4 of AS 4254.2 is part of the requirement.

3. Is poor duct sealing common, and where is it typically found?

Poor sealing is common in all types of ducts. Usually the larger the duct size, the more problems we expect to find. Our experience shows that most ductwork we have tested leaks between 8% to 18%, with some extreme cases leaking 35%+ in the initial test. It is also worth to note that these values are obtained from pure random sampling of ducts to be tested on the day or during a feasibility study for energy efficiency retrofit works. Advanced notice on which sections of ducts will be tested, generally yield better performance as contractors usually put in extra effort to seal sections of sample ductwork.

4. Why are ducts not being effectively sealed? Is it skills, knowledge of the issue or care?

There are lots of factors contributing to the poor sealing of ducts. Usually, the main issues are related to limited accessible workspace around ducts. Problems occur when ducts are installed too close to the underside of the slab or structure, which restricts the ability to install cleats near the centre of the duct joints. In some cases, the installer can put the cleat in place with telescopic tools, but accessibility issues disallow an inspection to understand how well the cleat is holding the joint together.

The other common issue is, not having enough room to install the custom pieces after all the standard sections have been installed, connecting all the ductwork to the riser. Contactors have to push the already installed ducts to the sides while slotting the custom piece in. This installation method can easily create situations where the foam seal on one side of the duct is “rubbed” off or rolled up. As a result, the seal ends up being not effective. Not enough margin/tolerance ends up leading to a situation where the contractor has to ’jam in’ duct pieces.

When ducts are in a subfloor, duct are damaged by trades walking on them.  Improper transport and storing the duct usually ends up bending the corners of the joining rims or in some cases one side of the rim is deformed. These can easily create gaps that cannot be properly sealed by the typical foam seal used. In one instance, a gap caused by such deformation leaked 6% of the design air flow at test pressure.

5. Is the lack of enforcement contributing to poor practices?

The more likely issue is that the contractor did not receive feedback on non-optimum practices, thereby unknowingly carrying on the poor practices. The other reason is that the industry focuses on the volume of air being delivered at various outlets as a performance indicator for ducts. As long as the HVAC contractor meets the air delivery specified, it’s a job well done, regardless of the level of leakage. Sometimes the slight oversized air handler (safety margin) also reduces to incentive to improve duct sealing as the installers knows there is going to be enough slack.

6. What does the NCC require, and how is duct sealing measured?

NCC refers to the AS 4254.2 2012 for rigid duct sealing. There is some confusion because it says that ducts in the very last room being served don’t have to be sealed. This made it unclear whether any duct pressure testing was required.

In addition, there are a few issues regarding details of the duct leakage test no being defined clearly.

AS 4254.2 2012 2.2.4

“Duct systems with a capacity of 3000 L/s or greater shall be tested for air leakage at a static pressure of a minimum of 1.25 times the calculated design operating pressure in the tested duct section. Leakage shall not exceed 5% of the design air quantity for the duct system.”

AS4254 calls for type-testing of at least10% of the system, including longitudinal seams, circumferential joints, floor distribution, riser and plant room duct, and each type of seam, joint and sealing construction. The standard does not indicate whether the 10% relates to total duct length, surface area or length to seams but overseas standards use duct surface area. Designers and contractors should agree as to which sections are to be tested having regard to design intent and practicability on site.

The standard does not define how to calculate the operating pressure of the test duct section. Another key issue is how to apportion the 5% system wide leakage allowance to the sample test duct sections. It is not a problem if the entire system can be tested at one go. In larger system where 1 AHU serving a number of floors, there is no guidance for the tester to divide the leakages to various sections of ducts. For example, a system serving 3 floors. It is logical to break up to test into 4-5 parts: Ducts in the plantroom (directly connected to AHU and Riser), ducts in riser, level 1 ducts, level 2 ducts, level 3 ducts.

They all can have different operating pressure due to the frictional loss along ducts and the number of outlets on each floor.

One possible method to address this is the arithmetic averaging of duct pressure and leakage allocation by surface area. This is simple method, though often underestimates the leakage in the plantroom and risers while overestimating at floor levels past the variable air volume (VAV boxes).

The other method you could use is pressure adjusted area weighting, where the operating pressure difference between various parts of the duct systems are taken into account. To use this method, a ‘pressure map’ along the duct system needs to be provided to the tester.

7. How should duct be sealed? What are the best methods?

The conventional method of combining foam seals in transverse joints and mastics can effectively seal the ducts. However, the inspection and verification of seal being applied can cause issues. Visual inspection can only do so much, especially when space is limited or when the seal is covered by other materials such as insulted ducts or attenuators. Pressure testing ducts can reveal the issues and pinpoint the leaks with the help of tracer smoke. In some cases, traditional method cannot be applied, such as sealed masonry risers. A more advanced solution is automated aerosol-based sealing, which internally pressurizes the duct and seals leaks from the inside.

8. What are the implications of leaking ducts (energy loss, exhaust air re-entering supply air duct)?

Despite the view expressed in AS 4254:2002, a few simple calculations suggest that there is reason for concern about the impact of duct leakage.  Consider a typical air conditioning system in which the designer follows AIRAH DA09 [4] and assumes a supply duct leakage rate of 5%. To deliver the design air quantities to the spaces served, the fan must handle 1/0.95 times the sum of the room air quantities or 105.3% of the nominal air flow. Applying fan laws gives an increase in fan power of 117%, so the widely accepted leakage rate of 5% has added 17% to supply fan energy, for every hour the plant operates. At 10% leakage the extra fan energy is 37%.

But that’s just the beginning—duct leakage also affects the energy used by heating and cooling systems. The impact depends on where the ducts are located:

• If the ducts are within the conditioned space, low leakage might not be a major issue because some lost air still contributes to heating or cooling.

• If the ducts are in a return air plenum or unconditioned space, the leaked air offers no useful benefit. It simply circulates inefficiently, wasting fan energy and reducing return air temperature.

• If the supply ducts are outside the conditioned space, like in a ventilated roof cavity, leakage results in direct energy loss. In this case, the 17% fan energy increase is compounded by additional losses in heating and cooling—and increased greenhouse gas emissions.

The analysis for return air ducts also depends on where the return air duct is located. If the duct is in the conditioned space, leakage has little or no effect since the air leaking into the duct is the air that would have been returned anyway. If the return air duct is outside the conditioned space, the effect is more pronounced.

For example, let’s say the system uses 15% outside air. If there’s a 5% leakage rate in the return ducts (from outside), this effectively increases outside air intake to 19.3%—a 28% rise in outside air load. For a typical Sydney cooling system, this could add 5% to peak cooling demand. It will have less effect on a VAV system with an economy cycle but more on a constant volume system with a lower percentage of outside air.

In summary a 5% leakage rate can contribute to the following:

• +17% fan energy use (supply side)
• +5% heating/cooling loss (supply ducts in unconditioned space)
• +5% extra load from return duct leakage pulling in outside air
  → Combined, this can easily add 10–15% to HVAC energy use and greenhouse gas emissions, depending on system type.

We do not have published data for the effect of duct leakage in Australian systems but there have been of a number of overseas studies dealing with the issue. One [5] estimated the heating energy wasted by duct leakage in Belgium at 15 GW.h (0.054 PJ) per annum and 0.75 TW.h ((2.7 PJ) per annum for the rest of Europe. Another study of VAV systems in large commercial buildings in California [6] calculated that, compared to “tight” duct systems (2.5% leakage), systems with 10% leakage had annual HVAC system operating costs 9 to 18% higher, while those with 5% leakage used 2 to 5% more energy.

Additional wastage occurs if supply and return air risers are collocated in the same structural riser shaft. This can cause short-circuiting, direct energy wastage and danger to HVAC control due to the miss matching of room temperature and return air temperature. The room will continue to call for cooling, but the return air sensor will be telling the chiller to reduce output.

Other symptoms such as poor distribution of air, uneven temperature across floor, and increased occupant complains are all related to leaky ducts.

9. How big an impact can this have on performance and longevity of the duct?

In humid areas, the leaks in supply ducts can result in condensation which accelerates the deterioration of the sheet metal. The same applies to kitchen exhausts where the humid air leaks to cool spaces inside or outside of the building.

10. How difficult and costly is retrofitting leaky ducts?

11. How can the industry improve its performance in this area?

• Mandatory pressure testing

• Feedback loops to installers

• Better off-site duct fabrication

• Improved on-site QA

• Innovative joint design

• Adoption of aerosolized internal sealing methods

Building performance is the single most important thing for ALL buildings.

With poor building envelope performance, comes higher energy consumption. Simply installing more efficient heating and cooling systems does not address the root cause of energy inefficiency.

Air tightness impacts three out of the six key criteria in the ASHRAE performance measurement protocols, highlighting its importance for:

• Indoor air quality
• Occupant comfort
• Energy efficiency
• Building longevity

To illustrate, Building Performance comes from achieving the following:

• Construction air tightness
• Consistent insulation (with no thermal bridging and high build quality)
• Energy or heat recovery ventilation systems (ERV/HRV)
• Well-designed HVAC systems that are effectively integrated with the building envelope, including dampers located close to the building envelope.

Air tightness effects on a building

Building performance is difficult to model precisely, but the benefits of a high-performance building can be measured and observed in real-world scenarios. Accurate computer modelling is often limited due to complex, unpredictable physical phenomena, including:

• Stack effect:
  Taller buildings often experience greater performance penalties depending on the vertical connection between floors, the building’s height, and climate conditions. The location of leaks (top vs. bottom) influences how much conditioned air is lost through the chimney effect. In warm climates, the reverse stack effect can occur.

• Wind effect:
  Air leakage caused by wind pressure and wind tunnels can severely degrade insulation performance through a process known as wind washing.

• HVAC imbalance:
  Changes made to meet occupant needs during the building’s lifespan can result in imbalances that amplify stack and wind effects.

These dynamic effects make it extremely difficult to model building performance reliably using software alone.

Efficiency Matrix has experience in some of the largest commercial buildings in Australia and abroad. We specialise in build quality, during both design and construction, to ensure issues are addressed at the most cost-effective time.

We pride ourselves on getting results for our customers. Contact us for our capability statement. Visit other pages on our website regarding building airtightness and insulation consistency.

Getting results

Important tasks for enabling good building performance:

• Building airtightness testing (blower door testing)
• Review of the building design.
• Constant building inspection for air barrier continuity and insulation consistency.

Presentation on Building performance

Why targeting ACH is not quite right

Using Air Changes per Hour (ACH) at 50Pa or even at ambient pressures like 4Pa — which are volume-based measurements — rather than a permeability rate (which is surface area-based) misrepresents how leaky a commercial or industrial facility really is. When ACH is used as the metric, larger enclosures will appear to perform better simply due to their size — not necessarily because of better build quality. This means that a consistent standard leakage rate cannot be reliably specified when ACH is the target.  Compare the ACH for these 3 examples.

A standard single-storey house

Floor Area: 125m2

Volume: 300m3

Surface area: 389.5m2

A building at 5 m3/h/m2@50Pa would be 6.5ACH@50Pa

OR

A standard single-storey Passive house

Floor Area: 125m2

Volume: 300m3

Surface area: 389.5m2

A building that achieves passive house .6ACH@50Pa needs to achieve .462 m3/h/m2@50Pa 

Smaller volumes or components of buildings trying to achieve .6ACH@50Pa can be next to IMPOSSIBLE to achieve when services penetrations and windows are a part of the building fabric.

Not much of a problem for now, the permeability rate and air change rate are very similar, due to these types of buildings usually being quite small. Volume and Surface area numbers are largely similar.  (However, very large homes, can enjoy the benefits of ACH)

Small Commercial BuildingVolume: 20m x 20m x 12m = 4800m3

Surface area: 1760m2

A building at 5 m3/h/m2@50Pa would be 1.83 ACH@50Pa

Identical Build quality as the home but it enjoys over THREE times lower ACH Rate

Large Commercial Building

20m x 20m x 260m = 104,000m3

Surface area: 21600m

A building at 5 m3/h/m2@50Pa would be 1.03ACH@50Pa

Identical Build quality as the home but it enjoys a FIVE times lower ACH Rate.  This building could be Passive house air tight if passive house didn’t have the rule that buildings larger than 4000m3 in volume need to use a permeability rate.

Using ACH on Commercial buildings, Oil & Gas rig refuges or any type of large enclosure larger than 3000m3 in volume, means the larger the enclosure, the easier it is to pass a particular target inadvertently.  For build quality, a permeability rate which uses the surface area of the building, requires a target based on air flow through a square meter of building envelope with simulated airspeeds of 32km/h winds.  This standard methodology ensures results are meaningful, reproducible, and comparable across buildings of all sizes.

Find out more about the Building Code of Australia and its guidelines here.

BCA Infiltration Requirements

The Building Code of Australia (BCA) specifies the following infiltration rates for energy modelling:

• (aa) For a perimeter zone with a depth equal to the floor-to-ceiling height, when the pressurisation plant is operating: 1.0 ACH

• (bb) For the entire building when the pressurisation plant is not operating: 1.5 ACH

These two clauses are highly ambiguous and leave room for interpretation. Let’s unpack some key oversights.

Please note that some of these effects are almost impossible to model, which throws a massive spanner in the works. EM0017 Revision 1 attempts to address this gap by deriving a permeability rate for commercial buildings using the ATTMA blower door testing methodology.

Not taken into consideration in the BCA, our calculator attempts to quantify this.

Not addressed in the BCA and not reflected in our calculator.  Quite difficult to accommodate, due to the lack of understanding of how big the hole is at the top and bottom of the building, as well as in between floors.

Not addressed in the BCA, but if duct work is done to Australian standards, EM0017 will be grown to include HVAC Leakage energy losses simplistically.

Air leakage in the building envelope can get extremely complicated depending on design, environmental conditions outside, build quality inside, and HVAC duct air tightness.  It’s a complex beast, and some serious energy savings can be realised, from focusing on improving the performance of a building envelope and how airtight the HVAC system connects to the building envelope.

To improve this lack of information in the BCA, some ESD consultants have attempted to improve specifics by including additional information into their specification which references ATTMA as a testing guide.  It does not help to add detail because in their specification they still reference unrelated measuring units with 1.5ACH(ACH) @ Ambient (plant not in operation) and 1.0ACH on perimeter zones with plant running.

As a comparison

• ATTMA: Requires all HVAC systems to be turned off. All vents and intakes must be sealed to isolate the building envelope. The permeability rate is tested at a pressure differential of 50 Pa.

• BCA: Refers to ACH at ambient conditions with plant off—but with openings exposed to external weather pressures.

Why ACH @50Pa Fails for Large Buildings

There is a good reason why ACH@50Pa is not used to understand air leakage in commercial buildings in ATTMA.  As the size of the building increase, the volume increases faster than the surface area. Two buildings with identical permeability rates and construction could show vastly different ACH values. In a residential setting ACH can work quite well, but it’s still not as accurate as a permeability rate (m³/h/m² @ 50 Pa) when it comes to comparing apples to apples.  Because the BCA uses ACH@ambient as its reference, you can’t reliably translate it into the leakage rate that ATTMA uses. In fact, using ACH at ambient pressure to derive a permeability rate becomes increasingly unreliable for buildings over 30,000 m³, which could equate to a leakage area of 12 m².

Building Name	Width Length	Number Floors	Volume	Perimeter Volume Zones	Envelope Surface Areas	After Hours Permeability m3/h/m2@50Pa	Plant operation permeability m3/h/m2@50Pa	Size of Hole in the building using ELA 4Pa
Small building	20	3	4800	3648	1760	14.6	16.8	.54 m2 Hole
Typical Suburban Office	50	4	40000	18832	8200	30.8	18.6	3.2 m2 Hole
CBD Office	50	20	200000	65936	21000	54.8	25.4	11.2 m2 Hole
Melbourne Central	55	58	701800	198148	57090	70.1	28.1	33.767 m2 Hole

Note the difference in permeability rate with plant running and not in operation.

Here is an excerpt of a specification used in Australia that uses BCA in conjunction with ATTMA as a guide.

Building Sealing

All heating and cooled spaces (other than spaces defined by BCA Provision J3.1) shall be sealed in accordance with the requirements of the current version of the BCA Part J3 ‘Building Sealing’ and to the degree necessary to reduce air leakage through the building envelope to a rate of: 

• 1.0 air change per hour (AC/hr) for perimeter zones of depth equal to the floor-to ceiling height when pressurising plant is operating; and
• 1.5 AC/hr for the whole building when pressurizing plant is not operating. 

The term ‘building envelope’ in this context shall be as defined by the BCA: ‘The parts of a building’s fabric that separate a conditioned space or habitable room from the exterior of the building or from a non-conditioned space.’

All sealed buildings shall be suitably pressure tested to adequately prove performance in accordance with BCA Part J3 ‘Building Sealing.’  Guidance on an appropriate procedure for determining building sealing effectiveness is provided in the Air tightness Testing and Measurement Association Technical standard L2 – Measuring air permeability of building envelopes, (Non-Dwellings) Oct 2010 Issue. (Pressure testing for the building sealing shall generally be carried out irrespective of building size. 

Using wind only Efficiency Matrix EM0017 Revision 1, attempts to predict a permeability rate. To determine the pressure differential between inside and outside we use a default of 4Pa, the standard adopted in Europe. In some locations of Australia higher winds may be present which means that this number should be revised upwards.  Understanding the pressure with which an HVAC system operates at is a tough question also, we have assumed a positive pressure of 2Pa to prevent infiltration.  These values can be adjusted for local wind conditions and building characteristics.

Problems with BCA Requirements

1.0 ACH for perimeter zones with pressurising plant in operation.  

1. What pressure does the HVAC operate with?  It is undefined.
2. What is the ambient pressure?  Varies with building height, external temperatures, and wind the structure is exposed to.
3. 21 degrees can be assumed for internal temperature.

1.5 ACH for the whole building when pressurizing plant is not in operation

1. Are HVAC dampers still open to outside?  Or is it ACH@ambient with the building prepared to what ATTMA requires?
2. What is the ambient pressure? Varies with building height, external temperatures, and wind the structure is exposed to.
3. 21 degrees can be assumed for internal temperature.

Let’s look at Section J

Section J says that you need to weatherproof your building everywhere other than in the Northern Part of Australia so climatic zones 1, 2 and 3 are excluded.  Here is the excerpt…

• Roofs, ceilings, walls, floors and any opening such as a window frame, door frame, roof light frame(skylight) or the like must be constructed to minimise air leakage in accordance with (b) when forming part of –
  1. The envelope; or
  2. The external fabric of a habitable room or a public area in climate zones 4, 5, 6, 7 or 8.
• Construction required by (a) must be –
  1. Enclosed by internal lining systems that are close-fitting at ceiling, wall and floor functions; or
  2. Sealed by caulking, skirting, architraves, cornices or the like.
• The requirements of (a) do not apply to openings, grilles or the like required for smoke hazard management.

Definition of close-fitting

“Closely constrained or constricting—designed to narrow gaps or resist airflow.”

Joins should be tightly abutted; if not feasible, constrictive sealing materials should be used.

Commonly asked questions…

1. Is there a way to test for air leakage which provides a result in ACH, instead of air permeability (ATTMA) in m³.h^-1.m^-2@ 50Pa?

ATTMA is just a guideline, and the BCA refers only to ACH, not an air permeability rate.  They are not compatible with each other.

2. If we do follow the ATTMA guideline, can one convert AC/hr (ACH@Ambient) to a permeability rate target?

Taking the wind into consideration, EM0017 tries to do just that…  At the bottom of this page, there is a link to the calculator.

3. Should a building be tested by each level separately instead of doing the whole thing?

No, pressure testing with blower door equipment should encompass a whole building, not just a single level.

As it stands, BCA compliance could theoretically be demonstrated using tracer gas. However, the outcome will heavily depend on the weather conditions on the test day.

For any architects out there, if you need a commercial air tightness specification, don’t hesitate to contact us at qu****@**************ix.com

Click here to use our BCA permeability conversion calculator.

Note: Our calculator does not yet factor in stack effect from tall buildings or HVAC losses. These may be included in future updates in a simplified form.

Authored by Joseph Chun Kit & John Konstantakopoulos