BCA and Air Tightness using EM0017 Revision 1
How can one make sense of this mismatch? In this article, we will discuss how the form of the building and the weather conditions that the building is in, completely discredits the ability for AC/Hr(ACH) to work on commercial buildings.
The BCA requires the following infiltration rate for energy modelling…
(aa) for a perimeter zone of depth equal to the floor-to-ceiling height, when pressurising plant is operating, 1.0 air change per hour; and
(bb) for the whole building, when pressurising plant is not operating, 1.5 air change per hour
These two sentences seem to leave everything quite open to interpretation, so let’s talk about some of the things that are left out.
Please note that some of these effects are almost impossible to model, which throws a massive spanner in the works. EM0017 Revision 1 will attempt to break this down and derive a permeability rate for commercial buildings to be tested using the ATTMA methodology for testing with blower door equipment.
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 air tight 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 also references ATTMA as a guide to test. It does not help to add detail because in their specification they still reference unrelated measuring units with 1.5AC/Hr(ACH) @ Ambient (plant not in operation) and 1.0AC/hr on perimeter zones with plant running.
As a comparison
- ATTMA, requires for the plant to be turned off, Ventilation plant to be turned off, fresh air intake and ventilation taped up and sealed, so that the building envelope itself is pressurised/depressurized, to be tested using a permeability rate with an elevated pressure of 50Pa.
- BCA, talks about Plant off, and plant left open and untaped/exposed to weather pressures outside using an ACH@ambient to determine its air tightness.
Let’s review surface area to volume ratio
There is a good reason why ACH@50Pa is not used to understand air leakage in commercial buildings in ATTMA. The bigger the building, the lower the ACH, even though the construction quality (permeability rate) of these different sized buildings are the same. Proportionally a larger building has a lot more volume than surface area, as the building gets bigger. In a residential setting, ACH can work quite well, but it’s still not as accurate as a permeability rate when it comes to comparing apples to apples. Due to the BCA’s reference to ACH, there are limitations on converting this value at ambient to a permeability rate, where ATTMA TSL2 can then be used as a guide to testing Australian commercial buildings. Any building with volume larger 30000m3 will allow a building to be very leaky, which assumes a hole in the building roughly the size of 12m2 in size.
|Number Floors||Volume||Perimeter Volume Zones||Envelope Surface Areas||After Hours Permeability|
|Plant operation permeability|
|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.
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-toceiling height when pressurizaing 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 seperate 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 pressure differential of 4Pa which seems to be internationally adopted in Europe, but 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 keep out infiltration. These numbers can be adjusted in our calculator.
The problems of both these air tightness requirement for energy efficiency:
1.0 AC/hr for perimeter zones with pressurising plant in operation.
- What pressure does the HVAC operate with? How long is a piece of string?
- What is the ambient pressure? Building height, temperatures that the building is exposed to, and wind the structure is exposed to.
- 21 degrees can be assumed for internal temperature.
1.5 AC/hr for the whole building when pressurizing plant is not in operation, is the leakage of the building when the plant is off, but the dampers of the HVAC system are still open to outside? Or is it ACH@ambient with the building prepared to what ATTMA requires?
- What is the ambient pressure? Building height, temperatures that the building is exposed to, and wind the building is exposed to.
- 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 –
- The envelope; or
- 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 –
- Enclosed by internal lining systems that are close-fitting at ceiling, wall and floor functions; or
- 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 constricted or constricting
make narrower, especially by encircling pressure
A fair interpretation that can be made regarding ”close-fitting”, is that, all joins should be abutted, snugly in order to restrict air flow, if direct abutment is not possible, then something that constricts air flow should be used to restrict air flow in the “not so” “close fitting” gap.
Commonly asked questions…
- Is there a way to test for air leakage which provides a result in AC/hr(ACH), instead of air permeability (ATTMA) in m3.h-1.m-2@ 50Pa?
ATTMA is just a guideline, and the BCA refers only to AC/hr, not an air permeability rate. They are not compatible with each other.
- 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.
- 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, ACH could be proven with the use of Tracer gas, and depending on the type of day you undertake this exercise in, will determine your chances of passing the BCA requirement.
For any architects out there, if you need a commercial air tightness specification, don’t hesitate to contact us at firstname.lastname@example.org
Our calculator translation into a permeability rate does not take into consideration ACH due to stack effect of large tall commercial buildings, nor does it take into effect HVAC losses, but future revisions of this calculator may expand simplistically into these areas.
Authored by Joseph Chun Kit & John Konstantakopoulos