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River Studio by Sjolander da Cruz Architects

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Three Passivhaus projects: Hattie Hartman on Sjolander da Cruz Architects’ River Studio

Repurposing an asbestos-clad barn outside Leamington Spa for their own offices gave six-strong practice Sjölander da Cruz its first opportunity to try Passivhaus. ‘We didn’t want to be a slave to the physics and end up with a box facing south with a few holes in it. We wanted to strike a balance between architectural quality and environmental performance,’ says founding director Marco da Cruz.

In consultation with Nick Grant of Elemental Solutions, the architects opted for Passivhaus Enerphit, a less rigourous version of the full standard, adapted for refurbished buildings. Numerous approaches to the structure and fabric were explored before the decision was taken to retain the existing steel structure and wrap it with SIPs panels. Glulams were priced but would have increased the budget by £70,000.

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The bold architectural move was to leave the steel structure exposed and allow for a 60mm gap between the existing columns and the new external wall, comprised of SIPs panels wrapped with an additional layer of insulation to achieve the desired U-values. Battens for cedar cladding were fixed through the external insulation on cantilevered screws. ‘It works really well but I wouldn’t have risked it for a client,’ admits da Cruz.

Every junction with the existing structure was carefully detailed to minimise thermal bridging. An airtightness and taping workshop established guidelines for the contractor, also new to Passivhaus, and the taping was done in a day and a half.

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Construction

Our concept was to wrap the existing structure using SIPs to provide a continuous airtight and windtight layer outside of the structural zone, limit thermal bridging and reduce depth of construction to provide an elegant detailing solution to meet Passivhaus Enerphit standard.

Headroom was restricted so we had to minimise insulation in the floor. As a result, window, roof and wall insulation had to be enhanced. We used Kingspan TEK panels to walls and Kingspan Unidek Aero to the roof for the main fabric to minimise construction depth and optimise U-values.

Limited on the thickness of the TEK panel, an additional layer of Kingspan Thermawall insulation was used to enhance the U-value to walls achieving 0.1W/m2K. The TEK panels were set back from the structure by 60mm to allow for a seamless plaster finish internally.

EJOT SS screws allowed battens to be fixed through the depth of the insulation back to SIPs panels. Western red cedar cladding was used horizontally to reduce thickness of construction. Sto Render on high density EPS insulation is continued below the dpc to limit thermal bridging around the existing concrete slab and column bases.

The airtight membrane sits between the existing structure and roof panel, lapping down and sealed onto OSB. The OSB acts as the airtightness layer for the walls.

Due to cracks and holes in the existing slab, the DPM forms the airtightness layer for the floor, creating tricky junctions with the steelwork, which required flexible SIGA Rissan taping around column bases.

Marco da Cruz, director

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