Ineffective and effective Air tightness testing specifications
Being a leading air tightness tester in Australia/Asia, we have the pleasure of seeing some exciting and strange air tightness testing specifications/targets. Their implementation was in good thought (no doubt), but the effectiveness of getting the builder to build an airtight building may not be so practical, because there are too many interpretations that can circumvent the original intent. Or it can be so hard, that everyone kills themselves trying to achieve something that may be impossible to achieve.
Defence and Commercial buildings, sometimes receive this clause in their specification:
All heated and cooled spaces (other than spaces defined by BCA Provision J3.1) shall be sealed in accordance with the requirements of the current edition of the BCA part J3 ‘Building Sealing’ and to the degree necessary to reduce air leakage through the building envelope to a rate of:
- air change per hour (AC/hr) for perimeter zones of depth equal to the floor-t0-ceiling height when pressurising plant is operating; and
- AC/hr for the whole building when pressurising 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 (ATTMA) Technical Standard L2 – Measuring Air Permeability if Building Envelopes, (Non Dwellings) Oct 2010 issue.
We see this one quite often, and it’s important not to compare BCA(2012) in its current form to ATTMA Technical Standard L2, because there is no similarity. They are not complimentary.
For starters, ATTMA Technical Standard L2
- does not allow one to derive a leakage rate in ACH at an ambient pressure
- nor does it allow testing of leakage with an elevated pressure from a HVAC system in operation and
- it also does not Allow ACH@50Pa for commercial buildings, which makes things even worst.
For commercial and industrial buildings a permeability rate is the only acceptable, reproducible and comparable leakage rate to be measured, using building envelope surface areas. See our article on volume vs surface areas.
By using BCA modelling ACH@Ambient or ACH with HVAC in operation, the following questions must be answered. (At what elevated pressure does a HVAC system operate at? How airtight is the ductwork? How tall is the building? Where are the holes located, where they may contribute significantly to stack effect? This type of spec has many questions to be answered, and a result using ATTMA blower door air tightness testing can easily be manipulated to create a pass, no matter how leaky the building is. Here is a an article on the subject and a calculator, that attempts to convert BCA to a permeability leakage rate which ATTMA talks about.
ATTMA uses a permeability rate to understand construction quality # m3/h/m2@50Pa, not an ACH modelled leakage rate from the BCA.
Moving on from this, we get many customers asking about compartment testing. It is important that if you are testing a large commercial building that you seek the expertise of a qualified Level 2 ATTMA tester. Compartment testing should ‘NOT’ be an option, and hiring an ATTMA Certified level 1 air tightness tester to undertake compartment testing is not an effective solution to your air tightness testing requirements. These Level 1 testers are only capable of advising and testing buildings smaller than 4000m3 in volume. They can only conduct air tightness testing with a single blower door fan.
Compartment testing Problems
- Offers no value for building performance to customers/building owners/builders. Leakage is detected, but no one knows whether this air is going outside or inside through non-air-tight internal walls.
- Qualitative air tightness testing only. Not Quantitative testing. Should always be combined with smoke testing.
- Internal leakage could be severe; the reported leakage rate could be horrific.
- ATTMA Level 2 Air tightness testers should NOT allow a builder or building owner to convince them to undertake compartment testing, and if they are, it should clearly be documented what areas ware tested, and they should only use the surface areas connected to the outside to distribute the air flow that was measured in the compartment. To improve the result, builders need to spend more money on labour and materials to make internal walls and floors more airtight (They are handicapped by a small externally connected surface area.)
- Due to small areas being tested with compartment testing, a very low percentage of the façade/roof is normally tested which does not comply with ATTMA L2
- The Builder learns nothing from compartment testing, other than airtightness is too hard and not achievable.
In 2012 there was Clarification for the Australian Defence Force Buildings which sorted out the BCA confusion in their original specification:
“Clause 3.4.1 Building Sealing
All heated and/or cooled spaces shall be sealed in accordance with the
requirements of the current edition of the BCA part “Building Sealing” and to the
degree necessary to reduce infiltration and/or exfiltration through the building
fabric to an air leakage rate of not more than 3 m3h/m2 of surface area including the floor surface area, at a static pressure of 50Pa.
Here is another interesting specification…
Equipment Requirement
Use fan system capable of raising room pressure to 200Pa (using VSD or damper control suitable for a pressure test
Differential pressure measuring instrument and tubing to measure room pressure to ambient condition. Smoke pencils and smoke generator to identify leaks during pressure test
Test Procedures
Implement the following test procedures based on CIBSE TM23 Air Leakage testing:
- Seal the supply and return air duct paths to/from the room
- Seal door openings but allow for a fan supply connection
- Seal joints between floor, walls and air grills. This includes all power outlets
- Run fan and raise the pressure of room to 100Pa (concerning ambient space outside of the room)
- If unable to achieve this pressure, then room has significant leaks. Investigate by using smoke pen/generator to identify and seal
- Once the 100Pa is achieved, switch fan OFF and shut dampers from fan to seal the room
- Record the time taken for pressure to drop from 100Pa to 60Pa. if less than 7 minutes then room seal test has failed
This is a serious procedure. It is an oldie but its also a goodie, I’m not entirely sure what the target might be, but for a building to hold onto 60Pa from 100Pa for a period of 7 minutes, you are talking about some amazing building airtightness. You may need to coat the whole building in polyurethane caulk. I’m not even sure if this procedure is physically possible to construct a building that could allow a pass. We may need to pass this procedure onto the Myth Busters, would make for great TV.
From time to time we see leakage rates specified that are confusing m3/h with l/s. ATTMA has recommended leakage rates for different types of buildings, but if m3/h is interchanged with l/s you allow a permeability rate that is substantially leakier. In fact, 3.6 times greater.
Building envelopes are measured in m3/h, ductwork air flows and façade lab tests are measured in l/s. It’s important we don’t get them confused.
Oil/Gas Rig Temporary Refuges(TR) or other buildings that Specify ACH@50Pa airtightness.
Appendix A in ATTMA on how to calculate ‘air change rate’ from measured values, specifically clause A.1.8.
“The Guidance on integrity testing for offshore installation of Temporary refuges” states that oil & gas rigs must target an ACH@50Pa for their accomoidation TR’s. In order to protect the Volume of air environment inside.
The purpose of the pressurization test is to determine the air leakage rate from the TR. This is usually converted to an air change rate measured in the non-SI units of ‘air changes per hour’ or ac/h. This measure of volume flow rate is widely accepted in the offshore industry and is more convenient than using the SI equivelant of m3/s, as in using ac/h the volume flow rate normalised by the volume of the TR. This allows the air tightness performance of different TRs with different volumes to be compared directly using the same variable.
HSE Offshore information sheet 1/2006 states that the air leakage rate is usually taken as .35ACH@50Pa, but that new build TR’s are constructed to a leakage standard of .25ACH@50Pa. However, there are many reasons why a duty holder may specify a different air leakage rate limit. For example, an accomodation platform with no production or processing functions is unlikely to be subject to high concentrations of gas or smoke products in the event of an incident, hence a larger air leakage rate may be appropriate in this case. Conversely, TR’s located close to production and process areas are more likely to be exposed to high concentrations of flammable toxic atmospheres and may require a lower limits on acceptable air leakage.
| Type of Building | Air Permeability m3/h/m2 @ 50Pa | Air Permeability m3/h/m2 @ 50Pa | Air Change Rate (ACH) @ 50Pa | Air Permeability m3/h/m2 @ 50Pa |
|---|---|---|---|---|
| Best Practice | Good | Reality | ||
| Offices | N/A | TBA | ||
| Naturally Ventilated | 3 | 7 | N/A | TBA |
| Mixed | 2.5 | 5 | N/A | TBA |
| Air Conditioned/Low Energy | 2 | 5 | N/A | TBA |
| Factories/Warehouses | 2 | 6 | N/A | TBA |
| Superstores | 1 | 5 | N/A | TBA |
| Schools | 3 | 9 | N/A | TBA |
| Hospitals | 5 | 9 | N/A | TBA |
| Museums, galleries and archival stores | 1 | 1.5 | N/A | TBA |
| Cold Stores | .2 | .35 | N/A | TBA |
| Passive House Standard | N/A | <1 | .6 | TBA |
| Passive House Retrofit | N/A | <1 | 1 | TBA |
| Oil and Gas EXISTING | N/A | N/A | .35 | TBA |
| Oil and gas NEW | N/A | N/A | .25 | TBA |
Refer to the article comparing ACH to Surface areas/permeability rate for large buildings.
Larger enclosures will be easier to pass, with oil and gas TR facilities.
Understanding ACH from an ACH@50Pa
CIBSE TM23 Explicitly states that the “divide by 20” method is for Dwellings (e.g. Residential homes) which are all very similar in size (see excerpt below). The applicability of this figure to commercial projects or facilities in the middle of an ocean is HIGHLY questionable considering the volume to surface area ratio alone. The average tall building or oil rig refuge in the middle of an ocean will be exposed to much higher winds than 4Pa.
6 Air leakage tests and infiltration rate
An air leakage test does not provide a measure of the air infiltration rate in a building, and therfore it cannot be used to estimate directly the infiltration heat loss. The test pressure of 50Pa is much higher than the pressure differences that drive infiltration due to eather conditions. A calculation can be carried out to relate the air leakage at 50Pa, say, to the air infiltration rate, but this will require some knowledge of the location and nature of the air leakage paths. If a direct measure of air infiltration is required, it involves a lengthy and complex test using tracer gases.
From a large number of measurements carried out on dweellings (and usually of similar voumes) it has been found that the air infiltration rate in air changes per hour (ACH) is approximately 1/20 of the 50Pa air leakage rate(expressed as air changes per hour rather than fabric air leakage index). The air leakage rate is defined as Q50/V, where Q50 is the leakage air flow rate at 50Pa, and V is the internal volume surrounded by the building envelope.
https://www.airah.org.au/Content_Files/EcoLibrium/2012/March%202012/2012_03_01.pdf
Infi
ltration LBL 4Pa Correction Leakage rate (ACH) <.1 ACH
-
- Average wind speeds in some oceans/land locations can be as high as 17.9km/h, which is equivalent to 4.972m/s which is an average ~15.45Pa applied wind pressure
- This criterion is more designed for residential land-based leakage rate. It is not a good idea to apply this leakage rate to a huge oil rig or a tall building, it’s not what these calculations were designed for.
Controlling the environment from outside, coming inside is of paramount importance for the safety of its personnel in the event of a gas leak or a massive disaster where smoke may engulf the whole facility.
Refer to excerpt
UNCERTAINTIES
In any qualitatively simple model such as the one developed above, the uncertainties will be large. They will be both systematic and random, as well as being hard to estimate. The size of the correction factors give an indication of some of the uncertainties (i.e., 85 10% -40%) depending on the building, in addition to the uncertainties from the LBL model itself. A good error analysis would involve a detailed simulation effort to model the distribution of buildings, climates, and other relevant factors. It should also include a large set of measured vs. predicted infiltration rates. Such an effort is beyond the scope of this report but as this
Model is used it may be possible to generate the necessary data for a good error analysis and validation.
CONCLUSIONS In this paper we have used a simplified, physical model of infiltration -the LBL model -to heuristically corroborate a simple infiltration estimation technique. The choice of model type and complexity is strongly a function of the application. A detailed description of how to choose the appropriate model for a specific application is beyond the scope of this report. (Currently, the Air Infiltration and Ventilation Centre is preparing a guide [13] to help users decide which kind of model to use.) However, it is clear that any time short-term or highly specific building specific estimates are required, the approach described herein is inadvisable. For large ensembles of annual averages or for rule-of-thumb estimates, the simplified method of indicators is appropriate. Of the two indicators used, one of them – the leakage indicator (ACH5o ) – is well known in the community. However, the climate indicator, No, the leakage-infiltration ratio is new. The way we have defined No is purely dependent upon the average climate at the site of interest. We have, however, defined a set of correction factors which use building- specific information to improve the estimate of infiltration. Future work should involve using field data to ascertain the accuracy of this method.
MAX H. SHERMAN
Energy Performance of Buildings Group, Indoor Environment Program, Applied Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 (U.S.A.) (Received August 6, 1986; accepted November 10, 1986; revised paper received December 12, 1986)
Air tightness testing is important, but when the building has been constructed airtight, it’s the easiest part of implementing building performance. It’s important you use a contractor who can help the building get the building to an airtight state during the construction phase. Failing a test and organising remediation works after construction is a very costly way of getting a building airtight. Having a repeatable and comparable specification is a good step in the right direction. Call Efficiency Matrix, if you need our assistance with specifications, airtightness testing and consultation during construction. We are passionate about air tightness, and empower our customers to build better. We are also the only company who has proven results from our consultancy service.
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