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Last edited 19 Jul 2021
Air tightness in buildings
Approved document F, Ventilation, defines airtightness as ‘…a general descriptive term for the resistance of the building envelope to infiltration with ventilators closed. The greater the airtightness at a given pressure difference across the envelope, the lower the infiltration.’
It suggests that air permeability is the physical property used to measure the airtightness of the building fabric. It is defined as air leakage rate per hour per square metre of envelope area at a test reference pressure differential across the building envelope of 50 Pascal (50 N/m2). The design air permeability is the target value set at the design stage.
The Air Tightness Testing and Measurement Association (ATTMA) defines ‘air leakage’ as the '...uncontrolled flow of air through gaps and cracks in the fabric of a building. It is sometimes known as infiltration or draughts. Air leakage is not to be confused with ventilation, which is controlled airflow in and out of a building'.
Approved Document E suggests that infiltration '...is the uncontrolled exchange of air between inside a building and outside through cracks, porosity and other unintentional openings in a building, caused by pressure difference effects of the wind and/or stack effect.'
Wherever infiltration occurs, there is a corresponding exfiltration somewhere else in the building. During the summer, infiltration can bring humid, outdoor air into buildings. In winter, exfiltration can result in moist indoor air moving into cold wall cavities and can result in condensation and ultimately mould or rot.
While air infiltration is not desirable, it is important for buildings to have sufficient purpose-provided ventilation. According to the Building Services Research and Information Association (BSRIA), 'Project teams should design and construct the building fabric to be reasonably airtight, and also provide natural or mechanical ventilation systems that maintain good indoor air quality while minimising energy use. In other words: build tight, ventilate right.' Ref BSRIA Topic Guide - Airtightness.
- Lower running costs through reduced heat loss.
- Fewer defects.
- Reduced condensation.
- Improved comfort.
- Reduced carbon emissions.
- Verification of build quality.
- Direction and strength of the wind.
- Orientation of the building.
- Ventilation strategy – mechanical or passive.
- Internal to external temperature differences.
- Behaviour of occupants.
- Frequency of use.
- Maintenance of buildings.
- Pressure differences between the top and bottom of the building (stack effect).
A significant amount of air leakage resulting in heat loss occurs in all buildings but much less in air-tight buildings. According to the Federation of European Heating, Ventilation and Air Conditioning Associations (REHVA), the energy impact is “in the order of 10 kWh per m2 of floor area per year for the heating needs in a moderately cold region”. REHVA also suggests there are a growing number of studies indicating that there is considerable impact on buildings in mild and hot climates.
|Air conditioned / low-energy offices||2.0||5.0|
|Factories / warehouses||2.0||6.0|
|Museums and archival stores||1.0||1.5|
NB: 'm3/(h.m2)@50Pa' is the flow of air (m3/hour) in or out of the building, per square metre of the building internal envelope at a reference pressure of 50 Pascals between the inside and outside of the building.
Air permeability testing is necessary as a means of demonstrating that airtightness targets used in energy calculations as Part L of building regulations compliance have been achieved in reality. In England and Wales, airtightness testing has been mandatory for virtually all new buildings since 2006.
The Building Regulations require that 50% of each home type in a development must undergo air tightness testing. The definition of home types was expanded in the 2010 revision, and includes differences in position – terrace versus end terrace, or top floor versus ground-floor flats. In addition, the rules require that half of the plots that are tested must be within the first 25% of completed properties.
- Receive notification of the intended work.
- Proceed to check drawings.
- Issue planning and building regulation approvals.
- Receive copies of the approved inspectors report, should one be appointed.
- Periodically inspect the works.
- Receive a copy of the air-test report.
- Issue a completion certificate.
- Apply for planning and building regulations approval.
- An agreement of sale / lease with tenant or purchaser may be in place.
- Have a duty / agreement with a funding party.
- Enter into warranties with design team, contractor / sub-contractor, and funder.
- May specify an air permeability rate within the contract documentation.
- If the building is not energy / fuel efficient, incur additional financial costs due to inefficiency.
- If the building fails the test, a completion certificate will not be issued, and the client may be able to claim damages from the contractor.
 Consultants / specifier:
- Take account of the statutory acts, building regulations, and Approved Documents.
- Ensure the building complies with both planning and building regulations' requirements.
- Consider buildability and compatibility of materials and methods of construction.
- Ensure the building is built in accordance with the contract documents.
- Ensure the building is inspected regularly.
- Adhere to the contract drawings and specifications and in accordance with building regulations.
- Procure and appoint a specialist to undertake an air test.
- Give notification to the local authority as to when and who will be performing the test.
- Schedule the test into the contract programme.
- Pass the air test to comply with building regulations.
- If the building fails the test they will not receive a completion certificate and the client may seek damages.
 Air tightness - managing the process
Research shows that the vast majority of pre-tests fail to achieve an air permeability of 10m3/Hr/m2 of the building envelope. Therefore, contractors should not be concerned whether the building passes or fails but what lessons can be learned and what problems can be identified from the test. If the objective is to achieve a 'pass' at this stage then the contractor may end up creating a false environment, by temporarily sealing openings and unfinished work, which could prove costly.
A balance has to be achieved in which it is possible to learn from an initial test by identifying potential problem areas and confirming the air tightness of completed sections of work by undertaking smoke tests during the pre-test procedure.
Initial tests could be undertaken once the building is weather tight to check the air infiltration through the fabric and the final test post-finishes. It may depend on how confident the contractor is with the building's design and on-site workmanship.
The client may see an increase in tender prices due to enhanced requirements, such as enhanced specification produced by designers, additional measures such as air barriers. They may consider whether conforming to Part L is adequate, and whether to amend contracts, warranties and employers' requirements. They may also consider specifying the number and timing of tests and whether to ask the contractor for an air-tightness strategy.
The contractor will consider who is responsible for conforming with requirements, how to price them, and whether they can pass on any exposure / risk to others. For example, should they amend sub-contracts to include specific reference to Part L or will this reduce the number of sub-contractors who will return tenders.
Conforming to Part L need not be a difficult issue if the parties approach the issue with an open mind and implement a strategy early-on in the design development process. As the industry makes advances in technology and legislation, both clients and contractors need to make equal advances in the way they approach and manage contracts.
- Ensure all employees and sub-contractors are suitably trained / qualified to conduct the work they are required to undertake;
- Ensure all employees and sub-contractors adopt standard working practices and are familiar with the materials / products / systems they are to apply;
- Ensure that the work of employees and sub-contractors is undertaken in an efficient and effective manner;
- Ensure that process controls relating to the activities of construction are performed under controlled conditions and undertake periodical inspections and testing of the work;
- Ensure that the design and specifications can be constructed under the operational and environmental conditions of the project;
- Ensure that all employees and sub-contractors fix, assemble and install all materials / products / systems in accordance with the supplier's and manufacturer's instructions;
- Ensure that the handling methods of employees and sub-contractors are appropriate to the materials employed;
- Ensure that employees and sub-contractors handle and store materials / products such that their accuracy and fitness for use is maintained.
The overall aim is to significantly improve the quality of workmanship on site. Materials and products in isolation very rarely fail; similarly, drawings and specifications rarely fail. Generally, problems arise due to poor workmanship and this ultimately is the responsibility of the contractor.
 About this article
 Related articles on Designing Buildings Wiki
- Accredited construction details ACDs.
- Accredited energy assessor.
- Air permeability testing.
- Airtightness of energy efficient buildings.
- BREEAM Testing and inspecting building fabric.
- Building performance.
- Building services compliance with the building regulations.
- Draughts in buildings.
- Effective ventilation in buildings.
- Energy audit.
- Energy Performance Certificates.
- Floor plenum airtightness.
- Indoor air quality.
- Leaks in buildings.
- Primary air barrier.
- Simplified Building Energy Model.
- Thermal imaging of the building fabric in the net zero world.
- Thermal imaging to improve energy efficiency in building design.
- Thermographic survey.
- The history of non-domestic air tightness testing.
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