Has Haworth Tompkins’ Stirling-winning theatre lived up to its sustainable promise?
When the Haworth Tompkins replacement for Liverpool’s Everyman Theatre won the 2014 RIBA Stirling Prize, the judges praised its naturally ventilated auditoria, applauded its use of concrete in cooling the fresh air and described its design as ‘exceptionally sustainable’.
Two years on and the exceptionally sustainable epithet has just been given added credence by the Chartered Institution of Building Services Engineers (CIBSE) after the Everyman Theatre won both the Project of the Year (Leisure) and Building Performance Champion in its Building Performance Awards this year. The win was in recognition of the scheme’s outstanding energy performance after it reported energy consumption figures of 159kWh/m2/year – beating the Carbon Buzz good practice benchmark figure for theatres of 275kWh/m2/year by over 40%, and significantly below the CIBSE TM46 theatre benchmark of 550kWh/m2/year, used in Display Energy Certificates.
To the Everyman’s environmental engineer, Waterman Building Services, this impressive performance will come as no surprise – sustainability was integral to the scheme from the outset. The initial plan was to build a larger theatre complex on a new site but Haworth Tompkins successfully argued for the importance of maintaining continuity by reusing the compact, city centre plot. To accommodate the new theatre the shell of the existing structure was carefully dismantled and the bricks salvaged for use in its reincarnation.
These reclaimed bricks make a striking reappearance as the walls of the 400-seat main auditorium, which is the heart of the new building. The space has been created to incorporate a thrust stage, which is encircled on three sides by audience seating to emulate the intimacy of its predecessor. The 25,000 exposed bricks give a familiar, comfortable ambience to the new space; importantly they also add a significant amount of thermal mass, which was critical to the auditorium’s natural ventilation.
'The client wanted a sustainable, low energy theatre so natural ventilation was seen as the obvious solution,' says Jonathan Purcell, director of building services for Waterman Building Services. This is easier said than done for a space which is, essentially, a windowless black box, where ventilation openings could, potentially, allow noise and daylight to enter the auditorium.
The method of ventilation adopted by the design team is to introduce fresh air at low level and then allow it to rise up, unobstructed, through the auditorium and out through its roof. Fresh air is drawn from a quiet road at the rear of the theatre. From an inlet louvre it passes through an acoustic attenuator and then into a giant concrete plenum buried in the ground beneath the workshop area at the rear of the main stage. In the summer this concrete cavern helps cool the incoming air.
From the plenum the air passes beneath the stage, through a second set of acoustic attenuators, and into a large horseshoe-shaped plenum hidden beneath the banked rows of audience seating. Perforated grilles beneath the seats allow the air to be drawn into the auditorium.
Inside the auditorium the largest single source of heat is stage lighting. The stage has 140kW of lighting installed, of which 65kW could be on at any one time during a show. In addition, the occupants and other sources of heat contribute another 50kW to the space.
The buoyancy of the air is increased by heat given off by the audience and theatre lighting, causing it to rise up through the lighting gantries to an acoustically attenuated exhaust air plenum, hidden above the auditorium ceiling. A giant duct, which doubles back on itself to help prevent noise entering the auditorium, carries the warmed air from the plenum to four giant cylindrical rooftop chimneys (nicknamed John, Paul, George and Ringo by the design team) and back outside.
Getting the height and diameter of the chimneys right was critical to the success of the ventilation system. The chimneys have to generate adequate buoyancy to pull sufficient fresh air through the space to keep the auditorium conditions comfortable and to meet the Building Regulation fresh air recommendation of 10 litres per second per person for the 450 capacity audience plus additional staff and actors.
Despite detailed modelling, the theatre trustees doubted whether a natural ventilation strategy would be capable of maintaining comfort levels throughout the year. As a consequence, two air handling units fitted with air source heat pumps to provide both heating and cooling have been installed inside the concrete air-inlet plenum. ‘In the two years the theatre has been open, these units have run once for a total of two hours and that was on one exceptional Saturday last summer after the stage doors were opened in the morning to bring in staging for a show followed later that day by two sell-out shows – a matinee and evening performance,’ says Purcell.
The rate of air flow through the auditorium is regulated by motorised dampers in the inlet and exhaust ducts. In winter fresh air supply rates are kept to minimum using carbon dioxide and temperature sensors and the heat pump in the air handling unit is used to preheat the auditorium when the boilers are not running. However, once the audience is in place the warmth it and the lighting generate means that no additional heat is needed.
The engineers have taken a pragmatic approach to the inclusion of the air handling units by using them to enhance the theatre’s natural ventilation strategy. This air blow is regulated by the automated control system as follows:
- Below 21°C the air handling unit supplies heat to the space and fresh air rates are kept to a minimum
- Between 21°C and 24°C, ventilation is progressively increased
- At temperatures above 24°C the Air Handling Unit fan is used to boost the ventilation rate.
- Above 26°C, the auditorium switches to mechanical cooling with minimal fresh air.
Since the theatre reopened in March 2014, monitoring has shown that the solution works and that it is possible to adopt a low-energy natural ventilation strategy, even for a theatre with high heat loads from stage lights and occupants.
Robert Longthorne, building development director at the Everyman, says the system does have to be ‘actively managed’ in autumn and spring when the weather flips daily between summer and winter conditions. However, he is pleased overall with the effectiveness of the auditorium mixed mode ventilation system: ‘The great thing is that you don’t get any complaints,’ he says.
It is because the ventilation solution is integral to the architecture of the theatre that it works so well. From the subterranean intake plenum to the unobstructed ventilation path through the auditorium and the four giant rooftop chimneys, all are essential features that have ensured the success of the ventilation strategy. Even the thermal mass of the reclaimed-brick auditorium walls is vital; in fact the engineers estimate that the mass they add to the naturally ventilated auditorium contributes to a temperature drop of up to 3.0°C in the auditorium. No wonder the CIBSE judges were impressed.
PERFORMANCE BY THE DAY
The plots below of carbon dioxide concentration and temperature show the auditorium system to be performing effectively on Saturday 5 July, 2014.
The times of the matinee and evening performances can be clearly seen on the carbon dioxide plot, as can the time of the interval. The plot shows carbon dioxide levels peaking briefly at 1000 parts per million (ppm), which is the CIBSE maximum recommended concentration, at the start of the second act before tailing off again.
The temperature plot shows the outside air temperature (blue line) peaking at 29°C, at about 2pm. Internally, the temperature remains remarkably constant at approximately 22°C, with a very slight increases at show times. The peak temperature is just under 24°C on the balcony (yellow line).