GreenSpec: Thermal Performance: Designing for airtightness

air tight

The 'Airtightness barrier'

• A continuous airtightness barrier system is the combination of interconnected materials, flexible sealed joints and components of the building envelope that provides the airtightness of the building enclosure and the separateness of heated and unheated spaces.

• The airtightness barrier needs to be designed into the building envelope during the initial concept design stage.

Developing an approach to airtightness

In the design office

• Define an airtightness performance target.
• Use a performance specification.
• Ensure all trade specifications include their requirements and interfaces with other trades.
• Ensure all ME&P service engineer’s specifications include airtightness requirement and measures to achieve it.
• At an early stage of the design, define the line of the airtightness barrier.
• It can be useful to take plans and sections and draw a continuous red line that passes through all the elements that separate heated and unheated spaces thus:

air barrier definition

• It is useful at an early stage to identify critical details that will have a bearing on the airtightness barrier.
• Details should be thoroughly worked out at design stage and not ‘left-to-chance’ later on site.
• Think in 3D and explore around every corner.
• Clearly identify the location of the air barrier on the drawings as an ‘airtightness line’.
• Tightly manage the design implementation by appointing an ‘Airtightness Champion’ to coordinate between consultants and to coordinate with the contractor’s ‘Airtightness Champion’ who will coordinate between trade sub-contractors.
• If the design team is inexperienced, it might also be prudent to appoint an independent adviser.
• Specify airtight components, membranes, seals and jointing methods.
• Check interfaces between components and between trades or work packages to ensure the continuity of the air barrier.

On site

• Tightly manage the implementation by appointing a contractor’s site ‘Airtightness Champion’ to coordinate between trade subcontractors and to coordinate with the design team’s ‘Airtightness Champion’ who will coordinate between consultants.
• Toolbox talks: Brief the construction team of the importance of airtightness and their collective role in achieving it.
• Clearly identify the location of the air barrier.
• Institute a regimen of inspection during construction, with particular regard to ensuring the air barrier is uncompromised by shoddy workmanship.
• Consider using airtightness testing kit during construction so all trades are aware of the effect of their work and encourage engagement in the quest for airtightness.
• Ensure that the air barrier is complete prior to covering up by other work and trades – failures found during testing can lead to expensive uncovering and remedial work.
• Ensure that air testing is scheduled in advance to achieve maximum benefit for purposes of remedial work.

The designer's work plan

The role of the designer is critical. Design for air tightness should be simple and buildable. Targets should be achievable. Roles and responsibilities should be established at an early stage. The Contractor should be made responsible for achieving the designed Air tightness levels.

RIBA Work Stage	Design Team Tasks
A Appraisal	Establish an airtightness performance target/air permeability rate.
B Feasibility / Briefing	Note Microclimate Identify procedure for review and testing
C Outline proposals	Consider air tightness issues in relation to decisions about form of construction
D Detailed Proposals	Identify requirement of additional consultants / design by specialists
E Final Proposals	Ensure co-ordination between Design Team to ensure airtight envelope & penetrations Detailed application of airtight materials, junctions, service penetrations
F Production Info	Select sub-contractors for specialist works (incl. testing) Careful specification of components, membranes, materials Emphasise methods for airtightness in documentation Show airtightness line on drawings and details Emphasise responsibilities in specification for dealing with ‘loose ends’ between sub-contractor interfaces
G Tender Documentation	Define Contractors’ responsibilities for coordinating work sequences
H Tender Action	Ensure selected tenders include adequate airtightness procedures
J Mobilisation	Brief all involved in areas critical to air infiltration before work starts Preparation of samples, training, testing and QA procedures
K-L Site Works	Co-ordinate inspection with Building Control if required Ensure inspection of areas to be covered Ensure audits and testing schedule is adhered to Ensure design changes do not compromise Air tightness performance
M Post Completion	Obtain feedback from occupants concerning comfort and energy consumption Carry out remedial work as required at end of Defects Liability Period.
Source: Design and Detailing for Air tightness, Chris Morgan, SEDA 2006

Testing

• Air testing is a crucial tool in determining the effectiveness of an airtightness membrane.
• Air testing is carried out when the envelope is complete. If possible, it is wise to test twice - once before the covering-up of the membrane when remedial work can easily be carried out, and again at completion.
• Testing will identify the overall 'leakage' of the building.
• Smoke generating sticks can be use to find air leakage points or lines and help in understanding air leakage paths.

Standards

British Standards Institute (BSI)

• BS 7386: 1997 Specification for draught strips for the draught control of existing doors and windows in housing (including test methods)
• BS 5925: 1991 Code of practice for ventilation principles and designing for natural ventilation
• BS 4255: Rubber used in preformed gaskets for weather exclusion from buildings Part 1: 1986 Specification for non-cellular gaskets .

International Organisation for Standardisation (ISO)

• ISO 6589: 1981 Nr permeability of joints, watertightness
• ISO 6613: 1980 Nr permeability of tests on windows and doors
• ISO 9972: 1996 Thermal Insulation – Determination of building Air tightness– fan pressurization method

Chartered Institute of Building Services Engineers (CIBSE)

• Measuring Air Permeability of Building Envelopes. CIBSE Technical Memorandum ATTMA TS1

Building Research Establishment (BRE)

• BR 359 Air tightness in UK Dwellings: BRE’s test results and their significance. 1998
• BRE Digest 306 Domestic Draught proofing: Ventilation Considerations

Further information

• Airtightness and intelligent membranes
• BRE, Air tightness and leakage
• BSRIA, Air tightness for Building Regulations