Approaches to airtightness that rely on basic workmanship coupled with secondary and remedial sealing measures during construction are likely to be much less robust than those approaches that are based on an explicit attempt at the design stage to concentrate on ensuring that there is an effective and continuous primary air barrier.
The first stage in the process is to ensure that the design gives consideration to the identification and location of the primary air barrier and that there is no reliance on secondary sealing. This involves identifying a line through the building that will form the air barrier. One way of achieving this is to mark the location of the primary air barrier on a set of General Arrangement drawings with a red line. This is known as a ‘pen-on-section’ test. The red line should provide a continuous line around or through all elements in the building that separate heated (conditioned) and unheated (unconditioned) spaces. This helps to identify areas where additional detailing will be required and allows the allocation of responsibility for design and construction of the air barrier.
Examples of 'pen-on-section' tests
Large scale drawings should be prepared of the key points of complexity in the design, identifying how continuity of the air barrier is to be achieved. Consideration needs to be given to areas that are inaccessible or out of view once the building is occupied (for example, behind fitted cupboards, kitchen and bathroom fittings, and in storage areas). All drawings prepared by third parties (such as heating and ventilation engineers) should contain detailed information on the air barrier and treatment of penetrations through it.
|An example, which illustrates the usefulness of the exercise, is shown opposite. In this example, the use of continuous ribbons of plasterboard adhesive at the wall/ceiling junction is critical in ensuring continuity between the wall air barrier (parging) and the ceiling air barrier (plasterboard).
Any gaps in the continuous ribbons of plasterboard adhesive will become gaps in the primary air barrier. In addition, it is also common for the continuous ribbons of plasterboard adhesive to be placed up to 150mm away from the edge of the plasterboard. This results in continuous channels around each board perimeter, enabling air to move freely around the corners of rooms and between boards.
||Eaves detail with primary air barrier marked in red
Problems in achieving continuity in the primary air barrier are often inherent in the initial design concepts if airtightness is low in the list of design considerations. To achieve continuity in the primary air barrier the design should aim to:
- Minimise the number of service penetrations to minimise the significant air leakage problem around areas such as service voids, wet rooms and cylinder cupboards
- Simplify as much as possible the primary air barrier by avoiding or minimising changes of plane, minimising the number of different materials used and avoiding complex details wherever possible. Design approaches that recognise the importance of buildability and simplify the construction process are less likely to result in discontinuities in the primary air barrier.
- If complex details are unavoidable, provide additional sections and detail design drawings that include detailing of the air barrier and its junctions, penetrations and changes of material and plane and that specifically identify how continuity of the air barrier is maintained.
- Minimise construction gaps by addressing tolerancing. Where different construction processes have different tolerances ensure that conflicts are resolved before construction.
- Ensure the air barrier is robust enough to withstand construction tolerances and is capable of inspection and repair prior to being covered by later construction.
SEQUENCING OF CONSTRUCTION PROCESSES
A more explicit consideration of construction sequence both as a design criterion and in planning construction would enable better levels of airtightness to be achieved. The following should be taken into account:
- The primary air barrier should be completed before it is obscured and its accessibility compromised. For example, where a parging coat forms the primary wall air barrier, it should be applied to walls before any subsequent trades go in to provide a clear air barrier.
- The use of intermediate floors as working platforms to lay the blocks around the floor perimeters should be avoided if possible.
- Wherever possible wall penetrations for services should be fitted with sleeves and sealed as construction proceeds to avoid the need for breaking out new construction
- Sealing of services penetrations should be robust enough to enable later fitting work to be carried out without damage to the seal. For example, electricity cables that penetrate the air barrier should be fitted with a grommet type seal that allows for the cables to be manipulated during and after the installation of terminal fitting without determent to the air seal.
- The air barrier should be installed over as large an area as possible in one operation. For example, putting the ceilings up first, rather than partitions, provides a clear and continuous air barrier at the top of the dwelling that minimises the number of wall/ceiling junctions to be sealed.
SITE SUPERVISION & WORKMANSHIP
High standards of workmanship are necessary to achieve good airtightness. It is impossible to divorce workmanship from design and other issues of construction management such as training, communication and quality control. There are, though, several ways in which high standards of workmanship can be promoted:
- Site and trade specific training on airtightness should be a compulsory part of the site induction, with explanations of why this is important, how it is being tested, what quality control processes are in place and what happens when things go wrong
- Operatives need to know what they are required to achieve and what constitutes an acceptable standard. The definition and visibility of the air barrier is crucial.
- The importance of high levels of workmanship in hidden areas should be stressed and quality control should be capable of verifying the standard achieved.
- Management processes at times of increased or accelerated production should assess the impacts on quality and provide additional resources and training to ensure that performance does not suffer.
Experience indicates that testing, monitoring, and feedback are essential to any quality control process. Specific ways that the process can be improved are:
- The quality control process must be formally described and different roles and responsibilities clearly set out with lines of reporting, recording, investigation and action established and applied consistently.
- Where there is a need to increase production this must be enable the quality control processes to remain effective.
- Checking the integrity of the primary air barrier, including measurements of airtightness, should occur at key stages of construction, before it becomes impossible to efficiently undertake remedial action.
- If a check-listing approach is to be used, it must be thorough and completed at the correct stage of construction. The list of key inspection points provided in Jaggs & Scivyer (2006) provides a general outline, but would need to be adapted to be site specific.
- Maintaining a photographic record of observations made during the construction process, not only allows a more precise retrospective analysis in the event of future investigations, but also provides useful material for training and improving the awareness among site staff of the impact of their actions.
- Testing regimes should be designed to enable developers to monitor the performance of dwelling production in general so as to identify performance trends that can be acted upon quickly and efficiently.
- As far as possible, construction specifications should ensure standardisation of detailing to enable site teams to become familiar with the materials, components and tolerancing needs. Where modifications are required these should be undertaken in a controlled way accompanied by effective detailed documentation.
The communication of detailed design information and feedback on airtightness performance is crucial if high standards of airtightness are to be achieved. To ensure an effective communication process requires that:
- Developers need to ensure that design information is available at the appropriate level of detail and that this information is communicated to subcontractors and their operatives through an appropriate mixture of documentation and detailed briefings.
- Design information should include procedural specifications as well as drawings depicting the final form. In particular, all drawings and specifications should define the primary air barrier and detail drawings should show how the air barrier is to be maintained at junctions and penetrations. Appropriate design information should be provided to all trades that may have an impact on the integrity of the air barrier.
- Changes to design information should be communicated quickly, consistently and clearly. Also they should be recorded and appropriate documentation reissued.
- Performance data (quantitative and qualitative) should be freely communicated within the developer’s organisation as part of a clearly defined process of quality control and improvement.
Performance data should include input from a range of sources so that all aspects are taken into account. This could range from data based on tool box talks relating to buildability to feedback on general trends in airtightness measurements for different dwelling types and construction forms.