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If a developer goes to all the trouble of building an office cloaked with 350,000 plants, then that greenery had better be more than skin deep. Fittingly named Eden, this 12-storey office, which stands out among the grey, metallic and dull red brick colours of Salford, is the first building in the UK to feature a NABERS UK 5.5* design review rating.

NABERS is the energy efficiency rating system that is gaining traction because it is a proven methodology for predicting energy performance which is subsequently vindicated with annual energy consumption monitoring.

Eden has also been designed to minimise upfront embodied carbon with an as built figure of 635kgCO_²/m_². This is just above LETI鈥檚 current target of 600kgCO_²/m_² for a new-build office, which is pretty good considering it was designed at a time when net zero guidance and carbon targets were being developed by LETI, RIBA and the UKGBC, but not yet published.

鈥淲e were setting ambitious energy and embodied carbon targets, but that was definitely not the norm at that time,鈥� says Simon Wyatt, sustainability partner at Cundall and a 好色先生TV the Future commissioner. Cundall has been responsible for the sustainability strategy at Eden, taking on the project at a time when NABERS UK had not yet been established.

Perhaps the most significant aspect of Eden, for promoting the net zero agenda, is the fact the 115,000ft² office has been built for 拢36m, just 2% more than the cost of a standard office building. New-build net zero offices are becoming the default in London but are more challenging to deliver in regional cities, where build costs are similar, but rents are a third of those in prime London locations.

Developed by The English Cities Fund, a joint venture between Muse, Legal & General and Homes England, how has the team, which includes Cundall and architect Make, managed to pull off this feat?

Eden is the fifth and last office to be built at Muse鈥檚 New Bailey development on the banks of the River Irwell, just to the north-west of central Manchester. Originally earmarked for residential development, the success of New Bailey prompted Muse to build an office instead.

Wyatt says Muse was formulating its sustainable development brief at the time and Eden was to be the guinea pig. Architect Stuart Fraser, who worked with Muse on two earlier buildings, 3 and 4 New Bailey, says Muse project manager Phil Marsden did not want a repeat of these buildings.

鈥淭he brief 鈥� and effectively challenge 鈥� was, what is the most sustainable building that we think we could deliver?鈥� Fraser says. 鈥淭he big challenge was there wasn鈥檛 the money to subsidise this. If the numbers don鈥檛 stack up, it doesn鈥檛 get built.鈥�

An embodied carbon calculation of the existing New Bailey offices revealed these came in at around 1000kgCO_²/m_² so a target of 700kgCO_²/m_² was set for Eden. Wyatt says a lot of time was spent on RIBA stages one and two (briefing and concept design) including exploring 17 different structural designs in a bid to get the most carbon efficient structure for least cost.

These included an all-timber, and hybrid structure with CLT slabs which brought the embodied carbon below 700kgCO_²/m_². 鈥淭he timber option was coming in at 拢2.5m more expensive, so it wasn鈥檛 cost viable,鈥� explains Wyatt. 鈥淎t the time there were only two or three other buildings using timber, and fire and insurance were major issues back then.鈥�

Instead, the team took the same structure as the existing New Bailey buildings 鈥� a steel frame and composite floor structure with concrete core 鈥� and optimised the structural grid, slab thicknesses and beam depths to make these as lean as possible. In common with the other buildings at New Bailey, Eden did not have a basement, which is particularly carbon intensive.

The facade was carefully evaluated as this has a significant impact on upfront carbon and operational energy performance. Wyatt says curtain walling was quickly discounted because glass and aluminium come with a high embodied carbon impact and energy performance is worse than a solid facade.

He describes the facade 鈥� which is mostly hidden by plants 鈥� as very basic, with half the embodied carbon of curtain walling.

Facade thermal performance was balanced against cost. The facade is 60% solid, the solid areas have a U value of just 0.15Wm²/K and the windows have a U value of 1.4Wm²/K. Airtightness is 2m³/m²@50pa. 鈥淚t鈥檚 not quite Passivhaus but, for a commercial office, as far as you need to go,鈥� says Wyatt. 鈥淲e pushed the fabric to what we think is economically viable and best practice.鈥�

Fraser describes the amount of solid facade as a challenge. 鈥淲ith 40% glazing, where do you position these to balance daylight and ensure there are still great views?鈥� he says.

The window positions were modelled against solar gain so there is slightly less glazing on the south elevation and more to the north. The core is offset and effectively shades the west side of the building against the low evening sun as this side is mostly solid.

Like the structural frame, the building services have been optimised to offer the best balance of performance and cost. The first step was to decide on what systems to use so electric water heaters for each floor were compared with a central CO₂ heat pump system which was the preferred option because of the number of showers on the ground floor for cyclists.

Air source heat pumps are used for heating and cooling in combination with fancoil units. Having decided on the services solutions, the next step was to assess each individual MEP element against a scale from one to 10 and push this as close possible to 10 on the grounds that marginal improvements add up to a big gain.

The team wanted to maximise the number of zones and associated controls to enable the servicing to closely match the way the building is being used at any one time as possible 鈥� if people are not using a zone, then the services in that area can switch off to save energy.

鈥淲hen the first cost estimates came in for the MEP, it was considerably over budget because we had probably gone a bit too far in terms of zoning and controls,鈥� Wyatt says. He adds that the cost of the base MEP equipment to achieve high NABERS ratings is cost neutral but goes up if the number of zones and associated controls is increased.

The number of zones was reduced to save money. Another element that was value engineered out was openable windows 鈥� useful at weekends if just one or two people are in the building as this saves the ventilation systems being switched on.

Wyatt says this does not affect the NABERS rating. He adds the temperature set points for the heating and cooling coming on were pushed further apart to reduce the possibility of one zone heating and the adjacent zone cooling to counteract that heating. The cooling setpoint is 26掳C and heating 20掳C.

Keeping tenant energy use down is a critical part of the annual NABERS certification. At the time Eden was designed, the British Council for Offices (BCO) Guide to Specification required a 25W/m² small power allowance to be built in. 鈥淢use were very brave with this and went away from the institutional investor standard and said 8W/m² was fine,鈥� Wyatt says.

Studies showed that this was sufficient to power a laptop and screen. Interestingly, this work was fed into a BCO position paper revisiting its specification for a greener age and the 2023 early update to the guide has now cut the small power requirement.

鈥淎 lot of the stuff that we included at Eden which were derogations from BCO are now actually BCO 2023 compliant even though that wasn鈥檛 published at that time,鈥� Wyatt says.

The upfront embodied carbon target of 700kgCO_²/m_² was reduced to 635kgCO_²/m_² through careful material sourcing and specification and was progressively reduced as the job progressed up to and including construction. A big help was sourcing reclaimed access flooring from a building being demolished in Glasgow. And the lift lobbies and part of the ground floor feature reclaimed timber flooring from a gym.

The embodied carbon and energy intensity target of the building 鈥� equivalent to NABERS as it had not been developed at the time 鈥� was written into the building contract. Wyatt says he has struggled to get contractors to sign up to energy ratings on other projects.

This meant providing contractor Bowmer + Kirkland with plenty of evidence that the design would work. Bowmer + Kirkland will update the rating using as built information once it completes later this year.

>>See also: NABERS: the energy ratings system that goes further than Part L

Regarding costs, Muse reckons it is getting a 10% rental premium compared with a standard, grade A office building. The focus on getting the maximum carbon reduction for the minimum cost has paid off 鈥� the building cost just 2% more to build than a standard office.

This proves that the additional cost of developing net-zero offices outside prime central London is more than offset by rental premiums. With increasing numbers of corporate clients demanding low or zero carbon office space, the business case becomes even more compelling as inefficient buildings are at greater risk of losing value.

There is a flip side to this: Ella Woodward, development manager at Muse, says one of the key embodied carbon and cost savings at Eden was using reclaimed access flooring, which is now in short supply. 鈥淭here are only so many buildings being demolished where we can get raised access floors from,鈥� she says.

鈥淚 know that other developers and contractors are struggling to get hold of it. That is an industry challenge: how are we going to keep costs and embodied carbon down in the future?

鈥淚f we couldn鈥檛 get hold of it and had to use brand new raised access flooring, we are going to have to think of other measures we can implement to keep the embodied carbon low.鈥�

She adds that Muse will share the lessons learnt from the project with other developers in a bid to help resolve some of these challenges. 鈥淲e need to collaborate; we are all in the same industry and, if we are going to make progress, we can鈥檛 just work in silos,鈥� she says.

What would the team do differently next time? Wyatt says he would go for a hybrid steel and CLT floor slab frame as the fire and insurance issues that ruled out timber when Eden was designed are no longer an issue.

鈥淭his would push the upfront embodied carbon down to the mid 500s kgCO_²/m_² and take 5% to 10% [of the upfront embodied carbon] off, but wouldn鈥檛 dramatically change the performance,鈥� he says.

Getting a contractor to sign up to binding embodied and NABERS targets is potentially more difficult to resolve as designs need to be fully developed and proven to keep prices in check. Wyatt says bringing the NABERS independent design review forward from RIBA stage 4 (technical design) to stage 3, when designs usually go out to tender, would help give contractors confidence although this means doing the calculations and simulation much earlier.

He adds that the detail behind the targets is also important. 鈥淟oose, woolly-worded requirements are meaningless. You have to state exactly what you want, when you want it and what it includes and the methodology,鈥� he says.

Once the building is finished, it is down to Muse and the FM team to fine-tune it to optimise energy performance, and the tenant to keep consumption down. Wyatt says experience from Australia, where NABERS originated, shows that the NABERS rating drops by one star initially.

鈥淭he idea is the rating improves over a couple of years as the building is fine-tuned,鈥� Wyatt says. 鈥淎ccording to the Better 好色先生TV Partnership, it is probably half a star incremental increase each year so if we got 4.5* in the first year, 5* in the second year and 5.5* in the third year, that would be a really positive story.鈥�