Two key strands form the environmental strategy behind Foster and Partners' British Museum project. First, is rationalising the engineering approach at the museum and allowing for future adaptation. Second, is to do this in such a way as to minimise the intervention in the building fabric.
The museum is a Grade I listed building and the museum is in operation throughout the project - so there had to be minimum disruption. To add to the difficulty, the Great Court and Reading Room, the foci of the scheme, are landlocked by the surrounding galleries of the museum. Museum officials were also keen to ensure that the scheme did not close off opportunities for the long-term regeneration of the engineering throughout the rest of the museum which would be carried out when adequate funds had been raised.
The biggest problem with the provision of services was presented by the movement of air. The aim is to bring 45m3 of air per second into the Great Court, galleries, retail areas, education areas and Reading Room. Four primary plant rooms are sited in roughly the four outside corners of the building. Air is taken from inlet grilles adjacent to access roads surrounding the museum, via the primary plant rooms and brought into the centre of the building to the secondary plant rooms under the Great Court where the air is refined. From here it is fed via a kind of distribution racetrack around the perimeter of the Reading Room to the restaurant, auditorium, Reading Room, seminar rooms and new ethnographic and temporary gallery spaces as well as fresh air supply to the Great Court. The air returns from these spaces to the secondary air-handling units and passes through an energy-recovery system. A proportion is recirculated and the rest is discharged via turrets leading to roof level. Only a limited amounted of air is recirculated owing to the predominance of displacement ventilation systems but in order to maintain a close control and reduce energy consumption the gallery areas revert to full recirculation outside occupied hours. The building has been divided into four quadrants to harmonise with the museum's long-term engineering strategy. This diagram of services distribution provides the museum with clearer circulation and will assist it with the long-term regeneration of the services engineering. It also means that everything can be fed to and from the centre of the building via the new infrastructure, thus avoiding the tortuous routes before. The diagram is sensible and fairly straightforward. What was less easy was turning this into reality.
The engineers were keen to exploit 'latent voids' in the fabric of the existing building. Some of these spaces have fallen into disuse, others are being used in a different way. The primary plant rooms are as small as they can be and are set in existing spaces. Many of the spaces were store rooms but some were departmental office accommodation which was decanted in a major enabling-works package. To get the air from the primary to secondary plant rooms, ducting had to be threaded around the existing foundations, which fortunately are not contiguous.
The four secondary plant rooms have been built just below the porticos on the east and west facades of the court. Each portico had two 2m diameter, stone spiral maintenance staircases which climbed from level 1 to the roof (the level directly under the main Great Court plaza). By stripping out the stairs, the remaining 'cylinders' will be used as the exhaust air ducts from the secondary air-handling plant. Only three will be used as such and one stair will be kept to maintain access to the balcony in the Grade 1 listed Kings library on the east side of the site. At present the only access is via the turrets.
The space below the Reading Room was another useful space and could be used for pipework and air ducts, providing a central 'hub' for the services. The air supply to the Reading Room mimics the original design but does not copy it exactly. In the original, the air was drawn up over heating pipes through brick arched ducts in the floor void, and then up through grilles between the desks and out of vents in the roof. In this new version which uses the Reading Room as an information space with large numbers of computers and screens, tempered air is being provided via insulated ducts connecting into the original desks from below .
There was no external finish to the Reading Room to speak of, so this left the way open for an external skin to be built leaving a 500mm void between it and the Reading Room wall. This void is used to take service risers such as the smoke extract.
Within each of the service runs allowance capacity has been left for future provision of service infrastructure.
The Great Court
Covering the internal court with 6000m2 of glazing has, as you might expect, some effect on the environment below but it also has a knock-on effect on the existing museum spaces. The basic strategy within the Great Court was to provide a tempered environment which is warm in winter and cool in summer. One of the bigger problems to be tackled was solar gain. Rather than adopt an out-and-out naturally ventilated solution incorporating extensive 'chimneys' and large louvred elevations, which current tendencies in environmental engineering might suggest, human comfort has been provided through a combination layering of several environmental mechanisms.
The designers have had to perform a careful balancing act between providing sufficient daylighting for the space, but not too much light for the existing galleries. Originally the bookstacks obscured much of the daylight and direct sunlight to the galleries. Given the conservation needs of the museum and that this project did not include work to the existing galleries, the design team had to be careful not to make the prevailing conditions worse. Solar gain in the space has been reduced through the use of low emissivity glass which has a varying pattern of fritting which helps achieve a shading coefficient of 0.25 (the lower the figure, the more shading you get, as a benchmark assume a plain sheet of glass has a shading coefficient of 1) while maintaining a relatively high daylight factor at about 40 per cent transmission.
Louvred clerestory windows around the perimeter of the roof allow natural cross ventilation at high level. On very hot still days the air can be pulled through by 40 fans which double as smoke-extract fans around the rim of the Reading Room. The fans will run at 1.5 m3 per second, allowing about 15 air changes per hour. Although this is more than would be needed for the number of people in the space below, it should guarantee a flow of air and provide guaranteed night-time purging to avoid 'heat soak' experienced by some heavy-weight buildings which are unable to 'breathe' properly in summer. The louvres are sensitive to temperature and humidity, and are controlled by the building management system with sensors at roof level inside and outside.
A small amount of heating is provided by an underfloor heating system within the slab. However, during periods of the year of high heat gain spare night-time capacity from the chillers is fed through the underfloor system, cooling the slab to about 17degreesc. The engineers predicted that without this intervention, on a normal day the slab would be about 30degreesc. Considering the amount of floor that a person's body would be exposed to, this 13 degree difference in temperature would have a significant impact.
Tempered fresh air is provided to the Great Court via specially developed displacement vents flush with the floor around the perimeter. To counteract the down drafts during winter and to provide a morning boost to the air temperatures, warmed air will be delivered via the displacement units and spill across the floor. In summer the specially developed flush floor displacement units will provide significant horizontal momentum to the chilled fresh air, allowing it to travel and delivering it to the occupants. Since the floor is chilled it will allow the cool air to 'stick' to it. Cool air sticks to cool surfaces better than warm ones. A warm surface warms the cool air, giving it buoyancy and causing it to detach. This effect allows the air to flow further across the floor than it could otherwise do. The air will have to travel 30m from the perimeter to the middle of the court which is very difficult to achieve even if ventilation is forced. The engineers modelled this using a full-scale model in Finland.
Modifying the environment in the Great Court has had a beneficial effect on the existing galleries around it. It has reduced the external surface area and so reduced air leakage and pollution infiltration through the fabric. The engineers reckon that the potential fabric energy loss to existing quadrangle buildings has been reduced by 55-65 per cent.
On the upper floor of the ellipse around the Reading Room the restaurant overlooks the Great Court. Here displacement ventilation is brought in through the balustrade with the floor slab being chilled at night. Although the restaurant is some 15m below the roof, solar glare and direct solar radiation will still be a problem for people so the engineers are working with Foster and Partners to design, a canopy to reduce this.
Underneath the main floor level of the Great Court, the auditorium, seminar rooms, new galleries and kitchen will have more-controlled environments so some chiller capacity is provided to feed the air-handling units in in the secondary plant rooms. The new galleries are essentially black- box spaces requiring close-control air conditioning to meet international standards. For these spaces the air handling units are provided with space to fit full carbon filters as required.
One of the consequences of removing the bookstacks around the Reading Room was that it would potentially change the environment inside. Suddenly these were large areas of exposed wall which had not existed before, changing the fabric loss and gain. To counteract the effect on fabric within the Reading Room, particularly the movement of the timber, a temporary ventilation system is controlling the environment.
Many clients are concerned about the adaptability of their buildings. Without being able to predict the future, the best we can do is to allow enough elbow room for it to happen.