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Commercial air tightness

RISERS/Ductwork: Not getting the air flow you expect?

Duct Work and Riser air leaks can compromise HVAC Energy efficiency and safety/health.

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

Ductless risers and even services risers are just big rectangles inside the building envelope that can become an avenue to feed the stack effect, turning tall buildings into giant chimneys.

Poor sealing happens in all types of ducts/builders’ risers, usually the larger the duct/riser size, the more problems we expect to find.  Our experience shows that most ductwork that we have tested leaks between 8% to 18% with some extreme cases some duct systems can leak up to 35%+.  It is also worth noting that the above figures are obtained from a random sampling of ducts tested during feasibility studies on energy efficiency retrofit projects.  Advanced notice on which sections of ducts will be a part of a tested sample, generally yields better performance as contractors can inadvertently put in extra effort to seal areas of sample ductwork.

Leaky ductwork and/or builder’s risers can contribute to stair pressurization commissioning issues, which can be very time-consuming to resolve and potentially cause significant problems with a stair pressurization system functioning effectively should it be triggered for occupant evacuation on a windy day.

For buildings that have carparks below them, especially in cold climates, any risers that have connectivity to an underground car park can suck carbon monoxide straight into the conditioned space of the building above.

Energy Consumption of Leaky risers or ductwork
Despite the view expressed in AS 4254:2002, a few simple calculations suggest a reason for concern about the impact of duct leakage.  Consider a typical air conditioning system in which the designer follows and assumes a supply duct leakage rate of 5%, to deliver the designed air quantities to the spaces served, the fan must handle 1/0.95 times the sum of the room air quantities or 105% of the nominal airflow.  Applying fan laws increases fan power by 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%.  This isn’t the end of the story because air leakage also affects cooling and heating plant energy consumption.  The size of the effect depends on where the duct is located.

Suppose the duct is in a conditioned space, and the leakage percentage is low.  In that case, one might argue that nothing needs to be done, that is, that the fan can safely supply 100%, not 105.3%, of the design because the leaked air produces valuable cooling or heating inside the building envelope.  This is not the case if the duct is in a ceiling return air plenum, as the leaked air will travel around the system producing minimal useful cooling and heating effect while increasing fan power and reducing return air temperature slightly.
In a situation where the supply duct is outside the conditioned space, such as in a ventilated roof.  In that case, the assumed leakage is lost, and the 17% increase in fan power is compounded by a 5% waste in cooling and heating effect and a corresponding increase in greenhouse gas emissions.
The analysis for return air ducts also depends on where the return air duct is located.  If the duct or riser 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.  The effect is even more severe if the return air duct is outside the conditioned space.
Assume that under regular (non-economy cycle) operation, the plant handles 15% outside air, in which case return air will be 85% of the design supply air.  Leakage at the rate of 5% into the return air duct will thus be 5% of 85% or 4.3% of the design supply air.  If the air that leaks in, is from the outside of the building, it adds to the outside air load.

The outside air percentage becomes 15% + 4.3% =19.3% of the supply air.  Since the outside air load is pro rata, the outside air load increases by 4.3% / 15% = 28%.  For a typical comfort cooling plant in Sydney, 15% outside air would be about 18% of the peak cooling capacity, so the leaked outside air will add 28% * 18% = 5% to the peak cooling load.  In summary, a 5% leakage rate implies a 17% increase in fan power and fan energy on the supply side plus 5% additional cooling and heating energy if the leakage is going outside the conditioned space plus another 5% waste in heating and cooling energy on the return side if it increases the outside air percentage.  The combined effects of these will depend on the detail of the system.  It will have less effect on a VAV system with an economic cycle but more on a constant volume system with a lower percentage of outside air.  For example, it is not unreasonable that a modest 5% leakage rate could add 10 or 15% of operating energy and greenhouse gas emissions.
We do not have published data on the effect of duct leakage in Australian systems, but some overseas studies have dealt with the issue. One 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 (excluding the former Soviet Union).  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.
In humid areas, the leaks in the supply duct can result in condensation, which accelerates the deterioration of sheet metal.  The same applies to kitchen exhausts, where hot and humid air leakage moves into cool spaces.
The major problem with the performance of the duct systems comes from the air not being efficiently delivered to the occupied space.  Short-circuiting of conditioned air is another common issue in leaky ducts where supply air enters the return stream by passing the occupied zone.

What are the best methods to solve air leakage?
The conventional method of combining foam seals in transverse joints and mastics can effectively seal ducts.  The issue is more on the inspection and verification of a seal being applied consistently everywhere.  Installers use foam seals and mastics but they do not necessarily deliver sealed ductwork.

Ductless risers can be quite challenging to troubleshoot, Efficiency Matrix has mining grade cameras for detailed visual inspections of builders risers to ensure no large holes are present, but to also audit the application of sealants to joins, and inspecting bracing for high pressure ductless riser systems.

 

Visual inspections can only do so much, especially when space is limited(Ducts installed close to the soffit) or when the seal/joins are covered by other materials, such as duct lagging or even other ducts or vermiculite.   Pressure testing ducts can reveal the issues, but it can still be challenging to pinpoint air leaks, even with the help of a tracer gas or theatrical smoke.  Sometimes, the timing for pressure testing of ducts means traditional methods of sealing ductwork cannot be applied.  Especially riser ducts enclosed in masonry shafts, masonry or speedwall shafts, builders’ risers can be difficult to troubleshoot when they have finished off with plaster also.  Alternatively, hole sealing with Aeroseal conducted by Efficiency Matrix’s automated sealing system can be used to achieve the desired air tightness level.

Can a building or a home be too airtight?

Some builders are concerned with building homes too airtight. Some reasons they give are:

  1. Humans breathe. Your building also needs to breathe.
  2. We don’t want people suffocating in our homes/buildings. You need to get fresh air from somewhere.
  3. We don’t want to have to install a heat recovery (HRV) or energy recovery (ERV) ventilation system if we build too tight.
  4. If I am going to naturally ventilate my building, why do I need to worry about air leakage?
  5. We don’t want our homes or buildings to grow mould.

To begin with, each of these concerns arises from of lack of ventilation, not airtightness.  Air leakage is NOT ventilation. So let’s go back to basics: what is ventilation?

Ventilation is the intentional introduction of air from the outdoors into a building. It can be natural or mechanical.  In apartments, it is highly recommended that you consult a fire engineer, and consider a pressure relief strategy in the event of a fire outbreak inside the building envelope.

Natural ventilation is the flow of air through open windows, doors, vents, and other planned building envelope penetrations, and it is driven by natural forces.

Mechanical ventilation is the intentional movement of air into and out of a building using supply or exhaust fans.

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? Some things to consider:

  • 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, lots of air exchange results, but on mild, calm days, there is very little transfer.
  • 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.

Mechanical ventilation is guaranteed fresh air from a strategically-located source, no matter what the weather is.

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

Condensation in wall systems and inside internal surfaces in a building.

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.

These issues may lie undetected until it’s too late. The mould that forms inside walls, with infiltration, can distribute mould spores through your home, and you cannot clean or maintain these surfaces inside your building fabric.

For buildings that are designed for natural ventilation, the intentional 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.

Reduces the performance of your insulation system.

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 for CO or other pollutants and ants, cockroaches, spiders, mites, mice, and other unwanted guests. Sealing up these paths is the cheapest and first method of pest control.

Breathing in air from a wall?To address concerns for airtightness in homes:

  1. When someone says that a house needs to breathe, ask them to close their eyes and… breathe. When they take a deep breath in, air only enters through their mouth or nose. They don’t need air to come in any other way. The same is true for a house. Breathe only through openings made for that purpose.
  2. You cannot build a building so tight that it is dangerous when you include ventilation.
  3. For typical homes, providing basic ventilation can be inexpensive and easy. A simple continuously running remote-mounted extract fan is quiet and energy-efficient.
  4. There is no good argument for airflow through walls. Truly fresh air should be filtered, volume-controlled, and distributed by design. Getting the fresh air from a mouldy wall cavity is not healthy or reliable.
  5. To reduce condensation and mould potential, add simple continuous mechanical ventilation and pay attention to thermal bridges in the construction of the home. Condensation is a complex phenomenon; increased ventilation alone may not solve the problem.  You can experience condensation and mould on a carport soffit that has no walls.

Under Floor Air Distribution Air Tightness Testing

Under Floor Air Distribution Air Tightness Testing

Efficiency Matrix tests to BSRIA BG65-2016

Underfloor air distribution systems have become a very popular form of air distributing in new builds throughout the world.  Efficiency Matrix has some of the most advanced equipment in the world for testing, troubleshooting, and documenting issues in UFAD systems.

Large Projects completed:

  • Nishi (Canberra)  27,411 sqm Floor Area
  • 150 Collins St 20,000sqm Floor Area
  • Deakin Uni Burwood (Deakin Uni Corporate Centre)
  • 60 Cremorne st 10,000sqm Floor Area
  • Latrobe City Council Library

And many other smaller projects.

We deal with ducted and non-ducted underfloor air distribution Systems(UFAD)

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

  • software processes,
  • troubleshooting capabilities
  • Overall understanding to enable underfloor plenum installations to be done in the most cost-effective and trouble-free implementations.

We have used a wide array of troubleshooting tools and techniques using our specialised equipment

We also have a specialist process when issues are hard to identified and rectify using conventional methods, that can be used to effectively and quickly seal up air leakage.

uncaulked plaster to lagging Uncaulked plaster ufad

Air Barrier Integrity Auditing

Leak FinderWhen trying to build airtight, the real magic is NOT the air-tightness testing itself, it’s actually the Building Design, continuous QA (Air Barrier Integrity Auditing) during construction and ensuring an “Air-tightness Champion” has been appointed by the builder.

The key features of conducting air barrier integrity auditing are:

  • Locating areas of concern with exact locations of the issues.
  • Verification of building materials being used, that will enable an air barrier that lasts
  • Timely reporting of the issues, so that remediation can be undertaken before finishes
  • Capturing photographs of remediation works
  • Confirming or finding issues in building facades/junctions for a continuous air barrier using Ultrasonic technology.
  • Recommendations of remediations required, with materials/products that comply with the Australian Building code.

Building inspectioinIf 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 remediation works 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, recommended fix and dimensions of the issue, with the direction of how the air barrier should be continued.

Photo evidence

 

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

Ultrasonic Sound Survey

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.Ultra Sonic Audit

 

 

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. Because ultrasonic waves are so directional, from the emission source surveys can be undertaken on partly completed wall details.   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, during a partly constructed building, which is the most economic time to organise a solution.

Curtain wall air leak

Recommended Remediation

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

Air Barrier Integrity Auditing, is a critical part of building commissioning, combination with an Airtightness champion who is an integral part of the construction team, coupled with good building design and detailing, airtightness targets of the likes of Passive house or even more stringent can be achieved as economically as possible.

 

Vapour permeable wall wraps, the devil is in the detail

The devil is in the detail

There are wall wraps, and then there are wall wraps. 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 – Stop rain from getting into the building during construction before cladding installation – Prevent damaging to insulation/internal services and finishes that aren’t designed to be exposed to the elements. Avoiding health and structural related damage
  2. Air – Once the building is finished air can impact/exacerbate water ingress, moisture build-up from water vapour, temperature ingress, sound ingress and fire/smoke pathways.
  3. Water Vapour – Condensing water vapour and high humidity can lead to health and structural damage in completed buildings.
  4. Not to Contribute to fire combustibility – limiting the amount of combustible material in key parts of our builds.
    • Some woven glass/aluminium laminates are deemed to be non-combustible by the building code
    • vapour permeable airtight wraps and plastic/foil laminate wraps usually are of low flammability rather than being non-combustible.

Control Layers

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 NCC.

Things have become easier since the NCC 2019 which extended the list of materials that may be used wherever a non-combustible material is required or where the requirement to be non-combustible does not apply.

C1.9 Non-combustible building elements

(a) In a building required to be of Type A or B construction, the following building elements and their components must
be non-combustible:
(i) External walls and common walls, including all components incorporated in them including the facade covering,
framing and insulation.
(ii) The flooring and floor framing of lift pits.
(iii) Non-loadbearing internal walls where they are required to be fire-resisting.
(b) A shaft, being a lift, ventilating, pipe, garbage, or similar shaft that is not for the discharge of hot products of
combustion, that is non-loadbearing, must be of non-combustible construction in-
(i) a building required to be of Type A construction; and
(ii) a building required to be of Type B construction, subject to C2.10, in-
(A) a Class 2, 3 or 9 building; and
(B) a Class 5, 6, 7 or 8 building if the shaft connects more than 2 storeys.
(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) The requirements of (a) and (b) do not apply to the following:
(i) Gaskets.
(ii) Caulking.
(iii) Sealants.
(iv) Termite management systems.
(v) Glass, including laminated glass.
(vi) Thermal breaks associated with glazing systems.
(vii) Damp-proof courses.
(e) The following materials may be used wherever a non-combustible material is required:
(i) Plasterboard.
(ii) Perforated gypsum lath with a normal paper finish.
(iii) Fibrous-plaster sheet.
(iv) Fibre-reinforced cement sheeting.
(v) Pre-finished metal sheeting having a combustible surface finish not exceeding 1 mm thickness and where the
Spread-of-Flame Index of the product is not greater than 0.
(vi) Sarking-type materials that do not exceed 1 mm in thickness and have a Flammability Index not greater than
(vii) Bonded laminated materials where-
(A) each lamina, including any core, is non-combustible; and
(B) each adhesive layer does not exceed 1 mm in thickness and the total thickness of the adhesive layers

NCC 2019 Building Code of Australia – Volume One                                        Page 67

In the Latest NCC 2019 some of the key additions that will help enable the construction of walls that can effectively manage water, air, vapour and thermal control are these materials:

  • Sarking-type materials that do not exceed 1 mm in thickness and have a Flammability Index not greater than 5.
  • Gaskets
  • Caulking
  • Sealants
  • Thermal breaks associated to glazing systems.

Prior to the NCC 2019 revisions, there was a conflict between meeting an:

  • air tightness target leakage rate,
  • water barrier and
  • vapour control when only
  • non-combustible elements could be used in type A and type B wall systems.

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 outside).

Holey umbrella constructionTo get around this issue of vapour control, some aluminium foil based products in an attempt to make them vapour permeable have holes punched into them.

This is a hard balance to strike. Making holes in a material compromises its effectiveness as a water and airtight layer. There is a reason as to why its hard to find an umbrella for sale that has holes pre-punched in it.

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.  Within the space of a year we have seen perforated deemed non-combustible sarking layer where quoted vapour permeance values have swung wildly from the realistic to the implausible and back again to just mildly astonishing.

Condensation on glazingWell 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 for non-combustibility of wall elements then in most cases this will inevitably require some compromise of water tightness, air tightness or vapour permeability.  The only realistic option to date has been to insulate external the non-combustible sarking, us a non-combustible vapour permeable sheathing board, or follow a performance solution / fire engineered pathway.

Moving on…  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 or 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?

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 hangover from the old building code to cover the old “perforated” foil wraps with holes punched into them(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 are increasingly vapour permeable toward the outside of the wall system.

TABLE 4

VAPOUR CONTROL MEMBRANE (VCM) CLASSIFICATION

Vapour permeance (see Note)

μg/N.s

ClassVCM CategoryMin. (>=)Max. (<)
Class 1Vapour Barrier0.00000.0022
Class 20.00220.1429
Class 3Vapour Permeable0.14291.1403
Class 41.1403No 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 combustible, and 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.

From NCC2019 building wraps/membranes that are less than 1mm thickness with a flammability index of no greater than 5 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.

Australian Slab detail with vapour permeable wrap.

Getting an air tight 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 or about this contact please contact us.

BCA – NCC 2019 – Duct Air Leakage Testing

Frequently Asked Questions (FAQ)

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

Testing capabilities for Builders risers(speedwall/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

1.  Why is ventilation compliance not being met across the industry, and what is contributing to this non-compliance?

It is Not a matter of compliance. The right question is more like, “Why the systems are not performing as designed?”

Traditionally, the compliance issue has always been dealt with using deem to satisfied methods. As long as the duct manufacturers and installers only have to follow the prescribed methods and procedures for compliance. In the past, only special buildings may prescribe pressure testing of ductwork by the mechanical consultants.

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 include the air leakage testing requirements of AS 4254.2 for all HVAC ducts. The NCC 2019 Guide to 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.

2. Is poor duct sealing common, and where is it more likely to be found (supply air ducts, exhaust ducts etc)?

Poor sealing happens in all types of ducts, usually the larger the duct size, the more problems we expected to find. Our experience shows that most ductwork that we have tested leaks between 8% to 18% with some extreme cases, duct systems can leak 35%+ in the initial test. It is also worth to note that the above figures 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.

3. 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 assessable work space around ducts. It can be caused by design issues or site constraints. Usually problems occur when ducts are installed too close to the underside of the slab or structure, which has restricted the installers 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 when the custom section is being slotted in place. As a result, the seal ends up being not effective. Not enough margin/tolerance, ends up leading to a situation were 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 leaks 6% of the design air flow at test pressure.

4. Is a lack of compliance checking contributing to this (out of sight, out of mind)?

It is more like the contractor did not receive feedback on non-optimum practices, therefore unknowing carry on the poor practices. The other reason is that the industry focus on the volume of air being delivered at various outlets as a key performance indicator for HVAC 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 reduced to incentive to improve duct sealing as the installers knows there is going to be enough slack.

5. What do the NCC and/or standards require to achieve compliance? How is duct sealing measured?

NCC refers to the AS 4254.2 2012 for Rigid duct sealing. There was a bit of confusion if the pressure testing of ducts is compulsory under NCC when it the explanatory not on the last section of ducts within the last enclosed room do not required to be sealed.

In addition, there are a few issues regarding details of the duct leakage test are not clearly defined.

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 apportion 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 namely

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.

If we assume we simply breakup an whole system test, then we test all sections using same test pressure (we use the arithmetic average of the pressure at the start and end of the duct system to estimate the operating pressure). Then, the leakage should be apportioned based on the duct surface area.

This method is simple but tends to under estimate the leakage in the plantroom section and risers and overestimate the leaks at floor levels especially the ducts downstream of VAV boxes.

The other method is use pressure adjusted area weighted method where the operating pressure difference between various parts of the duct systems are taken into account for apportioning the 5% leakage allowance. To use this method, a ‘pressure map’ along the duct system needs to be provided to the tester.

6. 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. The issue is more on the inspection and verification of seal being applied. As the previous questions covered the theme that installers applied foam seals and mastics do not necessarily delivered sealed ducts. 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 but sometimes still hard to pin point the leaks even with the help of tracer smoke. In some cases, the timing for pressure testing of ducts means traditional method cannot be applied. Such as riser ducts enclosed in masonry shafts. Alternatively, hole seeking fully automated sealing system can be used to achieve the desired tightness level.

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

2.1 Effect of leakage on energy and greenhouse gas emissions

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%.

This is not the end of the story because leakage also affects cooling and heating plant energy consumption. The size of the effect depends on where the duct is located. If the duct is in the conditioned space and the leakage percentage low, one might argue that nothing need be done, that is, that the fan can safely supply 100%, not 105.3% of design because the leaked air produces useful cooling or heating effect. This is not the case if the duct is in a ceiling return air plenum, as the leaked air will travel around the system producing minimal useful cooling and heating effect while increasing fan power and reducing return air temperature slightly.

If the supply duct is outside the conditioned space, such as in a ventilated roof space, the assumed leakage is simply lost and the 17% increase in fan power is compounded by 5% waste in cooling and heating effect and corresponding increase in 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 serious.

Assume that under normal (non-economy cycle) operation the plant handles 15% outside air, in which case return air will be 85% of design supply air. Leakage at the rate of 5% into the return air duct will thus be 5% of 85% or 4.3% of the design supply air. If the air that leaks in is from outside the building, it adds to the outside air load, the outside air percentage becoming 15% + 4.3% =19.3% of the supply air. Since the outside air load is pro rata, the outside air load increases by 4.3% / 15% = 28%.

For a typical comfort cooling plant in Sydney, 15% outside air would be about 18% of the peak cooling capacity so the leaked outside air will add 28% * 18% = 5% to the peak cooling load.

In summary, a 5% leakage rate implies 17% increase in fan power and fan energy on the supply side plus 5% additional cooling and heating energy if the leakage is to outside the conditioned space plus another 5% waste in heating and cooling energy on the return side if it increases the outside air percentage. The combined effects of these will depend on the detail of the system. 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. For the example discussed, it is not unreasonable that a modest 5% leakage rate could add 10 or 15% to operating energy and greenhouse gas emissions.

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 (excluding the former Soviet Union). 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.

#Ecolibrium p.48 May 2013

Additional wastage in case if supply and return air risers are collocated in the same structural riser shaft. Short circuiting of conditioned air not only is direct energy wastage but it can also mess up the HVAC control due to the miss matching of room temperature and return air temperature. The thermostat in the conditioned space keep calling for more cooling while the return air temperature sensor indicating the chiller can be turn down.

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

8. How big a problem 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 exhaust where the hot humid air leaks to cool spaces inside or outside of the building.

The major problem on the performance of the duct systems comes from air not being effectively delivered to the occupied space. Short circuiting of conditioned air is another common issue in leaky ducts where supply air entered the return stream by passing the occupied zone.

9. What are the difficulties of retrofitting or repairing non-complying ductwork? How costly can be this, particularly compared to doing it correctly the first time?

https://youtu.be/diLkegkIStc

Space availability and accessability are the key problems. It is not easy to seal all seams and hole while the ducts are on the ground before installation. It is also tough to apply mastic in tight spaces afterward. In certain situations, it is virtually impossible to retrofit duct sealing using conventional methods, such as riser ducts inside speedwall risers as well as ducts above highly ornamented plaster ceiling.

Efficiency matrix has advance fully automated duct sealing system can provide cost effective solution for all duct sealing needs, sealing from the inside.

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

  • Testing
  • Feedback
  • Improve factory fabrication
  • Improve onsite touch up and quality control systems
  • Advance duct joints
  • Seal ducts from the inside, using a vapourised sealant.
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