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Article from the September 2012 edition of the CIBSE Journal by Andy Pearson
Introduction
Building on a flood plain posed extraordinary challenges for the designers of the award-winning Brockholes Nature Reserve visitor centre. Andy Pearson finds out more.
It was a difficult brief. Design a visitor centre complete with shop, classroom, exhibition space and conference centre right in the middle of an ecologically sensitive site in the middle of a flood plain.
"The competition was advertised on the RIBA website and so I set about forming a team to win the project," says Hareth Pochee, an engineer at Max Fordham. He joined forces with Adam Kahn Architects, structural engineers Price and Myers, and quantity surveyor Jackson Coles to come up with a design for a visitor centre for Brockholes Nature Reserve, which is sited close to the River Ribble near Preston.
The team's winning design for the 67ha wetland site gave each of the centre's functions its own building. These are clustered together, village-like, on a 2,400m2 concrete pontoon that fl oats on one of the site's three lakes. As well as giving the centre significant flood protection - the area floods every 15 years - this innovative solution brings visitors close to the reeds at the water's edge: "The buildings are located where the nature is happening," says Pochee.
Once they'd won the contract, Pochee and the team sat down with the client to develop the scheme. "It was clear from the outset that environmental performance was top of the client's agenda," explains Pochee.
Initially, BREEAM was not a project requirement. However, as the design developed, a revised version of BREEAM was published that included a new "Outstanding" rating for ultra-sustainable developments. According to Pochee, both the design team and client were inspired by this new rating - and, as a result, BREEAM Outstanding became a key aim.
Concrete evidence
In addition to this aspiration, Max Fordham decided to conduct an embodied energy analysis of the £6.2m scheme's construction materials. The investigation was based on the Environment Agency's database, coupled with the consultant's own research. "The methodology is straightforward, but confi dence in some of the data is frustrating because different sources have different energy values for the same materials, so we had to make a critical assessment based on the accuracy of the calculations," says Pochee.
One result of these studies was that concrete, rather than steel, was used to construct the floating pontoon. This was cast in-situ, around polystyrene void formers. "Its construction means that, even if the concrete pontoon does spring a leak, it will still float," reassures Pochee.
The pontoon has a rustic appearance, thanks to a series of narrow, barn-like buildings with tall, pitched roofs and no ceilings. Their walls and roofs are formed from structural insulated panels (SIPs), topped by an oak shake roof.
Pochee is particularly pleased with the façade design, which he describes as "an environmental conditioning machine", and waxes lyrical about the part the windows, insulation, apertures, shading, glazing specification and air tightness have contributed to the environment and servicing strategy.
The buildings have been designed with areas of carefully orientated glazing to ensure good views out and excellent levels of natural light, minimising the requirement for artificial lighting. The windows are made from high-specification double-glazed units, combined with a low conductivity timber frame and thermal-break edge-spacers to produce a unit with an overall U-value of 1.1 W/m2 K. Pochee says this gives the units "a comparable thermal performance to triple glazing but with better light transmission". As a result, daylight factors exceed 4% in all the main spaces, and even in secondary spaces, such as WCs, the daylight factor is still above 2%.
Any artificial light required is provided by bare fluorescent tubes suspended beneath the roof. The lack of a diffuser significantly increases light output. A reflective panel mounted above the bare lamps helps direct light downwards. The fittings also incorporate daylight controls and presence detectors to turn off the lighting when there is sufficient daylight or rooms are empty.
In addition to daylight, the glazing also admits passive solar heat from the low winter sun. In summer, external awnings are used to prevent rooms from overheating. As well as providing shade, says Pochee, the awnings allow views out beneath the sloping canopy, while enabling air to flow freely through the open windows.
A breath of fresh air
A series of rooflights, positioned close to the roof ridge, open to allow warmed air to rise out of the buildings as part of a natural ventilation strategy. These are predominantly orientated north to avoid over-exposure to the summer sun, with retractable blinds fitted beneath to help prevent summer overheating.
The buildings that make up the visitors centre are naturally ventilated, with the exception of a few rooms such as the toilets and kitchen. In winter, fresh air enters buildings through a row of small, low-level vents positioned beneath the windows. The vents are opened and closed using a building management system (BMS), depending on room temperature and CO2 levels. When it is very cold the vents open far enough to allow 4 l/s/person of fresh air to enter the space increasing to 8 l/s/ when the weather is mild.
Ventilation rates can be increased in summer by manually sliding open the 2m-high windows because of the building's relatively low thermal mass. "We're implementing a marquee strategy: if you want cooling, you just open the windows," Pochee explains.
Despite its lack of thermal mass, Pochee has introduced what he terms "a nightcooling strategy". In effect, this is an option to flush the rooms with cool morning air. "Because the buildings are so well insulated and sealed, they will still be hot in the morning," he explains.
Cooking up carbon savings
Natural ventilation is also a feature of the commercial kitchen, which serves the centre's restaurant. "The caterer has the option of natural ventilation when the hobs are not in use," says Pochee. For the remainder of the time, however, a mechanical ventilation system incorporating variable speed fans will remove heat and moisture from the space.
Like the materials used to construct the buildings on the scheme, the kitchen equipment, too, has been selected to provide a low carbon solution. Max Fordham took the catering consultant's equipment schedule, which specified the types and numbers of appliances, such as ovens, deep-fat fryers and hobs, and analysed the efficiency of the different cooking devices.
"We worked with the catering consultant to specify equipment that was of the highest efficiency, used the lowest carbon fuel source, but was affordable within the client's budget," says Pochee. For example, the team would have preferred to use electric induction hobs because these were found to be the most efficient catering solution; however, they were also too expensive so liquefied petroleum gas (LPG) hobs have been specified instead.
Heat for the visitor centre is provided by a 150kW low NOx biomass boiler, burning locally produced wood chips - a cheaper solution than providing a gas supply to the site, according to Pochee. Air and ground source heat pumps were also considered but were found to be a more expensive option than biomass in terms of carbon saved per pound invested.
The boiler house is situated on land adjacent to the lake and is connected to the floating village by flexible, pre-insulated heating pipes. These have the capacity to cope with a 4m rise in water levels, and all services are flexible and waterproof to enable them to run under the lake.
Entrenched habits
On the pontoon, services distribution is through a series of floor trenches linking the various buildings. The trenches, which are up to 2,000mm wide and 500 mm deep, only provide access to the cables and pipes at corner junctions because the architect did not want access panels along the lengths of the trenches. Extensive co-ordination was required between the services engineer, architect and structural engineer to achieve this.
"As a solution, it was quite bold and daring enough to give me a couple of sleepless nights," recalls Pochee. To enable installation, all services are flexible to enable them to be pushed and pulled through trenches between buildings, although "it was not the most popular solution with the contractor," says Pochee.
The building's pontoon-based location also created some interesting challenges for the drainage design. Water for most uses is supplied from a nearby borehole while lake water is used to flush the WCs. Water consumption is minimised by low consumption sanitary fittings and low-flow taps and urinals.
The remote site does not have a mains sewer connection, so an onsite sewage management system was needed. Drainage pipework delivers wastewater to a pumping station housed within the pontoon, from where it is pumped to a septic tank on land and then to a reed bed treatment system designed to coordinate with the site's ecology. This solution has about 25% of the energy demand of a municipal sewage treatment system, but it does require a similarly-sized site to the footprint of the building, adds Pochee.
In addition to the service trenches, the pontoon houses heating pipes beneath its polished concrete surface. This is the underfloor space heating system. Pochee says space heating loads are minimised by high levels of insulation in the building envelope, which has a U-value of 0.1 W/m2 K, combined with low levels of air permeability. On average, air leakage is about 6 m3/m2/hour @ 50 Pa. "It's not as good as we wanted, because rectifying errors was expensive", admits Pochee.
Control buttoned up
Users control heating through "iPod controls", so termed because they have been designed to be intuitive to operate without the need for instructions, although the team did draw on symbols to help users.
"In general these have worked well," says Pochee. "What could have been improved is the time delay between pressing a button and the equipment responding," he says. Max Fordham is now monitoring the building post-occupancy. Overall it is performing well: monthly reports show the temperatures to be comfortable. However, the consultant is looking into the amount of biomass being burnt, which is "a little higher than we anticipated," says Pochee.
The building has achieved an A-rating Energy Performance Certificate. Its energy efficiency measures alone reduce its carbon emissions by 35% compared to Building Regulations Target Emissions Rating (TER). And when the biomass boiler is included, carbon emissions drop by 85% compared to the TER.
Brockholes Visitor Centre, Preston - winner of the New Build Project (Private Sector) Award 2012
Max Fordham LLP