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Saturday02 August 2014

AHMM’s Angel of north London

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Allford Hall Monagham Morris’s choice of curtain walling for its Angel building in Islington pays homage to Mies with floor-to-ceiling glazing and monchrome colour scheme

Don’t expect flamboyant splashes of yellows, greens and oranges à la Barking Central and Westminster Academy on Allford Hall Monaghan Morris’s latest office building. The five-storey Angel building, which is under construction at the Angel Islington in north London, is pure monochrome. It is encased in curtain walling that has black frames and spandrel panels, clear glazing without tinting and light-grey fritting. And its recessed top floor is pure white.

The Angel building is disciplined neo-Miesian in style. The bays to its curtain wall are framed in what look like exposed steel I-beams that pay homage to the Seagram Building and other 1950s wonders by Mies van der Rohe. In similar vein, it has generous expanses of clear glazing which stretch nearly from floor to ceiling, separated by the shallowest of spandrel panels.

The Angel building is an attempt by spec developer Derwent London to make good a large rectilinear blot on Islington’s landscape perpetrated in 1980. The new, bespoke-designed curtain walling was demanded by the expansion of the lettable floor area by 29% on two sides and the roof. It also contributes to a low-energy sustainable building that has earned a “very good” Breeam rating. And though it overlooks a busy central London traffic intersection, it even incorporates windows that can be opened by occupants.

Sustainable skin

The curtain wall plays a major part in controlling the Angel building’s internal environment in a sustainable and largely passive manner, which has helped merit a “very good” Breeam rating.

“We wanted as much natural daylight as possible,” says AHMM’s associate director Wade Scaramucci. “We also wanted bigger windows to see the mature trees on all sides.”

Scaramucci could also have added that generous daylight and views are essential to bring relief to what would otherwise be gloomy deep-plan office space, with floor plates of up to 23m in depth between the perimeter wall and central atrium and up to 19.5m backing on to opaque circulation cores.

The architect’s solution was to run clear, see-through glazing from a low sill height of just 810mm right up to the ceiling height of 2.885m. The light transmittance through the glazing was increased by using low-iron glass. And the light penetration into the deep-plan space will be enhanced by ceiling lights that respond to changing daylight levels.

Glare from sunlight is reduced by fritting on the glass above eye level, with 15mm wide strips of light grey enamel alternating with 15mm strips of clear glazing. The fritting also reduces solar heat gain, which is further cut by a colourless low-emissivity coating that was applied to the glass to reflect heat-generating infra-red rays.

The curtain wall even comes with opening windows that offer occupants direct control of the ventilation and temperature of their own office space. Openable windows are compatible with the building’s low-energy system of displacement ventilation, whereas they would have conflicted with full air-conditioning. As they are 1.5m wide, top-hung and heavy, these windows are mechanically operated.

Finally, the curtain wall packs high thermal insulation, with an overall U-value of just 1.5W/m2K that easily beats the building regulations’ Part L requirement of 2.2W/m2K. Thermal insulation is achieved partly by a 16mm-wide air-filled cavity in the double-glazing units and partly by efficient solid insulation behind opaque panels.

Five facade variants

Five different variants make up all the facades enclosing the Angel building. They are:

Typical new office floor

A harmonious facade has been created by applying identical fritting of horizontal stripes across adjacent stretches of clear and opaque glazing. In each 1,200mm-deep band of fritting, the lower 450mm stretch runs across the top of the clear-glazed window, while the upper 750mm stretch runs across opaque glazing which conceals the full floor zone.

When viewed from outside, during daylight hours at least, there is no distinction between transparent window and opaque floor zone. The recessed spandrel panels of black powder-coated aluminium cover only the 810mm upstands from finished floor to sill level.

Typical existing office floor

The facade to existing office floors appears identical to that covering new office floors. The only difference is that the same 1,200 mm deep band of fritting covers no more than the deep downstand edge beams of the existing concrete structure.

Ground floor retail

An air inlet band 335mm deep runs across at transom level.

Outward sloping fourth floor

The fourth floor slopes outwards by seven degrees to reflect the mature trees below.

Recessed fifth floor

A new fifth floor of office space has been added. To make it less prominent, it has been recessed behind roof terraces and clad in a flush, pure white facade of structural glazing.

Working with Scheldebouw

The distinctive bespoke design of the Angel building’s curtain walls was the product of an intense collaboration over several months between AHMM and Dutch supplier Scheldebouw. Part of the Italian Permasteelisa cladding empire, Scheldebouw was one of four cladding firms awarded pre-contract service agreements by Derwent London in advance of the main contract being agreed with contractor BAM.

One upshot of the collaboration was the prefabrication of huge panels up to a full storey height of 4.7m and two window bays, of 3m wide.

Project architect Steve Smith explains: “Cladding installation was on the critical path of the programme, and using 3m rather than 1.5m wide panels saved installation time and reduced the overall construction period.

“The 3m-wide bays are also important architecturally in allowing us to achieve the generous proportions of glazing that maximise views and daylight.”

The large panels were prefabricated complete with glazing, insulation and finishes at Scheldebouw’s works in Middleburg, south-east Netherlands. On site, up to 16 panels a day were dropped straight into place by three tower cranes without the need for scaffolding.

The precise profile of the frames were also designed in collaboration with what Smith calls the “cladding craftsmen” of Scheldebouw. The prominent mullions that resemble steel I-beams were actually extruded out of aluminium alloy in two L-shaped half sections — one for each adjacent bay. This fabrication is only slightly more contrived than Mies’s famous curtain walls of the 1950s. True enough, Mies exposed real steel sections in many of his facades, but in most cases these were not part of the building’s actual structural frame, which had to be covered by fire protection.

Project team

Client Derwent London, Architect Allford Hall Monaghan Morris, Project manager Buro Four, Structural engineer Adams Kara Taylor, Cost consultant Davis Langdon, Services engineer Norman Disney Young, Contractor BAM Construction, Cladding contractor Scheldebouw

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