Making net zero building the reality

Decarbonising the UK's building stock quickly enough to keep temperature rise within the critical global 1.5°C threshold requires radical action. 

This is common knowledge, yet inconsistent interpretations of what a net zero carbon building is, and how it should perform, still cloud project briefs and expectations, raising the spectre of yet more leaky and inefficient properties.

Industry efforts to dispel the confusion and clear the way forward include the recent launch of a pilot UK Net Zero Carbon Buildings Standard, which sets out for the first time how to robustly prove that built assets are net zero carbon and in line with UK climate targets.

The single methodology, developed and agreed by a cross-industry team, which includes RIBA, BRE, LETI, the Royal Institution of Chartered Surveyors and the UK Green Building Council, contains detailed requirements for all major building types, and both new build and retrofits. It is the most thorough attempt to date to set science-based limits on upfront embodied carbon and operational energy, with minimum targets for on-site renewables and other aspects.

The trailblazing standard consolidates and expands upon the RIBA 2030 Climate Challenge’s approach of asking signatories to meet key energy, carbon and potable water targets. It does so by defining a fundamental pass/fail requirement for fossil fuel-free sites, and setting operational energy limits based on in-use building energy consumption at 80 per cent occupation.

This is great news for architects wanting to cut through spurious claims around net zero and to accelerate the shift to reducing carbon. Lessons can also be learned from exemplar projects already delivered in the field. As the schemes explored below demonstrate, it’s never as simple as installing a few extra solar panels on the roof… 

Hermitage Mews, Crystal Palace

Architect: Gbolade Design Studio
In-use performance: Designed to exceed RIBA 2030 Climate Challenge targets for both operational and embodied carbon

Characterised by its distinctive sawtooth roofline and split-level floor plans, the mix of eight terraced and semi-detached houses at this south London development is fossil-fuel free, harnessing roof-mounted solar PV, air-source heat pumps for heating and hot water, and an MVHR system to ensure low energy use and good occupant comfort levels. 

A fabric-first approach with 450mm-thick insulated walls, triple-glazed windows, thick roofs and floors enable an average U-value of 0.10W/m2K. Initial plans to cut embodied carbon through a primarily timber construction required a rethink because of client concerns, following the post-Grenfell ban on combustible external wall materials in buildings taller than 18m, that the properties might not sell. 

The free design modelling tool FCBS Carbon was used in early design to compare the embodied carbon impact of either using masonry, brick and block construction, or a mix of brick and block walls with timber superstructure.

This ‘gave a clear steer that a timber superstructure was still going to help us move towards net zero reductions’, says Tara Gbolade, co-founder and director of Gbolade Design Studio. It also reduced the building’s weight, cutting the volume of concrete needed in the foundations, she adds. The focus was on using glulam beams, or hybrid steel plate/timber flitch beams, for structure. All internal floors and walls and the roof are timber.

Plans to use 100 per cent wood fibre insulation in walls and roofs had to alter after Russia’s invasion of Ukraine tripled timber prices. Luckily, there was enough waste wood fibre on site to finish the roofs, with wall insulation switched to an 84 per cent recycled glass product with low embodied carbon.

Gbolade says the use of ArchiCAD’s dynamic energy performance modelling gave a ‘strong idea of the direction of travel for materials’ as designs moved on. Working closely with MEPH consultants Mesh Energy became critical in order to move towards compliance with the RIBA 2030 Climate Challenge, which can be tested once the project has a year of bills to scrutinise.

‘As an industry, a shift to a more collaborative design process is necessary; the architect can’t keep designing in isolation, then issue the drawings to the structural and MEPH teams to develop,’ says Gbolade. ‘There needs to be a more iterative process until we are well versed in what we’re trying to achieve and how we need to achieve it.'

The Brambles, Hertfordshire

Architect: Bere Architects
In-use performance: Passivhaus Plus certified, generates ~3x more energy than it imports from the grid each year  

This all-electric timber-clad home, which was completed in 2020, takes most of its power from rooftop solar PV. This is topped up in the gloomiest winter months by a home battery that draws 100 per cent renewable energy from the grid via a unique tariff focused on wind power. 

A Passivhaus design, with a strong focus on thermal efficiency and airtightness, minimises the Brambles’ energy demand. A reversible electric heat pump provides hot water for bathing and kitchen use in the home, with an integrated 100 per cent fresh air ventilation system supplying supplementary heating or cooling by extracting warmth from the air as it circulates.

‘Space heating and cooling use so little energy that they are only a secondary requirement,’ explains Justin Bere, director of Bere Architects. ‘Peak power from the heat pump [a Pichlerluft PKOM4] is 1.8kW when the temperature is sub-zero outside – compare that to the typical electric kettle, which consumes around 2-3kW, and a comparable ordinary house would use a 10-12kW gas boiler.’

A focus on winter heat loss – which is by far the biggest factor affecting energy consumption in the UK’s domestic dwellings – was crucial to driving down operational energy. However, the Brambles’ complex building form posed a challenge when designing to avoid cold bridging. For example, maintaining a line of insulation around a deep timber beam, spanning above the ground floor living room and under a set-back balcony on the first floor, meant introducing a couple of steps up to the balcony in order to create space for extra insulation surrounding the structural element.

Embodied carbon was reduced principally through the use of a panellised engineered timber frame and beams filled with cellulose insulation. But Bere says operational energy was by far the biggest influence on energy efficiency, dwarfing embodied carbon over the building’s lifespan.

‘There is a terrible amount of carefully crafted misinformation that seems designed to make people confused and complacent about the relationship between operational energy and embodied energy,’ says Bere. ‘Where products and materials have been scientifically assembled in a building to minimise operational energy, as in a Passivhaus, any reductions in embodied carbon can undermine the building’s operational efficiency.’

The Brambles has been undergoing extensive post-occupancy evaluation to verify its energy performance. Bere admits that monitoring costs ‘a lot’ of money, adding that the ‘simplest thing a practice can do is get hold of the client’s electricity bills for a year and see if total energy consumption and total exported energy is what was expected’.

Entopia, Cambridge

Architect: Architype
In-use performance: Already complies with the 2050 energy requirement for a retrofit set by the UK Net Zero Carbon Buildings Standard, based on in-use data

This exemplar retrofit transforms a five-storey former 1930s telephone exchange into a vibrant headquarters for the University of Cambridge Institute for Sustainability Leadership.

Believed to be the largest EnerPHit-certified (the Passivhaus standard for retrofit) office building in the UK, the scheme cuts heating demand by 75 per cent versus an average office, and makes a 5x improvement on airtightness set out under building regulations. A canopy of photovoltaic panels on the rooftop terrace generates an anticipated 11,740kWh of electricity per year.

‘Central to achieving net zero, and the only reliable way to deliver a performance gap-free building, in my view, is Passivhaus,’ says Seb Laan Lomas, associate at Architype. ‘It’s a quality assurance methodology, measured in use – the airtightness requirement is a performance contract, so the contractor knows they’re going to be held accountable if they don’t deliver, which is a game changer on site.’

Plans to replace the building’s 97 mock-Georgian windows with more efficient continuous triple-glazed ones drew scepticism from planners, who had concerns this would damage the original character within a local conservation area. Architype used its modelling to show how removing transoms and mullions, and optimising the frames, would improve operational efficiency, thermal comfort, and daylighting. ‘Having accurate measurements enabled a data-driven dialogue with planners, which, combined with the fact the council had declared a climate emergency, meant we got consent,’ says Laan Lomas. Happily this approach is replicable, with 87% of local authorities having made such declarations.

The architect points to gaps around fire testing and the open availability of data related to bio-based materials as key barriers to delivering low embodied and upfront carbon projects. ‘We have gone on a big journey post-Grenfell around fire certification, and there are significant gaps in industry data – it’s a huge barrier that’s pushing timber projects to concrete and steel,’ he says. 

Any project targeting net zero needs early client and design team alignment, he adds. On Entopia the directors of all companies involved signed a document committing to targets for operational and embodied carbon and health. ‘That was vital to get everyone travelling in the right direction,’ Laan Lomas says. 

For more on the UK Net Zero Carbon Buildings Standard, visit: nzcbuildings.co.uk/home

You may also be interested in our RIBA Net Zero Carbon On demand Course available on RIBA Academy