Natural selection

Tackling the problems
M&E consultant Buro Happold and HOK were appointed at the start of the project, and they drew up outline designs for the M&E systems that were put out to competitive tender. Contractor Shepherd Engineering Services won the M&E contract and was able to give advice on buildability. "We assisted in the design because we knew what could be done on site," says Derek Joiner, senior project manager. The York-based firm was responsible for putting M&E packages out to competitive tender. Wherever possible, it used contractors on its approved supplier list.

Having ductwork fixed externally may have created more room for exhibits, but it also meant that specialist subcontractors had to design and fabricate air ducts that matched the appearance of the grey steel north elevation. "The traditional finish is a black weatherproof one, or tradition aluminium spiral cladding, but the architect didn't want either of these," explains Joiner. Instead, thicker gauge aluminium sheets were specified.

Insulation subcontractor Gill Insulation had to bend and join the aluminium cladding on site, so it was crucial that it informed the design team of the maximum thickness of metal it could work with. "Very early on we got invited to a meeting by SES and we talked through the external ducting. We took down samples of insulation and metal cladding and the project team chose metallic coated silver aluminium," says Gill Insulations director Sean Harrison. Gill later won the competitive tender to do the work.

"We had the advantage of knowing the job by then," says Harrison.

The choice of aluminium cladding made the ductwork more durable and gave a smoother finish as there were no spiral joints. The paint matched the colour of the perforated grilles that clad the north elevation.

"The quality of the insulation was very important," recalls Joiner. "The PIB [polisobutylene] vapour seal had to be maintained. We had to choose a subcontractor that could meet the performance requirements that prevented condensation on the inside and outside of the ductwork."

"Everybody was worried about the vapour seal," says Harrison, "so we made sure the ductwork was protected by double or triple seals." The final specification was a joint effort by Gill, SES, Buro Happold and HOK.

Foil-faced mineral wool insulation 40 mm thick with a Class O foil-facing joint, taped and sealed, was used to form the first vapour seal. This was then coated with the PIB and the joints were sealed to form the second seal. Finally, the joints of the aluminium cladding were sealed with clear mastic to form the third seal. The cladding was prefabricated by Gill in its Nottingham factory using aluminium.

One unanticipated problem with the specification was that Gill Insulations had to fit 2000 m2 of cladding to the external ductwork during one of the coldest and wettest winters on record. "It caused massive problems," says Harrison. "In cold weather, PIB goes rigid, so we had to keep it inside and bring it out in small batches. This was difficult as there were only two small entrances on each floor, and a small scaffold." Even worse, the metal fixers' fingers were sticking to the aluminium in the freezing conditions, so the metal also had to be stored inside.

The harsh conditions meant that Harrison got through three teams of metal fixers during the job, which led to problems of continuity. "The trouble was, the subcontractors could work in a nice warm boiler room for the same amount of money," says Harrison.

The ductwork exposed on the first floor of the atrium is not insulated but it has to look good as the public exhibition areas are situated there. HOK specified the same silver finish as the external ductwork cladding and opted for a smooth finish rather than the more usual spiral-jointed metal.

The design for the exposed ductwork incorporating the smoke baffle was HOK's. It fixed the glass baffle to a bracket, which was bolted to studding. This passed through the ductwork via a sleeve and was fixed to the underside of the slab. Buro Happold's consultant engineering company Fire Engineering Design and Risk Assessment, or FEDRA, made sure that the design met the required smoke extract specifications.

HOK also managed to conceal the electrical services for the emergency lighting, fire alarms and ventilation in its final design. They are hidden in trays behind the attachments that fix the ductwork to the ceiling.

Tight spaces and good looks were also the main drivers behind the specification of the ceiling spines in the laboratories and offices. These conceal the fan coils and services that provide scientists and staff with mini heating and ventilation systems, which they can control individually in the offices. In normal circumstances, the services and ductwork would have been hidden in a suspended ceiling but the low floor-to-ceiling height meant that the services had to be hung from the ceiling in a narrow spine.

HOK's outline design was for a series of curved perforated panels incorporating lighting, low velocity ductwork, heating and chiller pipework, fan coil units and re-heater batteries. Because of the limited space within the spine there had to be a lot of brainstorming between the contractors. "The team meetings were packed all the time," remembers Henry Onah, project designer at ceiling contractor James Rose Projects.

SES gave the job of co-ordinating the companies working on the spine to James Rose. "From a technical point of view, it was important that we assumed the lead role because if we couldn't get the panels up, the whole design wouldn't work," says Onah. "We had to work within the requirements of the other subbies and make sure that the other teams' fixings did not clash."

One of the requirements was that the services were easily accessible through the panels, which meant many of them were hinged from the ceiling. Onah says that the complexity of the job meant that some rival contractors pulled out of the bid. "I don't know what their difficulties were," muses Onah. "It comes down to the way designers look at a problem. Some say it can never be done, while others can look at it from another angle and find a solution."

One of the problems was finding a low-noise fan coil unit that could fit in the shallow space of the ceiling spine. The perforations in the panels, which allow for effective exchange of air, and the hard surfaces in the rooms meant that noise could not easily be dissipated. In the end SES found only one company that was able to provide a unit that could meet the specification (see "Seen and not heard: the fan coil specification").

HOK also had to design a distribution system for the methylated spirit. This means that specimen jars can be filled from taps in the offices, laboratories, spirit kitchens and dissection areas. Such efficient supply of spirit is in marked contrast to how jars were filled in the Natural History Museum's old building next door.

For many years, museum staff had to laboriously go from jar to jar topping up the 22 million specimens. Now, they can get on with more scholarly pursuits thanks to the efficient spirit supply and the success of the new air-conditioning system.