Passivhaus massivhaus: child's play at Sutton Secondary

Still seen as a fringe of wider construction, large & complex Passivhaus buildings are on the rise in the UK. Situated on the site of the former Sutton Hospital, Sutton Secondary School is set to become the UK’s largest Passivhaus school - With an internal floor area just shy of the University of Leicester’s George Davis Centre, currently the largest non-domestic Passivhaus in the UK.

Earlier this month the Passivhaus Trust & Building Performance network (BPN) held a site visit to the scheme in progress, with presentations & tours from PHT Patrons Architype, Willmott Dixon & Warm.

Sutton Secondary - Sports Hall

Christian Dimbleby - Associate; Architect and Chartered Engineer,Architype conducting the site tour

The £36m project for London Borough of Sutton and the Harris Federation is expected to be complete and ready for occupation by the start of the 2019 term in Autumn. The project, aiming for Passivhaus certification, was procured through the existing Richmond, Sutton and Merton Framework and forms part of the One Planet Sutton vision.

Studies conducted by Architype on previous Passivhaus & non-Passivhaus schools all acknowledge that classrooms with 30 children need a lot of ventilation! This is a key driving factor which has sold the Passivhaus concept to many school clients thanks to convincing data monitoring proving that the Standard delivers where most other options (many reliant on natural ventilation) do not. Occupant comfort coupled with closing the performance gap, low operational costs, and future-proofed buildings made Passivhaus an easy choice.

Key Stats

No. occupants: Approx 1400 pupils & staff
Completion due: Summer 2019
First phase of London Cancer Hub masterplan
Form Factor: 0.7
Construction: Concrete frame & CLT
GIA: 10,600 m²

Sutton Secondary School - proposed massing ©Architype

Originally faced with objections from neighbours who were concerned about the scale & privacy issues onto Chiltern Road, the 3-storey scheme has evolved after several iterations and is cleverly half sunk into the ground. As such, a concrete frame is used at the lower levels, with Cross Laminated Timber (CLT) on the upper storeys above ground creating a lighter structural load and helping to reduce the size of the required piles. The structural CLT accommodates large room spans, simplifies details and creates a degree of flexibility, allowing rooms to be divided as and when required for changing/ multiple uses.

Sutton Secondary School

The scheme adopts a simple palette of robust materials which include aluminium/timber composite triple glazed windows, raw internal finishes of exposed CLT, concrete soffits and walls. External materials include douglas fir timber and brick & copper cladding that hides guttering and drain pipes.

With such a large site there are complications - As heat loads are so large due to multiple devices for a high number of occupants, after careful consideration from the Passivhaus Institute & certifier Warm, the scheme has been granted an allowable energy demand slightly higher than the typical 120kwh/m²/a. Passivhaus target.

L: 2 small boilers supply all the radiator heating in the 10,600 m²building. R: View from a top storey window down into a courtyard.

Design Detailing

Sutton Secondary School - detail ©Architype/ Willmott Dixon There are several impressive factors on this scheme which have been learned and streamlined from the George Davies Centre (GDC) - Key lessons were to engage in a contractual agreement that allows early involvement of the contractor and their supply chain partners to help inform the detailed design – and to finish the design as much as possible before construction.

Getting everyone involved early assists in confirming value engineering issues and optimise construction sequencing to minimise changeover of trades. Importantly for a scheme with a high primary energy demand, it allowed coordination with mechanical electrical & plumbing (MEP), furniture fixtures & other equipment (FF&E), and end-user client to ensure appliances are as low energy as possible.

An extensive design programme was put in place to allow time to clearly detail every junction / penetration for airtightness. This involved colour coded 3D drawings with accompanying notes. Passivhaus details are more complicated and require more explanation.

Getting it right on-site

Training: Almost all the subcontractors on this project had never worked on delivering a Passivhaus. Fore-seeing issues, the key contractor Willmott Dixon made it mandatory for all key trades personnel to undergo a pre-construction Passivhaus induction and guarantee that at least 2 people from each company who attended this course would be available on-site. Regular tool-box talks are conducted, and a no blame culture is promoted – with emphasis placed on expectations of quality instead.

Sutton Secondary School - site details ©Willmott Dixon

Willmott Dixon has a key group of site managers, who have undergone the Certified Passivhaus tradesperson training, and who can tap into the knowledge learned from the George Davies Centre team. There are also several on-site Passivhaus Champions to check quality control.

Testing: Similar methodologies for testing were adopted as used at the GDC. Unusually, the airtightness membrane is on the outside of the structure (though still inside the insulation layer) to help avoid defects during construction. Early mock-ups were created off-site, which also helped with developing sequencing. Interim sectional testing helped address any quality or performance issues at an early first installation stage. Tests were conducted again at a later stage on larger sections. Any possible changes are run through the Passivhaus designer to check there is no significant Passivhaus impact.

Sutton Secondary School - Testing ©WARM/ Architype

Prefabrication: The scheme utilises its own on-site ‘flying factory’ to precision manufacture all the timber components. This helps maintain the quality standards of Passivhaus.

Sutton Secondary School - construction sequencing ©Willmott Dixon