Air tightness
Technical memorandum
Introduction 1
Legislation 1
What is already in place? 2
Procedure for testing 2
If the air test fails 2
Exemptions 3
Recommendation 3
Sources of reference 3
Air tightness Technical memorandum i
Version 1.0
Introduction
1. Air tightness is the resistance of the building envelope to inward or outward air leakage. Excessive air leakage results in increased energy consumption and a draughty building. Air leakage is driven by differential pressures across the building envelope caused by a combination of internal warm air rising, external wind and mechanical ventilation systems[1].
Legislation
2. The British standard for air tightness is BS EN 1382:2001 (thermal performance of buildings – determination of air permeability of buildings – fan pressurisation method).
3. Part L2 (conservation of power and fuel in buildings other than dwellings) of the building regulations, which came into force in 2002, relates to air tightness and was revised in 2006 to set maximum carbon emissions for buildings (including the renovation of existing buildings with a size of over 1,000m2). Part L2A applies to the construction of new buildings (with the exception of the exemptions listed below) while part L2B applies to existing buildings.
4. In a building of over 1000m2 where the proposed work consists of an extension, the initial provision of any fixed building services (such as heating, ventilation or air handling) or an increase to the installed capacity of any fixed building service will mean the whole building will be required to comply with part L, if this consequential improvement is technically, functionally and economically feasible.
5. Any extension in an existing building that is more than 25% of the total useful floor area is regarded as a new building and must follow the regulations set out in part L2A[2].
6. The total useful floor area is defined by part L2B of the building regulations as:
“The total area of all enclosed spaces measured to the internal face of the external walls. The area of sloping surfaces such as staircases, galleries, raked auditoria and tiered terraces should be taken as their area on plan. It includes the areas occupied for example, by partitions, columns, chimney breasts and internal structural or party walls. It excludes areas that are not enclosed such as open floors, covered ways and balconies. This equates to the gross floor area as measured in accordance with the guidance issued to surveyors by the RICS”[3].
7. The target emission rate is 10m3/hour/m2 at 50 Pa.
What is already in place?
8. Cornwall Council specifically refers to air tightness in the following documents:
• JCT agreement for minor building works
• Intermediate form of building contract
• JCT standard format building contract
9. The compliance section in each document details what is required under the building regulations and who is required to produce each piece of evidence. It states that where the works are not exempt, a contractors certificate of conformity and compliance shall be produced, stating that the results of the air leakage tests carried out in accordance with the CIBSE TM23 are satisfactory, together with all test results.
Procedure for testing
10. All areas of the building are sealed (this includes temporarily sealing all heating and ventilation equipment and closing trickle vents in windows). The air test will typically take around five hours and people can remain inside the building as long as they do not open a door or access hatch that forms part of the air barrier[4].
11. The test involves connecting a fan (or a number of fans) to a suitable aperture in the building envelope and pressurising it over a range of pressure differences. The fan speed is increased in steps up to a maximum and then decreased in steps. Air volume flow rate through the fan (equal to the air leaking through the building envelope) and the pressure difference through the building envelope at each fan speed are recorded[5].
If the air test fails
12. An air leakage test will be carried out to identify air leakage paths. These can include running localised smoke tests, running a smoke test on the whole building, carrying out a thermographic survey and physically checking over the risk area looking for holes and gaps. Air tightness experts suggest that to rectify problems which caused the air test to fail can cost ten times more than if the building had been correctly sealed before the test was carried out.
Exemptions
13. Temporary buildings with a planned time of use of less than two years are exempt from energy efficient requirements, as are buildings of less than 500m2 total useful floor area.
14. Further examples include greenhouses, buildings used as a place of worship, stand alone buildings (other than dwellings) with a useful floor space of less than 50m2, industrial sites, workshops and non residential agricultural buildings with a low energy demand.
Recommendation
15. Full compliance with part L2 of the building regulations and associated legislation.
Sources of reference
• Air tightness information pack - HRS Services Ltd
• Building regulations part L2
• Carbon Trust
• Air Tightness, Testing and Measurement Association (ATTMA)
Prepared by:
Chris Jackson
Policy and Training Manager
Property Services
14 June 2011
If you would like this information
in another format please contact:
Cornwall Council
County Hall
Treyew Road
Truro TR1 3AY
Telephone: 0300 1234 100
Email:
www.cornwall.gov.uk
Air tightness Technical memorandum 3
Version 1.0
[1] Air tightness information pack - HRS Services Ltd
[2] Building regulations part L2
[3] Building regulations part L2
[4] Carbon Trust
[5] Air Tightness, Testing and Measurement Association