The scale of the challenge is enormous. Humidity has to be controlled exactly – there are few second chances with documents almost 800 years old.
But if Twinn succeeds in regulating humidity without powered air-conditioning, the benefits will be immense: the space needed for plant is reduced, so the building costs less, about £330 000 less in this case, and running costs are lower. Without air-conditioning, the services will be simpler to maintain and operate, there will be less chance of plant failure, and energy consumption will be reduced. But can it be done?
Harnessing the "flywheel effect"
The British Standard for storing archive material calls for humidity to be maintained at 60%, give or take 5%, and air temperature to be 15ºC, plus or minus 2.5ºC.
Controlling moisture to these tolerances normally requires air-conditioning. But Twinn had other ideas. "Building materials absorb moisture during humid conditions, retain it and then emit it during dry periods," he says.
In this way, the fabric of the building can smooth out temperature and humidity fluctuations in a room, a process known as the "flywheel effect". Twinn knew if he could use the flywheel effect to his advantage, he might just be able to meet the demands of the British Standard for humidity with no mechanical ventilation.
However, nobody knew which materials were most appropriate or how long they would retain moisture. A sophisticated computer modelling programme was required. Unfortunately, none existed – so Twinn set about designing his own. The design team reviewed research methods for modelling moisture movement in porous materials, selected the most appropriate, adapted it and added it to Ove Arup's existing thermal software. This allowed the team to model the temperature and humidity in the archive simultaneously.
"The modelling showed that the key to controlling moisture was choice of construction materials, regulating the amount of air entering the room and ensuring moisture from construction had evaporated before the store was used," says Twinn. The software also showed that the archive material itself played the main role in absorbing or emitting moisture on a daily basis, whereas the flywheel effect of the building fabric on the store occurred over a longer time scale. With this information the team could start to plan the building.
The 700 m2 archive store is part of the much larger archive centre and will house documents, artefacts and photographs from the island's history. The location is a former quarry in St Hélier, with high rock faces to the north, south and east. The £3.5m building, designed by architect MacCormac Jamieson Prichard and local architect BDK Design Associates, includes an exhibition area, education rooms and a reading room. Support areas provide bespoke space for conservators and archivists.
Using the computer model, locally produced concrete blockwork was selected for the archive's walls. This high-density material had good moisture-retaining properties. Water penetration through the first 100 mm could be measured in weeks, but moisture held at a depth of 100-200 mm could take months to reach the surface, creating an annual "moisture flywheel". The team therefore opted for solid, 400 mm thick walls.
Walls this thick would also give the store some thermal stability, but this was enhanced by covering them with an additional 100 mm layer of thermal insulation, protected by an outer ventilated rainscreen that doubles as a brise soleil.
The roof, too, had to be specially treated to keep temperature swings to a minimum.
A standing-seam zinc roof forms a single pitched rainscreen, sheltering a 400 mm concrete slab ceiling covered by 200 mm of thermal insulation. To stop heat radiating from these surfaces, aluminium foil lines both the underside of the roof and the top of the insulation. Ventilation at the eaves keeps the void cool and dry.
Finally, the team decided to design the archive with no glazing to avoid excessive heat gain from the sun during the day, as well as heat loss at night.
The team took the unusual step of making sure the store met Swedish building regulations for air leakage, which is one change of air per day. "If the archive was constructed to normal UK standards, the moisture from the air entering the room would have overwhelmed the absorbent materials," says Twinn. It was crucial to stop air seeping into the archive through cracks and gaps. The floor slabs for the archive were allowed to bear directly on to the blockwork walls, reducing drying-out shrinkage and helping to seal the store. And the absence of windows meant that no air could enter through the joint between the walls and the window frame.
However, not all ventilation is bad. The designers knew that if the air was dryer outside the building, it could be used to replace moist air in the room, so computer-controlled, motorised vents were installed in the external walls to control the rate of ventilation.
A new standard
A low-temperature water pipe around the room's perimeter provides background heating in winter. "Of all the heating options available, this minimised local high temperature, and hence low humidity that electric heaters, for example, would give," explains Twinn.
As the computer model predicted, initial construction moisture exceeds the annual moisture movement in the fabric many times over. Work started on site in November 1997. The inner wall leaf and floor slab were completed by the end of 1998. The contractor then had to leave the building to dry during the summer. Work begins again this autumn with the installation of a continuous vapour barrier on the outer surface of the wall and the ceiling.
The building is due for completion in December. If the method developed by Twinn and his team works, it could significantly reduce the use of air-conditioning systems, not just in archive stores but in all buildings.