Design Principles For Airtight Buildings & Construction

Designing for airtightness is an essential part of building work in the UK and abroad, and involves a multitude of considerations, skill sets and experience to meet building regulations.

Fundamentally, the air barrier of a building comprises a set of elements that work together to deliver airtightness. These should be considered iteratively throughout the design process.

Here we look at the considerations that are integral to designing a building to achieve airtightness, how these are achieved and checked, how to get it right on site, and what can happen when it all goes wrong.

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What does a building’s air barrier consist of?

The air barrier comprises products and installation processes that:

  • Form an airtightness layer in the floors, walls and roofing (or top floor ceiling)
  • Seal the doors, windows and roof lights (if applicable) to the adjacent walls or roof
  • Link the interfaces between walls and floor and between walls and the roof, including around the perimeter of any intermediate floor
  • Seal penetrations through the air barrier, including:
  • Waste pipes & soil pipes
  • Ventilation ducts
  • Incoming water, gas, oil, electricity, data and district heating, as applicable
  • Chimneys and flues, including air supplies to wood burning stoves or similar
  • Connections to external services, such as entry phones, outside lights, external taps and sockets
  • Security cameras, satellite dishes

Simplicity is key to airtight buildings

Simplicity is key in airtightness design. The fewer junctions, balconies, dormer roofs and other features, the simpler the airtightness design will be. The images below shows a simple design with straightforward airtightness junctions, and a complex design with multiple junctions.

Some features can be added after the basic shell of the building has been designed – for example, balconies are less likely to affect airtightness detailing if they are externally supported.

Design for the health of the building fabric

The airtightness layer does not just reduce energy demand. An internal air barrier prevents warm internal air from moving into the building, cooling down and increasing the risk of interstitial condensation. This is different from a vapour control layer that prevents vapour diffusion.

Prevention of vapour diffusion and moisture risk management should have their own design strategy, but may have overlapping elements with the airtightness strategy.

Best practice is therefore to install the air barrier on the warm side of the insulation. In some cases this can be between the structure and the insulation – such as when external wall insulation is installed in a retrofit.

A wind barrier on the outside of the insulation is not an air barrier in this sense, although generally necessary for a PH project.2

Design for airtightness testing

Consideration of the sequencing of the interim and final airtightness testing must form part of the airtightness strategy. Ideally, the airtightness layer within all elements (walls, roof, floor) should be tested while it is still accessible, and therefore when remedial sealing works can be carried out if necessary.

If this is not possible, for example in larger projects, the effectiveness of the airtightness strategy can be checked by sample-testing small sections as the project progresses.

What to do for multiple properties or units in one building

Testing of multiple properties, or units within one building, can be complex if the air barrier has been designed around the whole building. For example, access to flats may be via an external or internal stairwell, possibly also an access balcony or an internal corridor. If flats can only be tested individually, then the airtightness layer should be designed within each flat.

If the airtightness layer is around the whole building, and the flats can only be tested individually then co-pressure testing of two or even more adjacent flats may be required. This is complex and can be difficult, time consuming and costly.

Design for simplicity

Consider the installation of the airtightness materials selected and the experience and skill set of the contractor.

For example:

  • For large spans of roof, it may be easier for the contractor to install an airtight board rather than a membrane
  • Many contractors are familiar with a plaster finish on traditional brick and block walls, so this could be the air barrier, and use a parge coat or airtight paint to seal chasing for services
  • Timber frame construction is likely to require an internal racking board – this could be upgraded to an airtight board of similar strength, sealed with tape between boards
  • In a retrofit situation, external wall insulation could include an air barrier that is installed on the building before the insulation layer

Drawing on a plan or a section in two dimensions with a red pen should be simple, but onsite, an air barrier must be continuous in three dimensions. Key details may need to be illustrated in three dimensions to ensure that sealing requirements are clearly communicated.

Reduce the number of junctions

Reduce the number and complexity of junctions in the building. For example, a dormer window can add many more junctions to a roof, all of which need to be made airtight. They can also be difficult to insulate well. Hence, opting for a rooflight may be advantageous from an airtightness perspective.

More complex architectural shapes can still be added to the external envelope (e.g. overhangs), but as long as they are outside the airtight boundary, then they do not make the airtight layer itself more complex.

Minimise interactions between the air barrier and services

This could be achieved with the use of a service void, for example. Ensure that the spacing between service penetrations through walls or other elements is sufficient to make effective sealing practical, usually ≥100mm.

Design for products and systems

Airtightness product supply is relatively new to the UK and new products are being developed and introduced to the market all the time. Ensure awareness of technical developments by checking websites, or attending CPDs offered by suppliers or airtightness specialists.

Build tight: ventilate right

An airtight building lends itself to a greater degree of comfort for the inhabitants.

Airtightness is also a precondition for effective sound insulation, consequently, benefiting from greater levels of sound reduction from the external environment. To maximise the thermal performance of a construction and to ensure the air in the living space remains healthy, some form of controlled ventilation is essential.

This can be in the form of mechanical ventilation with heat recovery or some other form of natural ventilation. When it comes to airtightness, ‘Build tight, ventilate right’, is the only mantra to ensure buildings are improved, while also reducing shameful, expensive and unnecessary energy waste.

What can go wrong in designing airtight buildings?

The UK has recently seen the completion of some extraordinarily airtight Passivhaus buildings, both domestic and non-domestic. A zero leakage dwelling (i.e. < 0.05 ACH @ 50 Pa) was achieved for the first time in the UK early in 2019.

Experienced teams (architects, contractors and specialists working together) are regularly achieving airtightness values of < 0.3 ACH @ 50 Pa, just half of the normal Passivhaus airtightness target. Yet, at the same time, airtightness is the thing that most commonly goes wrong in UK Passivhaus projects.

What are the main causes of these problems?

  • Incomplete designs
  • Lack of knowledge on site
  • Lack of attention to details

Why can airtightness go wrong in UK buildings?

Incomplete or inadequate design for airtightness is compounded by contractors lacking the appropriate site culture, without the necessary skilled operatives and management teams. This leads to delays, cost penalties and, in extreme cases, the failure to deliver a satisfactory level of airtightness.

Unlike the current building regulations, Passivhaus requires that 100% of builds are tested, both new and refurbished. Generally, there will be one or more preliminary tests and a final acceptance test. Passivhaus testing involves both pressurisation and depressurisation, which facilitates leak detection and is also more representative of real-world conditions.

The average of these will give the final airtightness result. The nature of Passivhaus is that it applies an absolute quality standard, both for airtightness and a range of other factors such as overheating. It is also absolutely evidence based; there is no scope for the uncertainty that sample testing leads to in volume housebuilding in the UK.

For major builds, airtightness coordination throughout the project will require the expertise of an airtightness specialist to ensure the process is effectively managed.

Airtightness is often considered to be the highest risk aspect of achieving Passivhaus certification as it is commonly perceived to be extremely difficult. However, this is primarily due to a lack of knowledge and expertise in the UK construction industry. With careful design and attention to detail both on and off site, and familiarity with the techniques and processes detailed in this guidance document, the risk of failing to meet the Passivhaus airtightness target can be dramatically reduced.

Avoiding these problems should be paramount

Mistakes cost money, sometimes a great deal of money. Avoiding problems involves all the trades.

Airtightness is a holistic process, it requires the linking together of a range of trades over different stages of the build, this begins with the architect’s drawings which are relied upon by all the participants to complete their allotted tasks. Ensuring that a secure airtight layer is not compromised on-site by a following trade is key.

If all trades are aware of what is aimed for and understand the most common areas for failure they are less likely to compromise the airtight layer even inadvertently, it can also help identify weak spots or problems as they arise.

Getting airtightness right on site

Procurement:

Timescales for delivery of airtightness products must be carefully planned and any change request should be managed and agreed with the design team. Airtightness specialists may be required, or training for existing site staff.

Management processes include the planning and delivery of:

  • airtightness training
  • change control processes
  • quality control systems (which may include checking, record keeping and risk registers)

It is usually the role of an airtightness coordinator to take on responsibility for airtightness management, education and training across the site. It is important to define the role as responsibilities relating to airtightness at an early stage, and communicate this to all site workers.

Training:

This may include training on the airtightness strategy and design, installation of airtightness products, airtightness testing or the implementation of airtightness management (see above).

Installation:

Only those site workers who have been trained to install airtightness products should work on the air barrier. Work should be checked and signed off by the airtightness coordinator or another suitably trained individual. Records should be kept of work on the air barrier before it is covered over.

Installers should make sure they have the correct tools and are working on clean surfaces. They should closely follow manufacturer’s instructions for installation of airtight products to get the best outcome.

Testing:

Interim testing is essential for any airtight building. The building should be tested at a stage where most of the air barrier is still accessible, so that remedial works can be undertaken. Smoke pens or thermal cameras can be used to aid leak detection. This can be done by an experienced airtightness tester, or it is possible to rent fans and train up site operatives in leak checking, which may allow for more flexibility and save money on larger sites.

The final test is undertaken upon completion of the building or retrofit and in compliance with Passivhaus procedure (ATTMA TSL4) for Passivhaus certification evidence.

A sufficiently airtight home is the only way that ANY insulation can actually work to 100% of its ability. Thankfully, currently, there is a justified focus on renewable energy sources for our homes and offices. This is to be commended. However, if the building is not airtight, the energy still escapes, in many ways defeating the initial purpose.

Until our building regulations seriously address this issue, a lack of airtightness will always be the primary leak in our heating budgets, causing unnecessary energy loss, the strain on our pockets, potential structural damage, and additional environmental damage in a time of precarious balance, not to mention the negative impact on our own health.

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