Feature

New model village

Thin-joint masonry is at the heart of Studio Partington’s low-carbon community, writes Pamela Buxton

For a project that’s just won a RIBA Award, Derwenthorpe near York is the antithesis of iconic. There’s no attention-grabbing use of form or showy materials. Nor is there any overly conspicuous display of renewable energy gadgetry that shouts the development’s low-carbon aspirations.

Instead, the focus is on creating a new community with a sense of place and identity that is both socially and environmentally sustainable. In the award-winning first phase, this has been achieved with the use of thin-joint masonry construction.

Derwenthorpe has been something of a labour of love for architect Studio Partington, which has spent 10 years on the Joseph Rowntree Housing Trust-led project. The aim was to establish a high-quality, energy-efficient, mixed-tenure community of 540 houses plus associated energy centre on the outskirts of the village of Osbaldwick.

Working with a PRP masterplan, Studio Partington drew inspiration from the nearby New Earswick model village established in the early 20th century as well as the English Arts and Crafts tradition and post-war rural social housing. In addition to 64 homes, the first phase also delivered the all-important landscape infrastructure that includes a lake, a series of public/play spaces and pedestrian-friendly “homezone” streets. Everyone has a garden but there is generous green space throughout too. Divided into four phased quarters, the site is intersected by the Sustrans cycle route leading into the centre of York. Power lines were undergrounded to avoid the adverse visual impact of pylons passing through the new community.

The development is purposefully low-density and tenure-blind, with the 40% social housing provision pepper-potted throughout all four quarters. There are 12 basic house types (with additional variants) including generously-proportioned, 85m2 two-bedroom homes with scope to add a third bedroom in the loft to cater for growing families. Dual-aspect living rooms and high ceilings of 2.6m and 2.7m contribute to a feeling of light and space. All share a restrained architectural and material palette including distinctive, steeply pitched roofs. “Rowntree and York City Council were really trying to make something very special,” says Studio Partington’s Richard Partington.

Low carbon was a priority, with the brief that homes should exceed Code for Sustainable Homes Level 4 and be capable of being upgraded to Level 5 with the addition of 4-10m2 of photovoltaics. Three homes are Level 5. Studio Partington generally advocates a fabric-first approach to creating a high-performing envelope with low air-permeability and good U-values. The post-financial crash slowdown gave Rowntree the opportunity to build and test two prototype detached houses using different modern methods of construction in order to analyse the comparative performances and investigate the often considerable gap between expected and actual performance.

These prototypes were factory-built SIPs (structural insulated panel system) and thin-joint masonry with a traditional trussed rafter roof. Both used MVHR (mechanical ventilation with heat recovery) systems. The practice had previously worked with thin-joint masonry construction and were confident that it could deliver the desired advanced U-values and quality of build. The thin bed incorporates large-format 100mm masonry blocks with joints of epoxy resin rather than traditional mortar. These are as thin as 2-3mm compared with the traditional 10mm, thus reducing thermal bridging. This inner blockwork leaf is combined with a partial fill cavity of 100mm rigid insulation and a 50mm ventilated cavity along with the final leaf of external facing masonry. Party walls have two leaves of 100mm dense concrete block with insulation in between and sealed at the perimeter.As well as the improved thermal performance, the thin bed offered the ability to build the whole inner leaf first to achieve an immediate watertight structure, followed by the windows and sheet insulation before the outer leaf of masonry is built. This allows easy visual inspection of the cavity side of the blockwork for the fill process, thus encouraging better delivery of both insulation and airtightness.

Another advantage of masonry over timber frame was that the former is more resilient to being left partially completed in the landscape – an advantage given the odd phasing that can occur on a tenure-blind site in the rush to deliver the affordable homes first. This structure also scores highly on fire and acoustic considerations.

Analysis by Leeds Metropolitan University’s Centre for the Built Environment showed that both prototypes performed similarly up to eaves level, achieving overall SAP scores of 89. Dwelling emission rates were 12.59kgCO2/m2/yr and 12.35kgCO2/m2/yr for masonry and SIPs respectively. However, neither performed as well as designed – while masonry had a 10% additional heat loss, the greater unpredicted heat loss was with SIPs at 20%, providing valuable lessons for future best practice. “The thin-joint masonry performed really well up to the eaves, where there was more heat loss in the rafters,” says Partington.

This outcome informed the decision to build out the first phase with a hybrid of the two tested prototypes – thin-joint wall construction but with a roof-cassette structure more akin to SIPS replacing the underperforming traditional roof. Ground-floor slabs were in-situ concrete or a proprietary insulated suspended system depending on ground conditions. All of the first phase houses subsequently achieved an airtightness of around 2m3/h.m2@50Pa.Thin-joint construction was however a challenging departure from the norm for housebuilder Barratt, which was more comfortable with traditional masonry and used mainly that system, as well as approximately 20 timber-framed homes, in subsequent phases.

The biomass energy and community centre has proved a strong focal point in the development, although the cost of heating is more than had initially been hoped – again research into this should prove useful for future projects. Post-occupancy studies were also revealing: while the performance on heat use was good, residents in the first phase had a markedly larger carbon use than those elsewhere in the city, mainly due to high transport mileage because many worked some distance away. “There’s a limit to what you can deliver with just the building. It’s also to do with occupancy and lifestyle,” says Partington.

The architects have a few misgivings about some aspects of the later phases in comparison to the exemplary first, mainly due to challenging skills and labour shortages. Nonetheless the practice is understandably proud of its achievement at Derwenthorpe, and how its emphasis on landscape and setting has produced a high-quality, pedestrian-friendly environment appreciated by those who live there. “It’s not a scheme liked by all the architectural elite – it’s too much in the English picturesque tradition. But the residents’ perception of it is really gratifying,” says Partington.

The final fourth phase is now underway and due to complete in 2019.

PROJECT TEAM
Architect Studio Partington
Structural engineer Alan Wood and Partners
Main contractor David Wilson Homes Yorkshire East Division

Photos: Kippa Matthews; Tim Crocker