The Treasury set a tough brief for revamping its Westminster hq. We report on the £112 million refurbishment and the challenge of securing an 'excellent' BREEAM rating.
The design of the Government Offices, Great George Street was considered advanced for its time. The building, known as GOGGS, was constructed in two phases. The first was available for occupation in 1906 while construction of the second phase began in 1912 when the balance of the site became available.

The design moved away from that adopted by the adjacent Foreign and Commonwealth building which was constructed with a central corridor between cellular offices, to one of corridors opening to internal light wells. Not only did the corridors of the new Treasury benefit from natural lighting but, the adjacent offices received supplementary daylight via above door opening lights. This also improved cross ventilation.

A major change implemented during the first phase of construction was that the open fireplaces were dispensed with and a below window radiator system installed. Some nine hundred steam fed radiators were installed with ventilation air introduced into the rear of each radiator via a manually operated vent.

As Britain prepared for war, a decision was made to reinforce the basement at the western end of the site to form what is now known as the Cabinet War Rooms. By March 1943 over £850 000 had been spent on the construction of the massive 'detonator' slab. This was formed of concrete 900 mm thick and reinforced with train rails and steel girders. Upon examination the current project team decided that this should remain in place, only to be removed where absolutely necessary to facilitate the refurbishment.

It is interesting to note that prior to the war, records show that the building housed 4000 people, but this dropped to 2820 in 1942 partly due to the space consumed in creating the substantial defence systems.

In 1970 the building was listed as Grade II*, signifying its place both in commerce and history. The completed refurbishment of the west end to accommodate the Treasury is valued at £112 million and was completed within budget. The work commenced in July 2000 with occupation achieved in July 2002. The refurbished accommodation has been particularly successful in creating a space capable of increasing available occupancy from 850 to 1200 people.

Natural ventilation
When originally constructed the building was provided with open fires although prior to completion a radiator based central heating system was installed, cross ventilation being provided through openable sash windows located in external elevations and into lightwells. It was part of the Treasury brief that the natural ventilation remain as the environmental strategy for the office portion of the project. Air conditioning and mechanical cooling was only to be provided to spaces requiring it for operational or demand led activities. This brief, as well as being part of government policy to promote green issues sat very well with a building of load bearing construction and which was subsequently thermally massive.

To improve operational efficiency and to create a modern ambiance, the majority of the cellular offices were removed and eight of the light-wells enclosed creating atria. This had the benefit of substantially reducing winter heat losses, creating occupiable spaces within the lightwells and enhancing cross-stack ventilation to the surrounding offices during summer months.

A thermal analysis of the building was undertaken by BDSP to optimise the ventilation strategy so that the summer overheating criteria set by the Treasury would be met.

There are several ways in which performance targets could be set for a naturally ventilated building. Probably the most well known, and recognised, is the guidance given in the CIBSE AM10 Application Manual for natural ventilation in non-domestic buildings. It recommends that the dry resultant temperature be used as the reference comfort index. The dry resultant temperature is currently considered to offer a reasonable assessment of environmental comfort conditions within internal spaces, at low air velocities, as it takes into account the influences of both the air and mean radiant temperature.

While the dry resultant temperature is harder to measure in reality, than say air temperature, it does take into account the reality of a radiant effect of heavy-weight structures, which is a significant factor in the Treasury for achieving specified comfort criteria.

The performance targets that CIBSE AM10 suggests are for space dry resultant temperature not exceeding:

  • 25°C for more than 5% of the occupied period;
  • 28°C for more than 1% of the occupied period.

For this project, 5% equates to roughly 150 hours, and 1% to around 30 hours. This is in-line with the Treasury occupier brief, which called for no more than 30 hours to be above 28°C. However, this approach, while suitable for prediction purposes when coupled with the named weather file, does not recognise annual weather fluctuations and hence is not suitable during the actual free-float building operation.

Another approach for setting a performance target was to establish a value of temperature differential of, say 2·5°C-3°C, between external and internal conditions.

In either case, weather data for which the building was tested had to be defined. In the case of the Treasury a benchmark weather file for the first set of criteria was agreed as Kew 1967 and for the second LWC 1997.

Important factors in the calculations were the influences not only of the increased occupancy and the internal gains of lighting and small power but the combined effect of windows, blinds and the lightwell enclosures. An early decision was the removal of the security curtains that traditionally adorn government buildings. Offsite testing indicated that the glazing to the sash windows could be replaced with Romag laminated glass. These windows when operated in conjunction with roller blinds give a high degree of user interface combined with good ventilation and improved daylight.

Many parameters were considered in the design of the lightwell enclosures. Functionally they had to embrace a roof structure that was translucent, lightweight and would not be hazardous to occupants should it collapse under terrorist attack.

Ventilation louvres were considered that, when open, would provide free area equal to 10% of the plan area of the lightwell for smoke venting and also the means by which natural ventilation to the offices could be supported. Planning considerations were significant with strategic sight lines of parliament from St James's Park imposing restrictions on the ability to construct high structures at roof level.

The solution was to use a combination of vertical louvres by Colt from their Seafire range with a Vector three layer inflatable ethyltetrafluoroethylene (etfe) foil roof. The 2·5 m high louvres are of twin skin construction for winter insulation and form a reservoir above each light well, which on analysis, proved sufficient to ventilate all but the fourth floor offices. In summer the temperature gradient within the enclosure indicated that windows to the fourth floor should remain shut, natural ventilation in this instance being supplemented by roof mounted louvred housings vents. To limit summer heat gain the top layer of the Teflon roof is printed with a silicon dot matrix of 65% coverage giving a 50% reduction in solar transmission. The roof cushions are pressurised up to 200 Pa with a 5 Pa difference between the upper and lower membranes. Each of the four louvred faces are capable of being controlled separately with shut, half open and full open positions. Weather stations sensing wind speed, wind direction and rain have been installed on the roof and the information from these, combined with that from internal temperature sensors, control the louvres as shown in table 1.

It is accepted that fine tuning of the ventilation control algorithms will need to be performed over a period of time as running-in knowledge is obtained from full occupancy and experience of summer climate conditions.

Fire alarm system
Driven both by cost and necessity not to be intrusive into the fabric of this listed building the use of a wireless fire alarm system was negotiated with the Westminster district surveyors' service.

The system fully complies with BS5839: Part 1 and uses radio communication between the trigger devices and the fire control centre, the voice alarm system remains hard-wired. Other successful installations were examined, examples of these being systems installed at Knole House in Kent, and the Royal Military Academy.

Wireless systems have been extensively refined in recent years, and do not now suffer from the perceived problems of limited battery life, equipment failure and false alarms. The equipment used at the Treasury was more expensive than that needed for a conventional fire alarm system although there proved to be a net saving in the total installation cost due to a significant reduction in wiring. A substantial saving in builders work was also achieved, as installations of this type are far less intrusive in buildings of a historic or sensitive nature. Battery life was also a consideration as maintenance costs of replacement could be high, research however indicated replacement will not be necessary under seven years with a unit cost of about £12.

Lighting control
The brief was for modular fluorescent type luminaires having non-dimmable ballasts with inter-changeability of Category 2 or 3 louvres. All areas use low brightness luminaires with some element of uplighting. Lamps selected are high efficiency triphosphor and polylux phosphorus colour rendering CIE Group 1B Ra 80-89.

The lighting control system provides full switching and time control of all lights to all areas, lower ground floor and above. Solar control has been installed where benefit is to be gained from daylight.

Switching is limited to a maximum of fifteen luminaires and located at the nearest entrance with two-way switching providing local control in accordance with Part L of the Building Regulations.

General office lighting provides a design maintained illuminance of 350 lux on the working plan level. Illuminance diversity does not exceed 3:1 across each floor of the general office area. The illuminance conformity does not exceed 1·25:1 across a projected task area of 2 m by 1 m located in any position within the floor core area as defined within the CIBSE Code for interior lighting.

The lighting control system is computer (pc) based, with luminaires in the perimeter offices being individually addressable and those in the racetrack (open office) being controlled in groups of two and three.

The control system uses a control module, giving two separate addresses, thus reducing the cost of both hardware and wiring as the module was installed in a selected fitting to which the data cable is run.

Due to the sensitive nature of the room finishes, the system is designed to operate using radio frequency wall switches with multi sensor passive infrared (pir) detectors. These are also the receivers for the radio frequency wall switch signals. To avoid the need, for chasing and extensive making good of the plaster details.

The system provides full switching and time control of all lighting to the user occupier areas, including but not limited to, general offices, circulation spaces and restaurant. The system incorporates daylight control as the use of multi function pir detectors are used to provide the switching functionality required. Each individual office is provided with a wall switch linked to the control system via the multi function pir detector by radio frequency signals. In the general office or 'racetrack' area multi gang switches are located at the entrance to the area with additional switches located adjacent to the task area, switching being limited to a maximum of 15 luminaires, these can be switched on or off from either location.

In addition, pir detectors are installed to work in conjunction with the manual switches to give occupancy control. If the switch is off the lights remain off even if movement is detected, if the switch is on the lights come on when movement is detected. If no movement is detected the lights switch off after a preset time lapse.

The system also utilises a central time control, which can be programmed with pre-set on or off times. It is possible to provide a delay to keep a group or groups of lights on in a room after the clock command, by the operation of a local switch, allowing the lights for that area to be switched on.

Corridor and escape route lighting can be maintained if any group or groups of lights are switched on after the clock command. All clock control times and addressing of pirs, switches and luminaires can be changed from the head end pc, from which each floor can be viewed with the current status of all controlled luminaries.

Power
The power supply to the building has been replaced, with the main supply being taken from 24 Seven's 11 kV ring main; metering of the electrical power consumption being provided at 11 kV.

Within the building the 11 kV supply is taken from 24 Seven's ring main unit, terminating in the consumer 11kV distribution panel consisting of one incoming and two outgoing 630 A Hawker Siddley vacuum mv circuit breakers.

A 1260 kVA Cummins automatic standby generator has been provided and connected to each of the two main lv panels via air circuit breakers with mechanical and electrical interlocks and operated by phase failure and under voltage relays. The generator system has a fuel storage/transfers system capable of sustaining three days continuous operation.

The generator provides power to 20% of the general office lighting and 100% of corridors, stairs and all below ground area lighting and small power. The ups system is also supported together with the security and IT equipment, all life safety equipment and mechanical plant necessary to maintain the heating system. Load shedding is achieved through the lighting control system and bems with some critical plants having hard-wire interface. A 240 kV ups system is provided to support all key computer areas and security.

The Treasury brief was for a flexible sub-floor distribution for small power with power facilities being capable of connecting six 13 A socket outlets per 10 W/m² with an overall provision of 25 W/m². This requirement is provided from floor outlet boxes set in the timber floor. The box layout being coordinated with the space planning consultant's drawings.

The floor void depth was very limited (100mm). Having to accommodate power, lighting and IT cables it was decided to use BICC general multi plus cable. This provided some mechanical protection, with shielding for the IT cables and at the same time being of a small diameter and relatively flexible. The cables were installed along a main route branching off between the joists to connect the floor boxes.

Flexibility is limited by the floor construction, which requires floor boards to be lifted for any repositioning of outlet boxes. An extensive matrix of boxes was necessary to avoid the need to add or relocate boxes except for extensive office replanning.

BREEAM rating
It was always going to be a challenge to establish an 'excellent' BREEAM rating for a building that was conceived in the reign of Queen Victoria and constructed having open fires for heating. However this was the task set for the design team by the Treasury for their refurbished building.

The first stage was for JB&B and Foster & Partners, together with ECD Energy & Environment, to undertake a study of the building to establish compatibility of achieving an 'excellent' BREEAM rating with the magnitude of refurbishment that was envisaged.

The initial assessment was undertaken in November 1999 with a preliminary analysis indicating a shortfall, but with potential to exceed the necessary 70% to achieve an excellent rating. It was with some satisfaction that in October 2001, after the combined efforts of all parties involved, the project received notification of an 'excellent' rating achieving a score of 70·7%.

Within the management category full credits were obtained as all the issues here, including provision for commissioning, were within the development team's control. Similar success was achieved with health and wellbeing, with the exception of daylight where the deeper office spaces could not fulfil the stringent criterion of 2% daylight factor. Satisfying the criterion for indoor noise also proved to be difficult as openable windows for natural ventilation are not quite compatible with London traffic noise. Full marks were obtained however for all the requirements for ventilation, lighting control and thermal comfort.

As the prime energy source for heating and domestic hot water originates from the chp plant of the Whitehall district heating scheme the CO2 emission category was based upon this central source. Under this assessment the project only obtained eight of the possible fifteen credits available.

The refurbishment scored highly in both transport and materials categories, the location of the site being optimal with regard to proximity to public transport. Specifying quality products for construction materials and the retaining of substantial elements of structure and facade ensured maximum credits for these areas.

HM Treasury, 1 Horseguards Road, London SW1A 2HQ

Exchequer partnership
Bovis Lend Lease
Chesterton
Stanhope plc

Mechanical suppliers
AHUs: Woods
Ceiling diffusers: Gilberts
Chillers: YORK
Coolant: R407C
Computer room a/c: Liebert
Ductwork: Hotchkiss
ETFE roof: Vector Special Products
Extract fans: Woods
Fan coil units: Diffusion
Floor grilles: Hewitson
Heat exchangers: Rycroft
Hot water calorifiers: Rycroft
Pumps: Grundfos
Pressurisation: Holden & Brooke
Radiators: Hudevad
Toilet extract: Nuaire
Underfloor heating: Multibeton
Valves: Hattersley
Ventilation louvres: Colt International

Electrical suppliers
Cable management: Kinstiut/Armourduct
Electrical distribution: Crabtree
Electrical accessories: MK
Fire alarm/detection: Tri-Star
Floor boxes: Power Plan
HV switchgear: Hawker Siddley
Lighting controls: Simm Tronic/Lutron
Luminaires: Whitecroft/Carvill
LV switchgear: Square D
Public address: Defenser
Standby generation: Cummins Plant
UPS: Universal Power Systems