Playing it cool

The Davies Alpine House at Kew Gardens is as counter-intuitive as it is contemporary. Glasshouses are usually about creating warmer or more humid atmospheres than the prevailing external climate, but not the Alpine House. Here, the Royal Botanic Garden’s internationally important collection of rare and precious Alpine plants, naturally occurring from the arctic to the tropics, require conditions that mimic those of upland pastures: as much light as possible, relatively low temperatures and constant air circulation to prevent overheating. As it turned out, this was a specification for which the concrete was as important as the glass.

The task of coming up with a design that technically fulfilled the brief and provided a strong visual aesthetic was no mean feat. The pressure was intensified by Kew’s status as a UNESCO world heritage site, which received 1.5 million visitors in 2005, and the fact that the Alpine House was to be the first glasshouse to be built there since the Princess of Wales Conservatory in 1987. Not only that, work on site was watched from a public viewing platform and a BBC film crew also dropped in from time to time to check on the project’s progress.

It fell to Wilkinson Eyre, the architect responsible for Kew Gardens’ long-term strategic development plan, to design a building that maximised the surface area of the small site while also falling within the similarly modest budget. Geoff Turner, Wilkinson Eyre’s project architect, believes he and his team came up with a solution that maximised value from the £850,000 available.

Wilkinson Eyre proposed a radical low-energy building, drastically different from the old low-tech Alpine House, which had relied on carbon-releasing refrigeration trays to keep the plants cool, noisy fans to cycle the air and primitive shading in the form of a layer of whitewash. Turner explains: “We wanted to dramatically raise the profile of Kew’s world-renowned Alpine collection and we wanted to do it sustainably, leaving not only an architectural legacy for the gardens but also an environmentally sound one.” It was at an early planning meeting in 2003 that the concept of passive cooling, using an evolved concrete labyrinth built beneath the glasshouse, first came to light.

The concrete blocks forming the labyrinth provide thermal mass Photograph: Pat Collins

Patrick Bellew, director of environmental consultant Atelier Ten, was instrumental in proposing the labyrinth solution, which works like a Roman hypocaust in reverse. Cool night air is drawn in at low velocity down an 80 m long tunnel and through a simple interweaving network of 3.5 N standard concrete blockwork. The thermal mass of the concrete cools the air further, and it is then recirculated through the Alpine House above through a series of displacement tubes nestled among the plants.

Crucially, the use of remote thermal storage, as this principle is known, completely eliminates the need for artificial cooling using refrigeration. It also means there is no need for artificial cooling in the labyrinth itself, which maintains a stable temperature even when that outside exceeds 30°C. Night cooling is used to purge the labyrinth of heat in readiness for the next day. Appropriately, the concept is borrowed from nature – termite mounds use exactly the same principle.

Above ground, the vertical space of the 10 m high atrium provides for a natural stack effect: air is passively drawn in through permanently open perimeter vents beneath the overhanging glass panels and flows up and out via a temperature-controlled ventilation exhaust at the top of the structure.

Two other cutting-edge features complete the Alpine House’s striking design. One is the virtually frameless system of glass cladding using stainless steel tension rods and clamps. A combination of special low-iron glass and thin silicone joints ensures light transmission levels of more than 90%. The other innovation is the inclusion of an automatic retractable shading system that employs polyester woven fabric “sails” that unfurl like a peacock’s tail and can cover up to 70% of the glass panels. The fans neatly disappear behind the concrete walls when not required.

The actual construction of the Alpine House was not without its technical challenges, either. The two curved walls form an ellipse in plan, and to make things particularly challenging, the inner and outer shutters for each are a different radii, the wall slope inwards and not only are they thicker in the middle but also at a higher level than at either end. This is because they had to fit with the complex geometry of the stainless steel pockets and plates from which the glass-supporting tension rods are secured.

Paul Noonan, project manager for contractor Killby & Gayford, recalls how tricky the concrete walls were to construct. “We baton-pumped the RC40 mix from either end and let it start to go off before scraping the top of the shutter out to achieve the desired 19° angle across the wall’s width,” he says. “Attaching the 240 × 120 mm twin arches was also a breath-holding moment, with enormous forces transferred to the holding-down bolts at either end. But I’m happy to say the structure went in and up on the first attempt.”

Following handover in late May 2005, Kew then planted a huge variety of delicate alpines, such as a blue crocus from Chile that until recently was thought to be extinct and the UK’s national collection of Juno Irises. This provided the plants with time to bed down before the Alpine House was opened to the public in April this year. It seems fitting that a structure built to protect plants at risk from climate change should itself have minimum environmental impact – and concrete in part can be thanked for that.