St Sidwell’s Point in Exeter is the world’s first Passivhaus-certified, multi-zoned leisure centre. Developed on a brownfield site, it comprises a 25m main pool and 20m learner pool, a confidence water pool, and a 100-seat spectator area. There is also a cafe, soft play area, gym, health suites, and spa.
The £44 million, world-leading project includes the A. Proctor Group’s Wraptite external air barrier as part of its highly efficient external wall construction. Designed by Space & Place Architects, alongside Passive House designers Gale & Snowden, the project was delivered by Keir. As well as Passivhaus certification, St Sidwell’s Point was designed and constructed to Building Biology IBN best practice in Healthy Building Design.
APPLYING PASSIVHAUS
Leisure centres are complex buildings with different internal environmental conditions to accommodate. Carefully managing the various requirements – ie, heating for swimming pools and cooling for fitness studios – is vital to the overall energy performance strategy. The starting point, especially in a Passivhausstandard building, is the building’s fabric performance. For St Sidwell’s Point, primary considerations for the facade systems were high levels of thermal insulation, minimal thermal bridging, and low levels of unplanned air movement.
The building featured steel frame construction, blockwork, and cross-laminated timber (CLT). Common to all three was the placement of the insulation and air barrier to the outside of the structure, contributing to the target U-value of 0.14W/m2 K.
Wraptite is a self-adhered membrane that is both airtight and vapour permeable. Positioning it externally moves the airtightness barrier away from the internal services zone, simplifying detailing and reducing the number of penetrations. It is also a Passivhaus-certified component, reinforcing its credentials as a high-quality product that can help projects to meet the standard.
The Passivhaus standard sets an airtightness requirement of 0.6 air changes per hour (ACH) at 50 Pascals pressure. As a pilot scheme for the Passivhaus Institute, a target for leisure centres of 0.4m3 /m2 /h@50 Pascals was set for St Sidwell’s Point. With the help of the Wraptite air barrier, it achieved a rate of 0.3m3 /m2 /h@50 Pascals – the equivalent of an opening the size of 1¼ sheets of A4 paper across the whole building envelope.
HYGROTHERMAL CONSIDERATIONS AND DESIGN
Highly efficient and airtight building fabric makes moisture management critical – especially in a warm, humid swimming pool environment. Placing insulation to the outside of the external walls means the structure is inherently less prone to moisture issues, but it is still essential to carry out risk assessments.
To verify the performance of the proposed wall build-ups, the A. Proctor Group technical team undertook a detailed hygrothermal analysis of the construction. The first step was to conduct a U-value calculation to BS EN ISO 6946 and an associated condensation risk calculation to BS EN ISO 13788. This process determined the temperature gradient and dew point throughout the structure and delivered a good performance overview.
Given the complexity of St Sidwell’s Point, including the moisture loads of the swimming pools, there was a desire to go into more detail about the performance of the facade systems. The technical team undertook a more advanced BS EN 15026 dynamic moisture assessment using WUFI software. The outputs from the assessment showed the hygrothermal characteristics through the facades. The benefit of a dynamic assessment is that it gives hour-to-hour results instead of simplified monthly figures. It also provides more detail on the temperature, relative humidity, and water content across the facade assembly.
As such, it was possible to determine that the humidity levels within, and on the outer surface of, the insulation layer were below the 70% threshold at which condensation risks would be expected. In addition, the hygroscopic sheathing board stored the majority of any moisture accumulations, albeit these were low. This means the insulation’s performance will not be reduced by excessive moisture uptake.
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