29 June 2021

Sutton’s sustainable schools: Hackbridge Passivhaus Plus Primary School

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Hackbridge Passivhaus Plus Primary School. Photo: Andy Stagg / Architype.

 

Hackbridge Primary School is a beautiful two-storey timber building, set into its landscape with the aim of regenerating the Metropolitan Open Land, acting as a flagship for Sutton Council’s ‘One Planet Living’ sustainability strategy.

 

Welcoming and modest, the school is a model for all future small-scale buildings, such as housing and community facilities, that are built to meet the 2030 climate challenge.

 

Cladding

Described as a ‘modernist version of a log cabin’, the building is clad inside and out with timber. Externally the facades are covered with sweet chestnut vertical battens in two different thicknesses, so as to provide a rhythm to the exterior, some of which run in front of the windows unifying the building’s whole form. Internally, birch-faced plywood linings, balustrades and furniture, together with softwood batten slatted ceilings, help the building enthuse a link back to nature and create an inspiring and healthy environment for children. The timber here is 21st Century precision-cut computer numerical controlled, manufactured from a 3D model to ensure perfect alignment and spacing on the building. It gives a crisp modern quality more associated with bespoke furniture, as opposed to the raw rustic features of 20th Century timber construction, which contrasts with the external courtyards where fallen trees have been carved into seats and sculptures.

 

Windows

With large amounts of glazing designed for both solar optimisation and connection into the landscape, the design uses triple-glazed composite timber and aluminium windows to frame the views of the Site of Importance for Nature Conservation and the iconic BedZED eco-village.

 

Timber frame

Hackbridge is constructed from lightweight timber framing allowing it to sit lightly on the ground on a shallow insulated formwork concrete raft foundation. Above this, up to and including the roof, the construction is fully timber based, using a panelised structural timber frame with glued laminated timber (glulam) columns and beams, a Larsen truss (timber boarding framing out the external insulation zone) and it is filled with Warmcel recycled newspaper blown insulation. The only non-timber based elements of the construction are the high-performance roofing system, intelligent membranes providing waterproofing and airtightness, the small amount of earth-toned cement boards that open out from the double height hall space and the anodized aluminium perforated artwork designed by the children at the entrance.

 

Net zero carbon

The timber structure is strong enough to support the bio-solar roof, which is made up of a variety of plants including sedum, ferns and wildflowers such as cowslips; this substrate secures the solar panels that are designed to generate more than 100% of the electrical demand. This detail, combined with a ground source heat pump and inter seasonal store (which provide free heating and cooling), make this building beyond net zero operational energy to form a climate positive building.

 

Secondary school Harris Academy Sutton, a four-storey 10,625m2 building, is an exemplar in achieving sustainability in a larger build. Although there are many differences in design compared to Hackbridge Primary School, the use of high-quality timber is consistent.  

 

It is the first building created as part of Sutton’s masterplan for the London Cancer Hub – a campus for research, treatment, education and enterprise. The school has been designed as a state-of-the-art science academy, with laboratories and technology suites, and there is a focus on health and well-being throughout. Natural light and excellent air quality (through low carbon dioxide levels) are achieved through the Passivhaus ‘gold standard’ of construction.

 

Structure

The school has hybrid cross-laminated timber (CLT) floors and walls, with external structural glulam columns above the first floor that has a concrete base due to the significant gradient on the site, giving level access on both ground and first floor.

 

In the main block, the external walls are typically non-loadbearing timber infill, but the sports hall stands alone with natural light falling on to exposed CLT on all sides. This includes the roof, which is supported with spliced glulam beams.

 

The biophilia strategy includes all classroom spaces having exposed ceilings (timber, CLT and concrete on ground), with acoustic rafts below and exposed services, giving a contemporary and sophisticated appearance. This strategy, alongside the exposed CLT wall in the corridor, brings a natural harmony to the building, as well as creating a calming and non-stressful learning environment. 

 

Solar shading

Considerable care was taken in how to protect the east- and west-facing windows from direct sunlight, as well as how to highlight the vertical nature of the circulation cores and break up the mass of timber of the ground scraper; large Douglas fir fins were mounted in front of the windows and curtain walling to provide shading and direction.

 

Summary

A new contemporary form of timber building is being established in Sutton, one that achieves low-carbon future design standards, and produces healthy interiors and aesthetically dynamic buildings of differing scales appropriate to 21st Century needs.

 

About the author

Christian Dimbleby, Associate, Architype

 

This is a new release from the 2020/2021 Industry Yearbook Online. Download the article, including further reading and a full list of references, here