Inside IWM's Duxford bunker

Say the word Duxford and architects are more likely to think of the great arc of Norman Foster’s 1998 Stirling Prize-winning American Air Museum in Cambridgeshire than they are the airfield it sits on. But there’s a lot of history to the site of the First World War aerodrome which also served as the WWII base for the RAF’s elite No.19 Spitfire squadron and the US Air Force – not to mention the Cold War and Frank Whittle.

It was all in the mind of Mark Barry, director at Architype, the firm appointed by the Imperial War Museum (IWM) to design a new-build repository for its 100-year-old  archive of artworks, photographs, letters and  diaries, to create a major European archive of aviation history. IWM had been looking to move the collection out of London and for reasons both logistical – the local council was keen – and perhaps sentimental (it worked up the proposal with Duxford Aviation Society), plumped for the airfield as a site.

IWM wanted to safeguard the collection for posterity in the best environmental conditions and according to the highest standards; and its interest had been piqued by Architype’s 2015, £8 million, Herefordshire Archive and Record Centre (HARC), the first archive storage building in the UK built to Passivhaus standards. The budget here was significantly lower but what’s been delivered for £2.8 million is arguably far more radical. With 0.03 air changes per hour, the store holds the world record for air tightness in a building; and its design strategies also obviated the need to adopt any failsafe backup. IWM, conceptually at home with the notion of risk and holding your nerve, ran with the proposition, bringing along the same consultant team for the project.

It involved big thinking from the outset, explains Barry. IWM had intended to refurbish one of the many disused, historic red-brick ‘working’ buildings on the site’s less glamorous north end. This was possible, said Architype, but meeting the technical challenges as a retrofit would be difficult and would cost. Why not a new exemplar facility, designed from first principles and for optimum performance, but which also gave an identity to a notable archive? Historic England, happy that a run-down shed was being spared an unsuitable iteration, concurred.

Park the store’s imposing Corten facade for the moment; that’s not where the real magic’s happening. HARC, for all its innovation, bore its own burden of history in an outdated performance specification that demanded 5% fresh air supply, a client balking and demanding back-up plant – and two storeys. But IWM felt that if the structure was so highly insulated and airtight, it wouldn’t need plant to deal with the temperature differentials and stale air pockets that necessitate air movement in the first place. And going from two floors to one meant no problematic detailing of wall/floor interfaces where tenths of a degree matter. So they did away with both. Instead, the idea was to keep the storage ‘box’ as cool as possible without any intervention at all. Of course, that meant thermal mass and the creation of a sizeable concrete structure with 215mm thick walls and 200mm thick roof slab, around which runs a continuous sheath of 200mm PIR insulation.

But Barry emphasises that the innovation of ‘ground coupling’ ensures the building fabric itself works as hard as it can. And here it’s not just the 300mm thickness of the slab that’s key, but where it is and what it’s doing. The whole ground area site was dug out 400mm below ground level, a waterproof membrane laid and then the slab cast directly on the ground. ‘Below the frost layer, the ground in the UK remains at a constant 12ºC,’ says Barry. ‘So by casting the slab here and running the insulation down past it, we were able to effectively turn it into a 12ºC chiller plate in the summer and a 12ºC low temperature radiator in winter.’ Insulation creates an unbroken thermal line while the slab transfers its heat/cooling up the walls and onto the roof. ‘It’s brings incredible thermal stability benefits,’ he adds. He knows, because of the sensors placed all around the perimeter.

The superinsulated, airtight box reduces the supply air requirement to a minimum but means that fresh air supplied at, say 20ºC, will cause a spike in the relative humidity of the internal air. Rather than installing a commercial dehumidifier – like using a hammer to crack a nut – Architype combined a chiller and MVHR unit, recovering coolth from the chiller to pre-treat the incoming air, lowering its temperature and reducing energy requirements even further. ‘All the chiller’s doing is dehumidifying the air in an efficient way at low volume.’ he says.

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As for the outer skin, Barry says there are thermal benefits in the cavity between it and the box, and that it hides the loading bay – but the sense is that it’s about the architecture. Why Corten? Why not some other high performance/low carbon rainscreen cladding? It’s only now that the architect talks about a feeling for the site and a respect for its materiality and history, and the formal play that arose. Each of its 100 panels represents a year of IWM’s centenary, and each perforation an archive acquisition; some blank, others shot through- each carrying its own trace of the past. Barry says they weren’t looking for an icon or a ‘jewelled box’, but to pick up on the colours and textures of Duxford’s wartime buildings, yet the store has assumed a sense of quiet monumentality all the same. He finally confesses: ‘What we liked about the material is that its outer surface becomes sacrificial to protect the rest behind it and we felt this resonated with the story of war.’

Inside this sacrificial skin, the store is gradually shedding its latent heat to go from 18ºC to its working baseline of 12ºC. Currently costing £2000/year to operate, it will be half that when it finally settles and 1/100th of what it would have once cost for an archive building of this size. It took concrete and a lot of embodied energy but IWM can enjoy the spoils. A battle won in a climate war we can’t lose.

IN NUMBERS

Gross final energy consumption 4.10
Whole building design life 200 years
Embodied / whole-life carbon (kgCO2eq/m²) 738.40
Sustainability rating Passivhaus Classic