Products in Practice: Wraptite at St Sidwells Point

Intro

Good morning everyone and welcome to today’s webinar, our fourteenth of 2022. If you’ve missed any of our series, which has been running for over two years now, you can view them all on-demand right here on you YouTube channel, or on our learning hub at www.proctorgroup.com. You can also now register for our online members area, where you can access product information libraries, personalised CPD certification and our free online u-value and condensation risk calculation.

As always, you can also request product samples, arrange follow up meeting to discuss the specifics of your project, or book one of our expanding range of RIBA assessed CPD’s covering a range of topics. This can all be done either face to face with our team of experts across the UK or online. Today we’re continuing our products in practice series of case studies with a look at St Sidwell's Point leisure centre in Exeter.

Designed by Space & Place Architects alongside Passive House designers Gale & Snowden and delivered on-site by main contractor Keir, St Sidwell's Point is the world’s first Passivhaus-certified multi-zoned leisure centre.

This 6700 square metre, 44 million pound project is a state of the art showcase for forward thinking sustainability focussed design, and uses our Wraptite vapour permeable air barrier as part of the highly efficient external wall construction.

So today we’ll take a look at the performance of this unique project, alongside the systems and technology used to achieve these results. We’ll also disucss the challenges involved in the on-site processes and how they were overcome through training and up-skilling.

We’ll finish up with our regular Q&A sessions where we’ll be joined by special guests Kate Swain and Stephen Booth from Keir Construction, as well as architects Maria and David Gale to answer your questions and disucss the project in more detail.

SSP Project Overview

Planning of the development at St Sidwells point began in 2011, with this site being the centrepiece of a wider regeneration project across central Exeter. This wider regeneration includes residential, retail and office developments and aims to enhance the city centre both economically and in terms of health and wellbeing.

Through their existing housebuilding program, Exeter City Council have over a decade of experience working with Passivhaus projects. So when planning to replace the city’s 50-year old Pyramids swimming pool, the Council was confident the passive principles could be applied to deliver not just substantial carbon emission reductions, but also a healthier and safer environment.

Developed on a brownfield site, the new centre comprises 25m main pool and 20m learner pool, a confidence water pool and a 100 seat spectator area. There also a variety of dry side facilities including a cafe, soft play area, gym and heath suites and a spa.

As well as being the worlds first Passivhaus certified multi-zoned leisure centre, St Sidwells point is also designed and constructed Building Biology IBN best practice in health building design, and is resilient against predicted climate change to 2080.

Leisure facilities, and particularly swimming pools are usually the most energy intensive buildings in a local authorities stock, so reducing consumption can have a major impact on an authorities energy footprint.

According to sibzee (i.e. CIBSE), a typical leisure centre will consume 1573 kilowatt hours per metre squared per year, or which 237 kilowatt hours is electricity for lighting and services, while 1336 kilowatt hours is used for space heating.

In contrast, the target energy use for Saint Sidwell's point is just 375 kilowatt hours per square metre per year, a 76 percent reduction over a typical design, and almost half the energy use of even recent “best practice” .

At the same time, it provides a 50% saving in water use, and the pool water contains less chlorine than tap water. In combination with state of the art ventilation and extensive use of natural materials provides a significantly healthier environment for the 500 thousand visitors anticipated annually.

Those visitors are important, as the project is funded by revenue generated, neither building cost nor running costs are funded through council tax. Prior to this years energy price rises, the predicted utility cost was just £20 per square metre against around £60 for a conventional design, saving 200 thousand pounds annually and giving a payback period for the sustainable design features or less than 10 years.

Energy Saving - Zoning & Systems

As leisure centres are large, complex structures with many different internal environmental conditions to accommodate, carefully manging these multiple zones was a key part of the energy performance strategy.

While the pool areas require a substantial amount of space heating, the dry-side areas such as dance studios and gym facilities often require cooling.

The building is therefore laid out with the pool areas on the glazed south and west sides of the building, to maximise the free passive heating from solar gain. These areas are also separated from the rest of the building with well sealed doors.

The gym and studio areas are on the north and east sides where solar gain is reduced. The heat loading produce by activities and equipment in these areas is removed by means of heat pumps, with the recovered heat used to raise the temperature of the pool water and other areas of the building.

Intermediate environments, such as administrative areas, are located in the centre, creating a thermal buffer between these heating and cooling zones.

Several plant rooms of varying sizes are distributed around the building rather than centralised, to minimise the length of duct runs required, and hence the associated efficiency losses.

The use of highly efficient air and water source heat pumps mean waste heat can supply the majority of the buildings space heating requirements. The environmental conditions in the pool areas are also optimised to minimised evaporative heat loss from the water.

The pool halls are heated to 31 degree and kept at 64 percent relative humidity, and the two smaller pools are also drained overnight. This reduced evaporation also reduces the require ventilation rates as less dry air must be supplied.

Energy Saving - Building Fabric

These efficient and precisely controlled internal environments make the performance of the building fabric extremely critical to delivering a good result. The foundation of passive house design is ensuring a robust, fabric first approach to efficiency, and this is no different here, just scaled up.

The five main principles of passive house design are:

Thermal Insulation Thermal Bridge Free Design Airtightness Ventilation with Heat Recovery and Passive House Windows, or more generically: high performance glazing.

We saw earlier how the ventilation and glazing systems play an important role in maximising space heating efficiency through using solar gain and waste heat effectively. Lets now look more at the build-up used in the façade systems.

The main considerations here are high levels of thermal insulation, with thermal bridging reduced as far as possible, and low levels of unplanned air movement through the fabric.

There are three main types of wall structure employed at St Sidwell's point, with some sections being a steel frame construction, others blockwork, and lastly cross laminated timber.

What the three type of construction have in common is that the insulation and air barrier layers are fitted externally to the main structure.

The insulation across all three wall types comprised 250mm of mineral fibre insulation with thermal conductivity of 0.034 watts per metre kelvin. Because this thick layer of insulation is placed outside the structure, thermal bridging is limited to just the fixings required for the outer cladding system.

These brackets are thermally isolate to minimise their effect on the u-value, which comes in at 0.14 watts per metre squared kelvin. The external location of the insulation also makes it relatively easy to wrap around the entire building, around junctions between walls, roof areas and floors.

This approach also extends to the air barrier.

Our Wraptite self adhered membrane is used as the primary air barrier across all the façade build-ups, and like the insulation is placed externally to the main structure. The build up is completed by the Fireshield vapour permeable membrane on the outside of the insulation.

This protects the insulation from weather during construction, while also providing a secondary airtight layer. Fireshield also has a Class B-s1, d0 reaction to fire classification and a unique intumescent coating, which as can be seen in this test video, helps limit the spread of fire in the outer cavity behind the cladding.

We’ll disucss the installation process in greater detail later, but in this location it’s far simpler to install the air barrier to a high standard, as there is less sealing and jointing work required around services and other penetrations.

This air barrier allowed the air leakage for the building to exceed the certification requirement, achieving 0.3 metres cubed per square metre per hour at 50 pascals, again a target of 0.4.

The final air test result of 0.3 metres cubed per square metre per hour at 50 pascals equates to an equivalent leakage area of 770 square centimetres or just over 1¼ sheets of A4 paper!

St Sidwell’s is a pilot scheme for the Passivhaus Institute and their Airtightness target for leisure centres is an Air Permeability of 0.4 metres cubed per square metre per hour. The St Sidwell’s project has meet and exceeded these airtightness requirements.

This is a huge achievement and is a really important scheme for the Passivhaus Institute, and the wider industry as it demonstrates that large scale, complex buildings can be delivered to meet the stringent Passivhaus certification requirements.

Each stage of the programme reflected the focus on achieving the standard. The programme delivery required full understanding of the sequencing, specification & detail of the build. The Air tightness strategy and testing regime was built into this with specific time allowances made for this within the programme.

The insulation (joints), taping, spraying, sealing, parge coats & setting up of air tests which typically would be non-critical path had to be included thought about & planned. There were also specific hold points set, for example testing of rooms or floors as a shell and again once completed. The early engagement with sub-contractors and collaborative approach was key to understanding the sequence, logic and methodology involved and ensuring the best performance was achieved with minimum remediation.

Hygrothermal Considerations

Maximising the thermal performance and reducing the air leakage also makes the management of moisture more critical, particularly when dealing with a warm humid environment like a swimming pool.

The warm frame configuration used here, with the insulation external to the structure, is inherently less prone to moisture issues, but it’s still important to ensure the system performs as expected.

In this case the use of wraptite over the sheathing with insulation externally meant that there was a vapour permeable membrane in the location where a vapour control layer would typically be installed.

To verify the performance of this build up, our team undertook a detailed hygrothermal analysis of the construction.

The first step in this process is to conduct a u-value calculation to BS EN ISO 6946 and an associated condensation risk calculation to BS EN ISO 13788. This process determines the temperature gradient and dew point throughout the structure and delivers a good overview of the performance.

We have a bespoke calculation system developed in-house to undertake these assessments, which is available to registered users on our website. The software uses of postcode specific climate data and can produce fully complaint pdf reports for submission to building control. You can also collaborate with you team across multiple saved projects, and submit calculation for review by our team of experts.

In the case of St Sidwells point, there was a desire to go into more detail about the performance of the façade system. Our team therefore undertook the more advanced BS EN 15026 dynamic moisture assessment we’ve discussed in our webinars before.

This assessment gives our-to-hour results instead of a simple monthly figure and gives more detail on the temperature, relative humidity and water content across the façade assembly.

These graphs show the hygrothermal characteristics through the SFS facade, with the external side on the left. The graphs on the right have a conventional vapour control layer over the sheathing while on the left hand side this is replace with he vapour permeable wraptite.

While there is a minor variation in moisture build-up between these two constructions, this is not of sufficient magnitude to cause concern.

We can further use this model to look at the moisture content at various points within the fibrous insulation itself.

We can see here that while there is a high relative humidity at the outer surface at some point of the year the average is significantly below the 70% threshold at which condensation risks would be expected. Humidity within the insulation layer itself is also substantially below this.

Again there is very little difference in performance between Wraptite and an impermeable VCL.

Finally we can assess the water content of the layers, which shows the hygroscopic sheathing board store the vast majority of any moisture accumulations. This mean the performance of the insulation will not be reduced by excessive moisture uptake.

So we can see from both the 15026 and 13788 calculations that this construction performs well hygrothermally, with or without a conventional vapour control layer. In practice a vapour control layer, our Profoil 861, was used in the pool hall areas.

Installation

So we’ve seen how the on paper performance works, but if the products and systems are not properly installed on site these ambitious performance targets will not be met.

Delivering Passivhaus requires collaboration at every stage to be successful and this was recognised early by the delivery team. Any part of the wider team could negatively impact the air test result or energy performance when working to such a tight standard.

Therefore the quality culture had to be set and consistently disseminated throughout the client, design and construction teams, throughout the supply chain. Collaboration and quality control were significant factors considered during procurement. Over 2,500 people worked on the project post-contract. One of the first challenges was to ensure a full understanding of Passivhaus and Kier introduced the Passivhaus Passport. The passport is awarded for the successful completion of Passivhaus induction and training. All operatives take part in the induction and the training is specific for each trade.

The supply chain involvement included 71 different trades and it was clear it wouldn’t be feasible to have site managers checking every detail, so the idea of the Passivhaus Passport is to empower the trades and also gain buy in to the principles. The modules were developed with consultants Warm and then rolled out across the wider team.

Some trades, for example groundworks, M&E and façade are integral to achieving the air tightness rating, which is essential for Passivhaus certification, so their training was very detailed and complex around minimising thermal bridging, for example. Developing a quality culture and pride in the work undertaken was a critical element and a key part of this collaborative approach was establishing a ‘no blame’ culture. It was essential from the start that if a mistake was made, that people felt comfortable to raise it, so it could be rectified, or a solution found.

These risks were further mitigate by undertaking extensive training using mock up panels to test significant details and work through interaction of different suppliers, as well as testing alternative build-ups, sometimes introducing risk mitigation measures such as secondary air permeability barriers in key locations.

Client, designers, contractor and supply chain have worked closely throughout to deliver a successful outcome. As part of this training, we delivered our Wraptite toolbox talk onsite and conducted practical training using the mock-up panels to demonstrate the optimum installation process for Wraptite.

The project team at St Sidwells have exceeded all expectations and delivered a building with almost no performance gap to the highest of design standards. The strength in collaboration with the client and wider project team and the commitment throughout the journey have always inspired the trust of everyone involved and the confidence that the project would be nothing other than a success.

The team have demonstrated exactly that, not just achieving the challenging airtightness requirements but exceeding it, which was essential for gaining Passivhaus accreditation.

Finally, the legacy for the local community includes the engagement throughout the build, delivering 925 hours of employment skills support and 34 T-level placements over 45 weeks, inspiring young people to join a forward-thinking sustainable industry.

St Sidwells Point represents the future of sustainable construction in both technology and working practices, and will benefit generations to come who will enjoy the unique quality swimming experience.