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| Air tightness FAQs |
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1. What is air tightness?
1. What is air tightness?Air tightness, air leakage or air permeability all refer to the leakage of ‘conditioned air’ (heated or cooled) through ‘unplanned/unintentional ventilation’ (gaps, holes, cracks, etc). By limiting the loss of this conditioned air, which requires the consumption of energy to produce and replace, it is possible to more efficiently maintain the temperatures of our buildings. Also, the infiltration of colder/hotter air from outside requires heating/cooling systems to work harder and consume more energy.
2. Why is air tightness important?Air tightness plays a significant role in the energy efficiency of buildings. Because:
By limiting the leakage of heated/conditioned air from buildings, it is possible to reduce energy consumption and costs. The government has made commitments to reduce carbon emissions through the European Energy Performance of Buildings Directive (EPBD) and the Kyoto Agreement. Part L is their method for addressing the conservation of fuel and power in buildings and as such places requirements on air tightness.
3. What impact does air tightness have on carbon emission rates calculated using SAP or SBEM?Air permeability is an important factor in assessing the overall carbon emission of a building via the appropriate calculation methodology:
Before construction: The TER is effectively the performance the building would have achieved under 2002 regulations, with a 20% improvement factor applied, in line with wider government targets for reducing carbon emissions. To achieve the TER, you may need to change the specification of products, orientation of buildings, heating system or commit to achieve air permeability lower than the basic regulatory requirement. If you want to view how you might achieve the TER using your existing methods and products you can do so at www.playtheregs.com After construction: The actual DER or BER must be lower or equal to TER. This is a regulatory requirement under Approved Document L.
4. Is air tightness testing mandatory?Yes, buildings need to demonstrate a minimum level of air permeability (10m3/h/m2) and in the majority of cases this is done through an on-site, pre-completion test. This is not an onerous requirement in itself but the design air permeability for a particular building may be lower to achieve the overall carbon emission rate for the building and/or client specification.
5. Which buildings need to be tested?
There are some exceptions to the above where buildings may be able to assume a poorer air permeability rather than undertake a test. The exceptions are explained below 1. Dwellings Refurbishments of and extensions to existing residential buildings do not require testing. Small developments (1 or 2 houses) may avoid the need to test by accepting an assumed poor value for air permeability of 15m3/h/m2 but this may add costs to other aspects of the building specification in order that the building meets overall targets for emissions. Alternatively it may be possible to re-use existing test evidence where building designs have been repeated within 12 months of conducting the original test. 2. Buildings other than dwellings ‘Small’ commercial buildings (with a floor area less than 500m2) may avoid the need to test by accepting an assumed poor value for air permeability (15m3/h/m2) but this may add costs to other aspects of the building specification in order that the building meets overall targets for emissions. 3. ‘Large’ extensions to buildings other than dwelling ‘Large’ is defined as greater than 100m2 AND greater than 25% of the useful floor area of the existing building.
6. Do I need to test every building?
For commercial developments, where testing is required, each individual building unit will need to be tested ie if the ‘unit’ is not connected to adjoining units and intended to be occupied/operated separately. However, in practice we have seen similar units being covered by a sample. On residential developments a sampling rate applies depending on:
1. Dwelling Type For housing developments, a sample of units from different ‘dwelling types’ are tested to prove compliance. Separate blocks of flats are considered as different developments. Dwelling types are defined based on: 1. Position – this is likely to be:
2. Construction type – where there is variation in floor, wall and roof construction details and/or construction method. For simplicity it is likely that this would be based on whether there is variation in those aspects covered by the Accredited Construction Details. This is likley to relate most obviously to variation in storey height ie duplex rather than single storey flats or 2.5 storey (warm roof) rather than 2 or 3 storey (cold roof). Plus whether similar units are being built using different methods ie traditional/masonry, timber frame, steel frame, etc. Other factors may also effect dwelling types based on ATTMA TS1:
Site plans and schedules are normally sufficient to define dwelling types and identify the number of each.
2. Accredited Construction Details Part L1A refers to Accredited Construction Details (ACD), these can be found on the Planning Portal website (http://www.planningportal.gov.uk/england/professionals/en/1115314255826.html). The details refer to insulation as well as air permeability and are divided into construction types:
They include details for the important elements of the dwelling, including:
The details take the form of a site checklist. Adoption of these details will need to be verified by Building Control or the Approved Inspector.
7. Who selects which buildings to test and when?In theory, it is the responsibility of Building Control or the Approved Inspector to randomly select which units to test. The aim is to prevent certain building units being constructed specifically for testing and therefore not necessarily reflecting the air tightness of similar units. Approved Document L encourages the testing of units as early in the construction process as possible to allow any problem identified under test to be addressed on future units, rather than completed units of the same type having to undergo costly and disruptive remedial work. These two factors are a little contradictory in so much that if you test the first completed unit of a dwelling type the sample will not be random (as there is only one to choose from). There is also a practical consideration when scheduling the testing, it will be significantly cheaper and less disruptive to test 4 dwelling types on a single day rather than over 4 separate days. It is yet to be seen quite how selection of units and timing of their testing will take place but I would suspect that common sense would dictate a balance of the above considerations. This might affect the build programme such that an example of each dwelling type is completed early in the construction of the whole development so testing can be completed early enough to address any issues but multiple tests can be conducted. There will need to be discussion between the contractor, testing body and controlling authority to achieve a pragmatic solution. In practice, It is likely that the air tightness testing body will propose a test regime based on information provided by the building contractor and the building inspector will approve this. The units to be tested are most likely to be the first completions of each type.
8. What are the most common air leakage/infiltration paths?In essence achieving an air tight building should not be too difficult. Identifying potential air paths is fairly straightforward and addressing them is often not complicated or may be already dealt with by measures to achieve acoustic or fire requirements, as well as achieving the basic finish that is likely to be expected by the occupants. Basically, the objective is to create a continuous air barrier between:
Creating this air barrier is the SHARED responsibility of:
Therefore communication is important. Ensure workers and sub-contractors are aware of the important areas and the impact of their work. Place responsibility on ‘subbies’ to maintain the air barrier and require them to pay remedial or re-test costs if found to be their responsibility. It may be helpful, having established the air barrier, to identify it on drawings so the site manager is made aware of critical areas and therefore operations. Areas to focus on:
Overlooking important details is one of the principal causes of failure and will also prevent buildings achieving an enhanced performance. A prime culprit is dot and dab plasterboard, a particular problem behind kitchen units where boarding may not meet the floor. There needs to be a continuous seal between wall and floor, for example a continuous ribbon of plaster on the bottom edge of boarding, or appropriate sealing around the skirting. Also, a parge coat of the block work behind plasterboarding should reduce leakage in this area.
Another common problem is leakage around service voids and risers opening into unventilated spaces, for example, soil vent pipes into loft spaces and services into plant rooms. Duct work, cabling or pipe work could be sealed at every point it enters the riser or void, alternatively the riser or void could be sealed where it exits the ventilated space. Generally, air should not escape in great quantities through walls, floors and ceilings themselves (plaster is an excellent seal) but it is worth checking the permeability of certain materials eg:
Finally, be aware of concealed areas to avoid an ‘out of sight, out of mind’ mentality, eg:
These may mask unsealed penetrations or other problems so site managers and agents will need to be vigilant when signing off sub-contract work On site, basically:
9. Who can carry out the testing?Part L requires testing to be conducted test engineers/bodies registered by the British Institute of Non Destructive Testing (BINDT). BINDT registration for Part L1 testing is based on training of individuals and calibration of their equipment or membership of the Air Tightness Testing & Measurement Association (ATTMA). Registration for Part L2 requires ATTMA membership. ATTMA membership requires technical review of training, reports and procedures and accreditation by the United Kingdom Accreditation Service (UKAS). UKAS accreditation verifies:
Testing can be carried out by a ‘suitably competent person’ but in the absence of a third party checking the accuracy and consistency of testing, the building inspector will need to take additional responsibility to ensure testing has been completed correctly. This should include checking the report especially the calculations and/or witnessing testing.
10. When does testing take place in the construction process?Basically when the building is completed, typically everything has been finished apart from the carpets being laid. But before handover. Inviting the test body to site too early is likely to jeopardise the result, mean testing is impossible or not satisfy the building inspector. This does place extra pressure on getting things right first time, remedial works at this stage may be awkward and cause expensive delays. It is important that the air barrier is complete and any penetrations have been fitted and finished. For dwellings we have produced a checklist to gauge whether the units are sufficiently complete.
11. What information is required to produce a quote for air tightness testing?For dwellings, sufficient information is required to identify the different dwelling types and the number of each eg General Arrangement/Site Plan and Schedule (including other important details such as variation in storey height or construction method). For buildings other than dwellings, the approximate envelope area is the key factor for quoting. It is required to establish the necessary fan arrangement. This affects the time on site and potentially the number of people. This can be calculated from drawings – floor plans AND elevations. The testing body may also need to identify the potential aperture(s) for the test equipment to be installed into, in some circumstances this may require additional time on site, extra people or bespoke templates to be made.
12. What preparation needs to take place in advance of testing?We provide a simple checklist of building preparation. This needs to go to site. The preparation would ideally be undertaken by the contractor n advance of the test body arriving on site to maximise the testing time in a day. For dwellings it may also be necessary to agree the test programme with the building inspector before arriving on site. Where possible, it is helpful to accurately calculate the envelope area and confirm the fan installation arrangements based on architectural drawings before coming to site.
13. What does air tightness testing involve?The objective is to measure the volume of conditioned air escaping through the building envelope via uncontrolled ventilation at an induced pressure difference of 50Pa. The following basic steps are typical:
14. What is the envelope area?The envelope area is the total internal surface area of the conditioned areas of the building ie the sum of floor area, wall areas and roof area (of the heated space). This should be calculated or verified by the air tightness test body as the accuracy of the envelope area has a significant impact on the final air permeability result. The building inspector should check this is being done correctly and/or approximately verify the area.
15. How long does an air tightness test take?Depending on:
Assuming the second 2 factors are within reasonable limits the 1st factor tends to mean a test time of between 1 and 4 hours:
16. How is air tightness calculated?Air permeability is essentially a function of the pressure difference between the inside and outside of the building and air flow rate through the fan(s) to produce that pressure difference. This is averaged out over the envelope area. The result takes account of environmental conditions. The final air permeability at 50Pa is based on a logarithmic graph of pressure difference and flow rate, the graph should:
These are aspects that the BCB should be checking carefully if choosing to accept air permeability results from non-accredited testing bodies.
17. When will I get the results?An indicative result is available immediately. Certificates can usually be issued on the same day, where required. Reports will certificates are typically sent within 5 working days.
18. What test evidence is required?A test certificate from a test engineer and/or test body that is registered with the British Institute of Non-destructive Testing (BINDT). The test certificate should include sufficient information to describe the building tested eg location, type and size (the envelope area is an important component in calculating the air permeability and must be accurate) plus the design air permeability as well as the actual result. I would also expect the certificate to refer to BINDT registration and/or membership of the Air Tightness Testing & Measurement Association (ATTMA). This can be cross-referenced via their respective websites eg www.attma.org BINDT registration for Part L1 testing is based on training of individuals and calibration of their equipment or ATTMA membership. Registration for Part L2 requires ATTMA membership. ATTMA membership requires accreditation by the United Kingdom Accreditation Service (UKAS) to check test engineer competency, accuracy of equipment and consistency/appropriateness of procedures, plus a technical review of training, procedures and evidence. The test engineer/body should also produce a full test report that meets the requirements of ATTMA Technical Standard 1. This will include the building pre-test preparations as well as full test data and should be available from the contractor if required.
19. What happens if the building fails?At Chiltern Dynamics, we will endeavour to help you identify the probable air leakage/infiltration paths. There are number of methods we employ to do this of increasing complexity:
In the vast majority of cases the first method is sufficient to identify air leakage paths. These can be temporarily or permanently sealed and the test repeated to quantify the effect of addressing these areas. The building will need to be re-tested, sometimes on the same day but usually at a later date. For buildings other than dwellings, the same unit is tested and must achieve the air permeability require to achieve the Target Emission Rate or for buildings less than 1000m2, until October 2007, a significant improvement. For dwellings, the same unit should be tested, plus another example of the same dwelling type (assuming one exists).
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