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Tales from the RivergreenDecember 2007

A naturally ventilated speculative office that beats best practice energy targets and houses happy and productive occupants - surely some mistake? Roderic Bunn heads to Durham to find out more.

Advanced naturally-ventilated buildings are often the most difficult to get right. Unlike a mechanically controlled environment, a naturally controlled environment is comparatively more difficult to manage without wasting energy. In a naturally ventilated building where conditions are more likely to float, the control of key elements like windows, blinds and lighting becomes critical to both energy efficiency and occupant satisfaction. Either the occupants can have local control, or systems can be automated and managed by a computer. Either way, there's lots of potential for error.

So to find a naturally ventilated, speculative office building that not only has remarkably good occupant satisfaction scores but also exemplary carbon dioxide emissions is unusual. But the Rivergreen Business Centre at Aykley Heads in Durham seems to have pulled it off.

Named after its owner-occupier Rivergreen Developments, the Centre is located on a leafy business park just outside the City. The 4852 m2 building was occupied from February 2006. Since September 2006 Arup Research and Development has been carrying out environmental monitoring, while occupant satisfaction has been analysed by Building Use Studies.

Rivergreen Developments had a hands-on role, being the promoter, financier, project manager and design team leader of the Centre. It also built it, and continues to own and maintain it - ideal conditions for a building to be a success.

Although the Centre is Rivergreen's headquarters, it is also home to around 20 tenants, varying in size from one-man-bands to Durham County Council, which occupies half of the building. Total occupancy varies depending on hot-desking arrangements and whether the 200-seat conference centre is in use, but averages around the 300 mark.

Rivergreen's principal Peter Candler believes in developing stable and long-standing relationships with professional designers and builders. Architect JDDK Architects and environmental consulting firm Armstrong Rhead had worked together on many of Rivergreen's projects before. There was no JCT-type contract, no RIBA stages, and no formal design team submissions.

“The big difference with Rivergreen projects is that the client leads the design team - it's not a case of the design team trying to find out what the client wants,” said Mike Rhead. “We spend many hours talking about it and discussing options.”

These discussions took placed in what Peter Candler of Rivergreen calls a pre-design period. “Before we got into the nitty-gritty of design we stood back for five months. We looked at examples of best practice, and visited the Centre for Alternative Technology to see if there were interesting things to learn, like rammed-earth walls and sedum roofs,” said Candler. “I wanted to make a major step-change” he added, “not just make this building five percent better than the buildings we've procured previously. Everyone bought into that.

“I told the architect David Kendal that the architecture was not important,” revealed Candler. “Instead, I wanted him to take the environmental challenge - and my commercial requirements - and make a project out of that. For me, the important architecture was the spaces and how they are organised, and how they could be naturally ventilated.”

Environmental design
The largely timber-framed building is cruciform in plan, with a double-height glazed central circulation space running east-west. A south-facing rammed-earth wall runs along the length of this space to provide the building with some useful thermal mass. The asymmetric roof is covered in sedum which attracts insects while reducing rainwater run-off.

The building is wholly naturally ventilated. Cross and stack ventilation to the 13m-deep offices spaces is aided by motorised clerestory windows that open out to the atrium. The atrium glazing has motorised vents under the control of the building management system, along with local thermostat control so that occupants can take control when they wish.

Office ventilation is provided by manually openable windows and vents. The windows provide rapid ventilation, while the vents to the side allow for background ventilation and night cooling. The high-level windows that open on to the glazed atrium are motorised, with override control switches in gangs by the core areas for the occupants to use.

Unusually, the conference centre is also naturally ventilated - with no comfort cooling. BSRIA carried out computational analysis for Rivergreen to determine whether the double-height space would work without air-conditioning, and concluded that conditions would be uncomfortable in high summer.

Rivergreen's response was to accept a limit of about 80 occupants on hot summer days rather than resort to air-conditioning. The conference suite is therefore ventilated by openable windows and by four roof-mounted Monodraught Windcatchers.

All space heating is provided by two Talbot biomass boilers, with rated outputs of 50 kW and 100 kW. These are used to deliver heating to radiators and underfloor heating circuits in four zones that correspond to the building's wings. The boilers heat a 1000 litre buffer vessel which acts as a low-loss header in the system and to allow the boilers to react more quickly to changes in load.

Domestic hot water is heated by six solar panels installed along the atrium roof, and by two biomass boilers when solar heating is not available. An immersion heater provides a back-up.

A 12,000 litre underground rainwater tank supplies water to the toilets and the sedum roof's irrigation system. The building has Airflush waterless urinals, and all WCs are low-flush units.

Lighting is a mix of daylight-linked T5 fluorescents in office areas, and compact fluorescents for feature lighting.

The Rivergreen Centre has a 6m high, 600mm-thick rammed earth wall, constructed in six separate panels inside the atrium. Being south-facing and subject to direct solar gain, the wall absorbs the sun's energy during the day and releases it at night and early morning to pre-heat the adjacent offices prior to occupation.

Approximately 80 percent of the material used in the construction of the wall is fine sand obtained from the basement excavation. The remainder consists of gravels and clay obtained from local quarries.

JDDK Architects commissioned the Department of Architecture and Civil Engineering at Bath University to develop the optimum blend of clay, sand and gravel. Sample panels were built prior to the main construction to test the physical characteristics of the wall and to provide training for the construction workers.

“The main risk is to do with controlling the shrinkage of the wall,” says JDDK project architect Ruth Walters. “At Rivergreen we were able to build in a controlled environment - it's much easier to build internally. But we still made the wall in six separate panels, with expansion joints between them.”

The building's features enabled it to achieve a BREEAM rating of 'excellent'.

Occupant survey results
An occupant satisfaction survey was carried out in March 2007 by Arup Research & Development using the Building Use Studies (BUS) methodology.

The Rivergreen Centre is in the top 17 percent of the BUS UK dataset. This is a good result, given the size and complexity of the building. Eleven of the main study variables are significantly higher or better than the UK benchmark.

The building is well-liked, and receives good ratings for perceived health, which BUS finds rare in a building like this. However, the building is excessively noisy, with ratings significantly below benchmark on the five noise variables measured by BUS. This puts the building in the bottom seven percent of the UK dataset for noise.

Some occupants complain about excessive glare from the sun and sky, and give poor ratings for control over the lighting and daylight. Some occupants expressed their dislike about the hot-desking, which some said made them feel uneasy, insecure or nomadic.

Nevertheless, BUS found that the Rivergreen Centre is the best building of its type in the UK - unusual for a building with a relatively large, open-plan floorplate with many green features and credentials. In general, BUS finds that bigger green buildings tend to score lower than smaller ones because it is more difficult to create good thermal comfort and acceptable noise conditions.

Environmental performance
Rivergreen's starting point for the building was not to use energy if it could avoid it. Surprisingly, this did not manifest itself in detailed energy targets for the building. “We just tried to adopt best practice,” said Mike Rhead. “We considered green issues but they weren't the overriding factors. If a green technology didn't suit the building - composting toilets for example - they didn't go in.”

Arup Research & Development were commissioned by Rivergreen Developments to conduct an environmental assessment of the Centre. Electricity consumption has been taken from utility bill records and from the extensive sub-metering of end use loads, which have been logged on the building management system between October 2006 and August 2007.

Analysis of the electrical maximum demand revealed a base load overnight and over weekends of 12-15 kW. This is made up of 6 kW of small power, 5 kW of lighting, and 2 kW from the kitchen.

The monitoring shows that the total annual electricity load is 53.3 kWh/m2, of which lighting contributes 17.8 kWh/m2, small power 18.1 kWh/m2, and IT small power 6.5 kWh/m2. This almost exactly matches the good practice benchmark in Energy Consumption Guide 19 (ECON 19) of 53 kWh/m2 per annum.

For lighting, the largest single component during the occupied period seems to be from the ground floor common lighting. Lighting demand is modest during peak occupancy at 3.5 W/m2 in spring and summer, and 6 W/m2 in winter.

Analysis of the office small-power loads show that the Rivergreen Centre is showing specific demands of around 5 W/m2. This is considerably lower than other typical offices where small power loads are in the region of 15-18 W/m2. This figure may be partially due to the effect of low occupant density.

Space heating and hot water is delivered by the two biomass boilers with a combined heat output of 150 kW. The biomass boilers are fuelled by wood pellets produced locally from recycled material such as wooden pallets and packing crates. The pellets have a calorific value of around 17,600 KJ/kg.

The biomass boilers have suffered from tripping-out on high water temperature, over-feeding of pellets, and difficulty in reacting to rapid changes in loads. To overcome these problems the supplier is in the process of upgrading the boilers.

Arup assessed the efficiency of the boilers based on energy input data from the bulk pellet deliveries, which occurs approximately monthly. The conclusions that can be drawn from this broad analysis are limited, but it appears that boiler efficiency has been around 24 percent.

The total fuel consumption data for the heating season from September 2006 to May 2007 is estimated to be 77 kWh/m2. In CO2 terms, this equates to 24.5 kg CO2/m2 per annum at a conversion factor of 0.025 kg/kWh of heat (from Approved Document L2A)

Overall, Arup's interim results indicate that the Rivergreen Centre emits 24.5 kgCO2/m2 compared with the 'best practice' benchmark of 37.7 kg CO2/m2 in ECON 19.

Overall assessment
On the basis of the building's carbon dioxide and occupant satisfaction score, the Rivergreen Centre is clearly an exemplar of its kind, despite the problems with the biomass heating system. There are one or two other minor bugs, such as a storage vessel for the solar panels located in the basement, which means a 5 degrees K temperature loss between the panels and the tank (“We should have put that on the roof”, admits Mike Rhead), and a blinds-down, lights-on problem in some areas. But overall, Rivergreen is a highly successful example of the genre.

Interestingly the building's good performance is largely down to an enlightened client that, while highly experienced in designing and constructing buildings, was not afraid to take risks in order to achieve a step-change in energy efficiency. Witness the rammed-earth wall - an innovative technology for which there are few equivalent installations in the UK, and none in a speculative office.

That said, Rivergreen knew where to draw the line. “Everything we do has to be responsible,” said Peter Candler. “We don't want to be a guinea pig. People might think that we were being reckless building an earth wall, but that's not true. We looked at it very carefully and made an informed decision on whether we should do it.”

The building's sustainable design extended beyond the building to the landscaping. The planting scheme used a healthy mix of apples trees, alpine strawberry bushes, and herbs like chives and dill. The produce is used in the sandwiches and cakes sold in the staff cafeteria.

So where does Rivergreen go from here?
“We are going to build another one like this in Northumberland,” says Peter Candler. “So I need to know what worked and what didn't work, and also what contextual things will be different in Northumberland that will influence changes in design.”

True evidenced-based design is what this is: repeating what works and reducing the risk of failure in order to make good performance routine. A worthy ambition.

Further reading (rammed earth walls)
Bunn R, 'Friends of the earth', Building Design, 12 May 2006.

BSRIA acknowledges the help of Rivergreen Developments, Armstrong Rhead Partnership, Building Use Studies and Arup Research and Development for the data used in this article.

For more information contact BSRIA:

Tel: +44 (0) 1344 465600
or email