Airtightness Part 2

Airtightness Part 2 Image 1

In the previous article I outlined the basic principles and advantages of Airtightness. These were primarily:

  • Avoiding drafts and discomfort
  • Avoidance of moisture related building damage
  • Avoiding high heat losses due to air leakage
  • Introducing a controlled ventilation system
  • Improved acoustic performance
  • Improved indoor air quality

While it is one thing writing about the principles of Airtightness, it is another thing achieving high levels of Airtightness practically, on site. The following are the primary steps to achieving higher levels of Airtightness in construction:

1. Forethought, planning of works schedule and clear simple design principles;

2. Quality control on site and good workmanship with the use of appropriate materials to achieve an airtight seal;

3. A final measurement which confirms we have achieved the required level of Airtightness which we were aiming for in the first place.

The glue that cements everything together is communication, coordination of works on site and a high level of consciousness and awareness regarding the importance of Airtightness from the design stage to the build process. Figure 1 summarises these key steps.

Airtightness Part 2 Fig 1

Figure 1: Design, Build and Test for Airtightness -the 3 key steps to achieving Airtightness on site

Designing for Airtightness:

To maximise the Airtightness of any building the Designer/Architect plays a pivotal role. The architect should select the level of Airtightness to be achieved from the outset. Table 1 taken from the Airtightness Testing and Measurement Association (ATTMA) in the UK, provides guidance regarding normal and best practice for Airtightness in various building types:

Type Air Permeability
m3/(h*m2) @ 50Pa
Best Practice Normal
Naturally ventilated 3 7
Mixed Mode 2.5 5
Air conditioned/low energy 2 5
Factories/warehouses 2 6
Superstores 1 5
Schools 3 9
Cold Stores 0.2 0.35
Naturally ventilated 3 9
Mechanically ventilated 3 5
Table 1: Normal and best practice Airtightness criteria for different building types (ref: ATTMA Technical Standard 1; Issue 2, 2007)

Airtightness sealing details for common leakage areas which are difficult for the builder to seal on site, can be greatly simplified at the design stage, leading to vast improvements in workmanship and assuring an airtight seal is practically achieved. The key is to keep the Airtightness detailing as simple as possible. Superior levels of Airtightness can be achieved and of course significant savings can also be made as a result of time savings on-site, at the installation stage.

The major cost in relation to Airtightness is generally not down to the material used, but the cost for labour and time on site sealing unnecessarily complex details. Designing effective, buildable Airtightness details does not add significant costs in terms of design, but can make dramatic savings in time, labour and cost on site. The more uncomplicated the detail, the more chance of success on site, the cheaper the cost and crucially the better the Airtightness result!

On the other hand, where details are complicated by unnecessarily intricate details, this leads to a requirement for more sealing material and more labour time on site, increased costs and may lead to a poorer Airtightness result.

From the outset the architect should clearly define the Airtightness layer on the building design in both a plan and elevation view. It is particularly important to clearly outline what forms the air barrier at each location and especially how various interfaces between different sections and materials are made (see figure 2 below).

If one can trace a continuous line defining the Airtightness layer around the external envelope, without lifting the pen once from the drawing, then we have perfect Airtightness layer. In reality this is impossible of course as we have to lift our pen at connections to windows, separating floors, service penetrations and so on. It is at these details that the designer has to clearly specify a buildable Airtightness solution.

The Architect should specify materials and propriety airtight sealing products which simplify the airtight sealing process for the installer and guarantee a clean, durable and lifelong seal. The Airtightness achieved at the build stage should last for the lifetime of the building, not just the day of the test. The following details provide a sample of some simple yet very clear and effective Airtightness details.

Figure 2: Some examples of effective Airtightness details

Airtightness Part 2 Fig 2a

2 (a) Overlaps of a vapour control layer sealed with an Airtightness tape at overlaps (shown in red)

Airtightness Part 2 Fig 2b

2 (b) Service penetration sealed with flexible service grommet

Airtightness Part 2 Fig 2c

2 (c) Window frame sealed reliably to vapour control layer

Airtightness Part 2 Fig 2d

2 (d) Separating floor detail sealed continuously

Building to achieve Airtightness:

There is no doubt that good workmanship and a high level of awareness and consciousness on site within various trades is a central part of achieving high levels of Airtightness on site. From the outset an “Airtightness Champion” should be appointed on site. An “out of sight out of mind” philosophy will lead to poor quality control and inevitably a very poor Airtightness result at the end of the project.

Typically this person will be on site for the length of the project and interact with the various trades on site at all times. The Airtightness Champion should have direct contact with the designer and the airtight system supplier to clarify any outstanding details over the length of the project. When the building is at the construction stage communication between trades is absolutely essential.

It is crucial to schedule a site meeting with key site personnel (i.e. plasterer, window installer, electrician, plumbers, etc) who will interact with the Airtightness layer as early as possible. It is also essential that the airtight installer maintains a high level of consistency and consciousness to continually seal the Airtightness layer on site. Scheduling of key steps in the build process and determining how the various trades interact with the Airtightness layer throughout the build process should also be planed early. Airtightness courses and training are now available from Airtightness system suppliers, such as Ecological Building Systems, or from an independent national training authority, such as FAS.

Intermediate Airtightness assessments are absolutely essential to ensure air leakage paths are identified as early as possible. This can be conducted by using a pro clima WINCON Airtightness quality control unit, or a conventional blower door fan, both of which are described in my previous article.

On site, potential air leakage paths can be categorised into 3 main areas:

1. Structural leaks

2. Service leaks

3. A combination of structural and service leaks

1. Structural Leaks: These can occur where two structural elements in the building fabric connect to each other. An example of a structural leak can be a leak between a window frame and a wall or where overlaps in a vapour check are ineffectively sealed, attic hatches or even cracks in walls.

2. Service Leaks: These can occur where pipes or electrical cables penetrate the external envelope and are ineffectively sealed. One of the worst offenders is the dreaded recessed light fittings. The simplest way to alleviate service leaks is to minimise the amount of such openings on the external envelope. Including a service zone on the inside of the Airtightness layer will also significantly reduce the risk of service leaks and lead to greater flexibility on site.

3. A combination of a structural and a service leak: This is where a structural leak interacts with a service leak leading to air leakage. These can often be the biggest contributor to air leakage and one of the most difficult to seal when the building is complete. An example of such a leak could be a separating floor joist which is poorly sealed at the connection to the external wall, leading to air leakage through the floor joist at light fittings on the ground floor ceiling, plug sockets in the internal stud partitions on the first floor and even air leakage throughout the first floor.

Finally, materials used to achieve an airtight seal throughout the external envelope should be designed to provide an airtight seal for the lifetime of the building, not just for the day of the test and be simple for the installer to apply. Expanding foam cannot be relied upon to provide a reliable durable airtight seal. Systems which have been independently assessed by recognised bodies, such as the Irish Agrément Board, should be specified (e.g. pro clima Intelligent airtight system), to guarantee performance.

Examples of poor and good practice details are illustrated in the following images:

Figure 3: Good Practice Airtightness detailing:

Airtightness Part 2 Fig 3b Airtightness Part 2 Fig 3a Airtightness Part 2 Fig 3c

(a) Airtight sealing of Airtightness membrane and surrounding attic hatch opening;

(b) Airtight sealing of surrounding timber rafter penetration and overlaps of Airtightness membrane;

(c) Airtight sealing of ceiling joist in refurbished block construction;

Airtightness Part 2 Fig 3d
(d) Joints and service penetrations through hollow core slab sealed with proprietary Airtightness sealing tapes and glues
Figure 4: Poor Airtightness detailing:
Airtightness Part 2 Fig 4a Airtightness Part 2 Fig 4b Airtightness Part 2 Fig 4c
Airtightness Part 2 Fig 4d Airtightness Part 2 Fig 4e
(a) Tear in a substandard vapour barrier;

(b) Poor sealing around gap between window frame and block wall;

(c) and (d) Large openings in a hollow core concrete slab leading to air leakage;

(e) Large gap to facilitate services in hollow core slab leading to air leakage

Figure 5: Insulation does not provide a continuous airtight seal
Airtightness Part 2 Fig 5a Airtightness Part 2 Fig 5b
(a) Gaps between timber structural elements and rigid insulation;

(b) Poorly fitted over compressed fibreglass is not airtight

Testing to confirm Airtightness:

When the building is complete it must be tested to confirm that the level of Airtightness achieved is as specified by the architect, and also to identify leakages in the building envelope. An Airtightness test is carried out by a blower door tester. The only way to confirm we have achieved the specified level of Airtightness is with a blower door test. Over the last decade the number of testers providing this service has increased dramatically. The principles of blower door testing were outlined in the previous article.


Good build practice on site is a precondition for high levels of Airtightness on site, however, good workmanship alone can only achieve a limited amount. The economics of achieving the highest levels depend on adopting a holistic approach from very early in the design process. The role of the architect is central in determining clear, concise, simple details, which the various trades can follow and carry out on site.

The Airtightness of the building is not just for the day of the test, but for the lifetime of building. Materials used to achieve Airtightness should be designed for that purpose and should have third party certification from an independent authority, such as the Irish Agrément Board.

To avoid costly delays and remedial action for sealing leaks both the architect and the builder must be aware of the importance of Airtightness and what is required. Reducing air leakage in buildings is one of the most cost effective means of conserving energy, improving building durability and workmanship standards on site and to minimise condensation risk within the building envelope. With Airtightness in practice there is no doubt that the devil is in the detail be it at the design stage or the sealing procedure on site!

Niall Crosson

Technical Engineer BTech, MEngSc, MIEI

Pictures and graphics provided courtesy of Ecological Building Systems

In the next article:

Avoiding condensation risk within constructions is a precondition for building durable, healthy, low energy constructions. Selecting materials which combine high levels of Airtightness with effective protection against condensation is desirable. In the next article I will focus on designing low energy, airtight, healthy constructions.