This month we’re celebrating news of an excellent air permeability test result at St Sidwell’s Point, the world’s first multi-zoned Passivhaus leisure centre, but two questions we often get asked are: why do we air test buildings? and, how significant is Exeter’s new state-of-the-art leisure centre? Before we answer these questions, let’s address some basics first…

What is Air Permeability?

Air Permeability is the air volume leakage rate per hour through the area of the envelope of a given building. The units for this metric are cubic meters per hour per square meter (m3/h/m2) and are standard practice in the UK. Passivhaus buildings typically use the alternative metric of Air Changes per Hour, which could be described as the air volume leakage rate per hour per net building air volume (m3/h/m3). The units for Air Changes per Hour can be abbreviated to h-1, and often stated as ‘ach’.

Why is airtightness important?

Heat energy is lost from our building through uncontrolled air ‘infiltration’, or leakage when air is forced through our building envelope by the difference in air pressure. The air pressure is caused by wind and the buoyancy of warm air. A higher rate of uncontrolled air infiltration through our building envelope means a higher rate of heat loss from our building. This is different to the air controlled through ventilation systems, which circulate air to provide an acceptable quality of indoor air for us to breathe!

How is air permeability measured?

Air Permeability and Air Changes per Hour are measured by pressurising the building to 50 Pascals and then measuring the rate of airflow needed to maintain the building at a constant pressure. The result of the test is then factored by using the building envelope area or net building air volume according to which metric is needed to determine compliance with the relevant standard.

What are the airtightness targets at St Sidwell’s Point?

The limiting fabric parameters in the 2016 edition of Approved Document Part L require an air permeability of 10.0m3/h/m2, with a welcome but meagre improvement in the 2021 edition. As part of the certification procedure for St Sidwell’s Point, the Passivhaus Institute requested an air permeability target of 0.2m3/h/m2, with a threshold of 0.4m3/h/m2 required for certification. Our final airtightness test result was 0.3m3/h/m2, testament to the exceptional efforts of the whole project team. For our building’s net floor area, shape and form, this equates to 0.1 Air Changes per Hour.

So just how significant is Exeter’s new state-of-the-art leisure centre?

Within a few weeks, St Sidwell’s Point will be the world’s first Passivhaus-certified multi-zoned leisure centre, following on from two predecessor swimming pools, in Bamberg and Lünen, Germany. In Exeter, our community can be proud to have a comfortable, healthy and low-energy facility available for use by the whole community. With the final airtightness test complete, we’re thrilled to be approaching practical completion. From our dual role on St Sidwell’s Point as Architect of the Building Fabric and Passivhaus Consultant, we at Gale & Snowden are already applying our Lessons Learnt to other Passivhaus leisure centres in the UK. Watch this space!

What technical design strategy did Gale & Snowden employ for airtightness?

Pre-tender, our technical design was based on Gale & Snowden’s 25-years’ experience as Architects and Mechanical Engineers undertaking design and consultancy appointments on award winning projects. Projects used various external envelope assemblies on schools, flats and other housing schemes, and in many cases, this resulted in plastered clay or concrete blocks as a robust airtightness layer. This is because, in a plastered or rendered wall, an airtightness perforation is clearly visible and can be accessed late into construction. This is particularly important should an on-site air test reveal a perforation that needs addressing.

St Sidwell’s Point makes use of an in-situ concrete primary frame, shear walls, ground floor slabs and retaining walls, and the original design included for internally rendered autoclaved aerated concrete blockwork walls. The areas of concrete frame are inherently ‘airtight’, but the areas of infill wall also required a level of airtightness to meet our targets. Post-tender, the contractor wanted to swap out areas of rendered blockwork, a wet, resource-hungry trade, for the much faster (and drier!) steel framing system (or SFS) and adhesive membrane-lined cement particle board. This assembly was already present on the building over a few isolated wall areas, but we were apprehensive to accept an amendment to the design where the risk of airtightness failure could significantly increase. After a period of investigation, redesign and on-site mock-up testing, the feasibility of this method of construction in achieving Passivhaus levels of airtightness was determined. We amended our design to increase the use of SFS to 70% of the external wall area.

What measures were put in place to reduce the risk of airtightness failure?

Ever wondered that failure might not be an option? On a £44m leisure centre, airtightness failure is definitely not an option we wish to choose, but it’s a real possibility without the correct design, planning, preparation and workmanship. The following critical factors can be employed to manage this risk and ensure the successful implementation of an airtightness design.

    • A design that considers the sequence of construction and the need to leave airtightness elements exposed for airtightness testing;
    • An airtightness testing strategy that includes interim, sectional and mock-up airtightness testing;
    • Coordination between the airtightness design and airtightness testing strategy as a whole;
    • Meticulous site control to capture penetrations caused by follow-on works; and,
    • An integrated approach to airtightness as part of the construction programme.

What were the greatest challenges in achieving an airtight design?

Challenges were faced by the whole project team, within different works packages and of different perspectives. It’s easy to understate the amount of human and material resources needed to install an extensive envelope to such low levels of air permeability. For example, to achieve an ‘airtight’ installation of a double door, the door should meet a specified air permeability class to EN12270 and the connection between the door and each of the adjacent elements must be sealed. If the door is glazed into a curtain wall, then similar attention to detail is needed at the transition between the curtain wall mullion and the threshold of the door, and then there’s door closer fixings! At St Sidwell’s Point, much of this was subjected to product changes, critique, review, consultation, testing, and so on.

So what were the lessons learned during the works?

We will try to briefly list and explain a handful of the many lessons learned that relate specifically to detailed design and construction of the airtight envelope at St Sidwell’s Point.

Partial air testing:

Works progress differently across large commercial sites than they do across typical domestic or residential projects. At predetermined points during the construction, the envelope must be available for air testing and leak finding. On a large building, it simply is not practical to carry out interim air tests on the whole building. A building designed for partial air testing, with logical places for internal airtightness zoning provides the contractor with convenient pockets to preliminarily test in stages as works progresses.

Fixings through sheathing boards:

The type of fixing, type of substrate and number of fixings of each type across the envelope are all critical factors in the total air leakage resulting from the fixings overall. Facade brackets, insulation, secondary structure, signage, and electrical items all result in fixings through an airtightness layer or component in our building. Imagine realising that standard insulation fixings could potentially result in a gross failure of the airtightness test, by design! Seriously, watch out for this one… this actually happened to us post-tender, and resulted in us requesting the submission of every fixing in the building envelope for our approval.

Material wastage:

One estimate from site was that 30% of adhesive airtight membranes needed replacing as work progressed. This was for a variety of reasons, including damage by follow-on works and temporary works. With such a tight target, it’s understandable why a contractor would rather replace so much membrane to control the risk of failing the airtightness test. Perhaps buildability of the airtightness around temporary works can be considered by the design team pre-tender, possibly using BIM and early contractor input, and perhaps this could minimise the impact of the works on material wastage?

Complex building geometry:

Consider this the curse of the curvy building. Put simply, simplicity in building form reduces cost, reduces variations in interfaces between elements and can still be designed, aesthetically, to fit within the context. Reduced variations between interfaces simplifies the airtightness design.

Concluding thoughts…

So why isn’t more being done to reduce energy losses through infiltration? The team at St Sidwell’s Point have this month demonstrated that it is possible to build to exceptional levels of airtightness in large, complex, multi-zoned buildings. The techniques developed by the design team and contractor can now be taken forward into future projects, and we commend everyone involved in accomplishing one of the most significant Passivhaus challenges to date.