CIBSE Case Study Angel Building Refurbishment

1 Introduction
2 Cooling
3 Façade
4 Heating
5 Carbon footprint: recycling the structure
6 Project team

[edit] Introduction

Energy reduction was at the heart of the transformation of a drab central London office block into a CIBSE award-winning refurbishment.

The key to the success of this project was to involve the design team to develop the building to reduce energy consumption in conjunction with the building services solution,’ says Danny Hall, associate director at consulting engineers Norman Disney & Young (NDY). Hall is talking about the £70m reinvention of the Angel Building – a transformation from an ugly, inefficient 1980s commercial property into a swanky, modern office that is also a showcase for sustainable refurbishment.

The scheme’s recent triumph at the CIBSE Building Performance Awards 2012 – where it was the Refurbishment Project of the Year – and its shortlisting for architecture’s 2011 Stirling Prize, are a testament to the success of this transformation.

Situated on a busy crossroads in Islington, north London, the revamped building has a sleek, curved slate-coloured façade. Inside the building, the street’s hubbub gives way to a cool, restrained interior centred on an impressive atrium that affords glimpses of five floors of modern offices.

Fundamental to the building’s reincarnation has been the retention of its concrete structure. NDY has been involved in the makeover from the outset, both as environmental engineers and sustainability consultants. It was the firm’s analysis – demonstrating how much carbon could be saved by retaining the structure – that convinced the design team this was an option worth pursuing. ‘It saved about 7,400 tonnes of CO2, so it made a lot of sense to keep the structure,’ says Hall.

To tease every inch of extra space out of the site, the architect discarded the original cladding and sympathetically extended the structure to maximise the floor areas. At the front of the building this extension takes the form of a curved, steel-framed addition that follows the bend of one of merging roads, St John Street. A smaller, rectilinear extension has also been added to the southern elevation, while a new fifth floor has been added to the along with two spacious roof terraces giving views over north London.

In addition to the perimeter extensions, the floor plates have also expanded into the large central courtyard. The remainder of this opening is now covered by a transparent polymer roof of ETFE pillows to form the central atrium. In all, the addition of a new floor and extensions to the existing office floors has added over 60% more lettable space for the developers, Derwent London.

The scheme now totals more than 24,000 sq m of high-specification office space, along with an atrium, retail outlets and new roof terraces. ‘We’ve been able to generate a significant amount of new space,’ Hall explains.

Energy reduction was at the heart of this transformation. The re-used structure saved a significant amount of embodied carbon; the challenge for the building services design was to build on this achievement and come up with an energy efficient, environmental solution to complement the structural solution.

The engineers were helped in their task by the existing structure, which was based on a significant floor-to-ceiling height of 3.5m. ‘We inherited a building with fairly generous floor slab to ceiling slab heights, which opened up a lot of options for the type of cooling and heating systems we could install,’ says Hall.

As part of the renovations, a new fifth floor has been added to the roof, along with two spacious roof terraces giving views over north London.

[edit] Cooling

The designers looked at a number of options to provide cooling to the office floors – including fan coil units, chilled beams and chilled ceilings – before settling on an air displacement system. ‘In the end we decided to exploit the exposed soffits by opting for a displacement ventilation system concealed within a new 450 mm raised floor,’ says Hall.

Displacement air is introduced to the offices through grilles set in the floor. This arrangement restricts the minimum temperature at which air can be introduced to 19C which, in turn, limits the amount of cooling the system can achieve. ‘Because we’re introducing air close to people it cannot be too cold or they will feel a draught,’ Hall explains.

The upside of supplying air at this temperature is that it allows the building to run in ‘free cooling’ mode for 80% of the time. ‘At 19C we don’t have to cool the outside air for most of the year, we don’t have to use the water-cooled chillers, so we save loads of energy,’ says Hall. Another big advantage of this approach is that using mostly outside air helps keep the office environment feeling fresh. The downside is that the fans still have to run to push all this air through of the offices.

Another potential drawback of the system is that NDY was concerned that the displacement air would heat up in contact with the floor slab. The problem arises because air temperatures can reach 27C close to the ceiling soffit, heating the floor slab of the offices above. NDY had data from tests carried out in Germany showing that, depending on the temperature difference and the distance air travels in contact with the slab, the supply air temperature could increase by up to 2.5C. A temperature increase of this magnitude would significantly reduce the amount of cooling available, particularly at the perimeter of the floor plates, where the system has to cope with solar gains in addition to occupancy gains.

Hall’s solution is to deliver the air from the service cores to the floor perimeters in insulated ducts concealed within the raised floors. Each of the building’s floors is divided into quarters to allow the building to be subdivided for letting. Each of those quarters is served by a riser. Hall’s solution allows the temperature and quantity of air reaching a specific floor area to be targeted precisely. It also allows air to be ducted to cellular offices, where these have been installed by tenants. Air is extracted from the offices at high level through grilles set into the cores, and then ducted back to the roof-mounted air handling units.

[edit] Façade

Design of the building’s façade was critical for the displacement system to perform effectively. ‘We knew the maximum cooling capacity of the system, which was about 75W/sq m tops, so once we’d taken off the cooling needed to deal with the loads of a modern office, we were left with the maximum cooling load we could handle at the façade,’ says Hall.

To keep the cooling load within this limit the engineers worked closely with the project architect, Allford Hall Monaghan and Morris (AHMM), to develop a façade solution that balanced high performance glazing with solid elements. In the end the designers opted for high performance neutral-coloured glazing with a G-value of about 0.3 and low level solid elements on each floor. However, even with this type of glazing, to keep the solar gain within manageable limits a 500 mm band of fritting had to be added to the top of the glazing to provide additional shading.

The windows are fitted with internal blinds for glare control. To prevent heat build-up close to the glazing, NDY and AHMM developed a neat return air detail. According to Hall, when the sun hits the blind, the blind heats up and the heat form changes from radiant to convective. Their solution has been to incorporate a return air slot detail in the box housing the blind to remove this convective element as it rises upwards.

Careful detailing has also been used to extract air from beneath the soffit in areas where the floor plates have been extended. An existing downstand beam marks the outer limit of the original floor plate; this protrudes into the offices from the soffit. The intrusion impacted the flow of warm air along the soffit back to the return air grille at the cores. Tests by NDY in Germany had shown that without intervention, returning warm air at high level would dislodge from the soffit at the obstruction and fall into the occupied zone.

In another neat solution, to overcome this particular challenge the engineers designed an extension to the extract system to draw air through a shadow-gap created between the plasterboard ceiling panels of the new floor plate extensions and the downstand beam. The extract air is carried from high level to the floor void through ducts concealed within the webs of the steel columns that support the new structure. Once in the floor void, the warmed air is routed back to the main return air ducts in the cores.

The scheme also includes openable windows controlled by the occupants at will. About half of the windows can be opened. Hall says that because the displacement ventilation system is designed to use outside air for much of the year, the system is able to cope with additional air ‘better than most other systems’. The windows do not form part of the night time ventilation strategy; instead the air handling units run at night, with the cooling system off, to purge the structure of the day’s residual heat.

The design of the building's façade was crucial to the performance of an internal air displacement system.

The lighting scheme, too, was influenced by the cooling system. NDY restricted the heat output from the lighting to a maximum of 12W/sq m. ‘It was quite a challenge to maintain the design at that level but if we’d gone over, the lighting would have impacted on the cooling available for the space,’ explains Hall. The solution developed uses suspended high efficiency fluorescent fittings with daylight and PIR controls as part of a DALI system.

The lighting solution has been enhanced around the atrium, where the new infill has created deeper floor plates. The number of fittings has been increased in this area to give the impression of higher levels of light entering from the atrium than is actually the case. ‘The light in this area has been enhanced to make the offices feel light and open,’ says Hall.

Lighting was carefully designed to avoid impacting on the cooling load limit

[edit] Heating

In winter the return air from the offices, which can be at temperatures as high as 27C, is mixed with the outside air to achieve the 19C supply air temperature without having to use additional heat. The solution can recover up to 65% of the heat that would otherwise be thrown away. ‘It’s a really efficient system,’ says Hall.

When heating is needed, it is provided by a combination of three gas and two biomass boilers. The biomass was installed because, at the time the scheme achieved planning approval, the rules required a 10% reduction in the building’s carbon emissions from renewable technologies.

‘We tried to minimise energy use by making the building as lean as possible through the improved façade performance, our system selection and the plant efficiencies,’ says Hall. As a result, the biomass boilers are the scheme’s only renewables.

The two biomass boilers, located in the main ground floor plant room, provide 15% of the overall heat demand. In summer, a 550 kW boiler is sufficient to meet the hot water demand. Hot water is delivered to each toilet block via a hydraulic interface unit complete with a heat exchanger. In winter, a larger 900 kW boiler will supply additional heat for the ventilation system heater batteries, office perimeter trench heating and the below-floor fan coil unit for the main entrance door. The gas boilers provide supplementary heating when necessary. Variable speed pumps on both the heating and chilled water systems help match the circuit flow rate to demand.

Unlike gas boilers, which can be turned on and off instantly, biomass boilers are less responsive. ‘If the biomass boilers are servicing a big demand and that demand suddenly drops you cannot turn them off instantly,’ says Hall. To allow the boilers to consume any remaining fuel, Hall has included a giant 20,000 litre hot water storage vessel in the heating/hot water primary circuit to act as a thermal buffer, ensuring biomass boilers can run efficiently.

The hot water store also helps prevent corrosion in the boilers by keeping the water circuit warm. ‘Biomass boilers don’t like being fed cold water, so the hot water store helps provide back-end protection by maintaining a minimum return temperature,’ explains Hall.

Proof of the effectiveness of the building services solution is in the scheme being awarded a BREEAM Excellent rating. It has also achieved a B rating Energy Performance Certificate. ‘It’s an impressive performance from a 1980s building,’ says Hall. ‘But then it was a pretty heavy refurbishment,’ he adds.

[edit] Carbon footprint: recycling the structure

Re-use of the existing structure helps limit the carbon footprint of the development and significantly reduces the transport and disposal of demolition and construction waste, which reduces emissions and minimises dust creation.

The omission of unnecessary finishes and fixtures such as suspended ceilings helps expose the thermal mass and also reduces the amount of energy embodied in the finishes and their maintenance.

Using the concrete carbon calculator and data from the University of Bath, NDY estimated that the retained structure accounts for 39,500 tonnes of concrete, or about 30,000cu m, which is equivalent to about 7,400 tonnes of CO2 or running the operational elements of the building for 13 years.