Goldsmith Street and beyond: Mikhail Riches’ award-winning timber-frame housing

With a strong track record delivering Passivhaus at scale, TDUK members Mikhail Riches have carved out a special niche for themselves designing large, net-zero carbon housing projects.

Although Mikhail Riches‘ catalogue of timber projects dates back to Clay Field in Suffolk in 2009 – built with timber frame, lime-based insulation and shingle cladding – it was their social housing development Goldsmith Street that really consolidated the practice’s agenda to build with low carbon materials such as timber.

The rows of terraced houses on Goldsmith Street are bookended by low-rise flats

The rows of terraced houses on Goldsmith Street are bookended by low-rise flats; image © Tim Crocker.

Trailblazing timber social housing

Completed in 2019 and providing 93 sustainable homes for social rent in the heart of Norwich, Goldsmith Street has become a national exemplar of timber frame housing built at scale. With seven terrace blocks neatly arranged into four rows, the development provides a combination of family-sized houses and low-rise flats – all built to rigorous Passivhaus standards.

As the first new social housing development undertaken by Norwich City Council in 20 years, it was important to the council that the scheme aim for high quality as well as quantity.

The existing city-centre site comprised low-density sheltered housing that no longer met modern standards. Wanting to increase provision of much-needed, good quality homes – that would alleviate fuel poverty as well as housing need – the council ran a RIBA competition with a brief requiring a mixture of houses and flats. They also wanted to see innovative ways to provide public and private amenity space to enrich the lives of inhabitants and foster a sense of community.

Mikhail Riches developed a winning proposal based on a Victorian terraced community.

With tightly arranged but well-proportioned houses, inward-facing back gardens and pedestrian-friendly streets, their design allowed for increased development on the site, while maintaining good spacing between buildings.

From their early design plans, the architects carefully laid out the terraces with a southerly orientation, excellent form factor and sloped roofs that would maximise daylight. By embedding their solar strategy into the masterplan, they were able to adapt their initial design with minor amendments to target full Passivhaus accreditation at the council’s request.

Street view of Goldsmith Street: brick-clad timber housing.

Many people don’t realise that, under the brick exterior, Goldsmith Street is built with timber; image © Tim Crocker.

Material choices

For the design team, it was clear that timber was the optimal way to meet the projects’ aims. “Whichever flavour of timber you want to work with, ” Mikhail Riches’ associate director James Turner explains, “whether it is Propassiv OSB board or CLT, there are lots of case study projects that demonstrate that you can deliver Passivhaus levels of airtightness robustly and with confidence.”

Use of a prefabricated timber frame system insulated with Warmcel was decided upon early on as a low-cost way to achieve the required high quality and Passivhaus specifications. It was also the most suitable option for the architectural arrangement, which has a high frequency of large window openings and irregular roof forms.

Although brick and tile were used for the skin of the buildings, in keeping with the local Norwich vernacular, the use of timber frame meant that the embodied carbon was able to be kept relatively low – to 311 kg CO2/m2.

Passivhaus-certified construction

The hybrid MMC timber frame was designed, manufactured and installed by specialist timber frame subcontractor Cygnum, working closely with Mikhail Riches. The system comprises a twin-wall, sheeted on both sides, with a membrane on the inside forming an airtight layer.

Drawing by Darc Studio showing Mikhail Riches' scheme for Burnholme - one of the City of York sites currently under construction.

Drawing by Darc Studio showing Mikhail Riches’ scheme for Burnholme – one of the City of York sites currently under construction.

The ground floor build-up features 89mm internal load-bearing studs and a second 67mm stud, providing an external wall cavity of 350mm filled with blown insulation, designed to be thermal bridge-free. The intermediate floors and roofs are built using premanufactured cassettes. The engineered timber roof joists are typically 400mm deep, providing plenty of space for additional insulation.

The timber frames were partly pre-manufactured in a factory before being ballooned out on site, then filled with Warmcel blown cellulose insulation made from recycled newspaper. Each unit was tested three times for airtightness, during the build and before handover, ensuring the required quality was met. Passivhaus detail design was provided by consultant WARM, who worked closely with the architects to achieve incredibly high levels of airtightness and prevent thermal bridging.

The scheme was delivered under a traditional contract with Mikhail Riches acting as contract administrator throughout the build, for a budget of approximately £1,875/m2 construction costs (excluding professional fees). By demonstrating the affordable achievability of environmentally conscious, beautiful housing, this pivotal project has had a far-reaching legacy, setting a new benchmark for both social and Passivhaus developments in the UK.

Providing light-filled homes with impressively low fuel bills (of approximately £150 per year), this high-density scheme (83 dwellings/ha) has been duly celebrated with multiple architectural awards, including the RIBA Stirling Prize and Neave Brown Award in 2019.

What’s happened since?

With Goldsmith Street so convincingly demonstrating the suitability and viability of timber-frame in creating affordable and energy-efficient low-rise housing, the question arises: why haven’t more local authorities followed suit? Part of the answer may be rooted in resistance from main contractors – with some companies unwilling to take on the perceived associated risks of timber construction, and others, unfamiliar with timber systems, seeing it as a step into the unknown.

Another major frustration to council-led timber housing is funding, with the Greater London Authority placing a blanket refusal to fund even low-rise timber housing in the capital. For Mikhail Riches, however, there is plenty both on the drawing board and under construction, with ambitious projects across the country – not least a vast multi-site programme of work for City of York Council.

Net-zero housing in York

Just before their Stirling Prize win for Goldsmith Street in 2019, Mikhail Riches were appointed as architects for City of York’s groundbreaking Zero Carbon Housing Delivery Programme. This first-of-its-kind local government development will provide 600 homes across seven sites in York, targeting both Passivhaus certification and net-zero carbon in use.

Leading on from the approach developed at Goldsmith Street, the practice was asked by York council if they could do the same, but at a greater scale – and taking it all the way to net-zero operational carbon. Like Goldsmith Street, the York houses will be built using brick and render cladding with timber frame underneath, provided by timber frame specialist (and TDUK members) Roe Ltd.

The first two of Mikhail Riches’ sites – Duncombe Barracks (34 homes) and Burnholme (78 homes) – are currently onsite and due for completion this year. Caddick Construction have been appointed as the principal contractor to deliver the two schemes. The third site, Ordnance Lane, will provide almost 100 new shared ownership/social rent homes, with construction expected to start shortly.

Both the scale of the development and its ambitious set of deliverables represent a major commitment to housebuilding and sustainable construction in the region, boosting local skills and providing certainty to the market.

This article appears in Designing Timber issue 7