poorly mechanically ventilated existing theatre building, extending it with new foyer, rehearsal room and studio theatre; then cutting into the existing fabric in order to bring air in and – with heightened architectural drama – let the warmed stale air out.
As a theatre it does however work well in terms of our qualifiers and exhibits successfully some of the characteristics of a naturally ventilated auditorium.
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1. clear ventilation paths as part of the site strategy. Note the Contact Theatre has compromised site ventilation routes
2. use of acoustic dampers to lower noise ingress at intake and extract
3. use of a plenum to balance out air flow across the auditoria with exposed thermal mass within the plenum and auditorium for summer temperature control
4. height of auditorium to allow space for stratification of hot air to collect above the occupant zone
5. extract stacks above auditoria to take advantage of thermal buoyancy
6. BEMS for lighting, heating, ventilation and external weather condition monitoring
Figure 90: Natural ventilation elements of the Contact Theatre
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The Contact Theatre – like other low energy auditoria – relies on the design of the plenum in order to attenuate, distribute and warm in-coming air. This is in effect an under croft set beneath the seating area, constructed out of heavy materials with exposed thermal mass in order to act as a heat sink. It is recommended to split the plenum into chambers – as in the Contact Theatre - to allow for a more balanced distribution of fresh air into the auditorium. Ideally this plenum is served by
louvered inlets to opposing directions in order to facilitate wind pressure ventilation by avoiding localised negative pressures. The Contact Theatre is unable to do this as it is blocked by the foyer space –should this have been planned differently or does it indeed matter as the main auditorium ventilation appears to function well. The thermo-dynamic of the warmed rising air in the space draws the cooler fresh air through the plenum; thermal buoyancy driven ventilation solutions such as in auditoria are generally more robust and less complex than those relying on wind driven alone (Cook M. & Short A., 2009).
The Architect’s response to the site was driven by the placement of the existing theatre building, the position of a large car park and the need for visibility from the main road. The design has endeavoured to make the best of this in terms of natural ventilation with the use of high inlets for the Studio theatre and the air inlet to the main theatre space is orientated out into a courtyard, at present a paved and planted space. There is a risk here however, in that future development of the courtyard could introduce a source of pollution and the main theatre auditorium then has few options beyond a redesign of the intake strategy. As a general rule we can assume that some control of the external space for sourcing clean air is important and in-built
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flexibility to allow for contingency actions if required. Alternatively, the use of high level wind scoops could have been an option considering the stack driving force of the warmed air.
Figure 91: the pedestrian courtyard to the rear of main theatre showing louvered air intake beyond fire escape door
Cook and Short (2009) note that fly screens, grilles, dampers, acoustic attenuators and insulation will all reduce pressure along air inlet paths and that it is important to check that the total effective inlet area equates to what is required in design. Acoustic attenuation in particular reduces effective duct area by up to 50% - however, it is a necessary attribute particularly in an urban environment.
We can conclude that the plenum is critical to the architectural section of the naturally ventilated auditorium; it is however a place of restricted access, low head height and tightly packed acoustic attenuators. This space is not impossible to clean but it is not easy to do so and inevitably the incoming air brings with it dust and dirt.
It does appear that one of the criticisms of ducted mechanical ventilation – that of the
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potential for poorly maintained filters – could be levelled equally at the restricted design of the plenum in naturally ventilated buildings. However, there is little evidence as yet to show poor air quality in the theatre as a result.
The auditorium itself need not suffer the conflict between balanced acoustics and exposed thermal mass as this mass is largely effective in the plenum to temper the incoming air. The upward evolving heat issued by the audience and the lighting is an important part of the thermo-dynamic of the ventilation system. Again looking at the architectural section, height is important – keeping the pool of warmed stale air above the heads of the audience; Short A. notes (2017) that the required volume may be in excess of what makes for good acoustics, not apparently a conflict in the Contact Theatre but one which may be of future concern.
The exhaust stacks must have a free area that equates with the supply (Cook and Short 2009 report that the open area should be 3 -4 % of floor area of the space to be ventilated) and, like the inlets, require acoustic attenuation. Both the Contact Theatre’s two auditoria and Bedales Theatre (see Literature review) have low speed fans within the stacks to assist in the event of summer stagnation temperatures.
Curtis S. – the facilities manager – noted that in the Main Auditoria the fans are positioned below the acoustic attenuators which assists with ease of access but does give some acoustic disturbance when in operation; within the Contact Studio Theatre they are positioned in reverse and so present less of an acoustic issue but require dedicated access. It is also noted that the stacks ideally are constructed with thermal mass in order to retain heat or insulated if made of light weight construction – the
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Contact Theatre design has both examples – insulated zinc-clad stacks to the Main theatre and thermally massive brick ducts to the Studio theatre.
Figure 92: brick stacks to studio theatre Figure 93: insulated zinc-clad stacks to main theatre
In contrast to the Main Auditorium, the Contact Studio Theatre was wholly new build and not compromised in terms of air intake. Air is extracted from this space through four H pots which do not always extract but sometimes, depending on wind pressure, blow in. Woods and Fitzgerald (2007) in their POE showed that the Contact Studio Theatre is not fully operating in a displacement mode as cold air also drops down one of the stacks back into the studio space however this is sufficiently dilute in a 5m high space not to cause a problem for audiences. Originally there was to be a further mezzanine gallery in this space and access to an external theatre space
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on the roof (Curtis S. 2017). This would have further helped to justify the perceived construction expense of the brick H pots.
The stack terminations must be resilient to wind direction changes so as to always have the benefit of a positive pressure gradient from intake level. The H pot at the Contact Theatre achieves this being of an omni-directional design; a higher stack gives better thermal draught but there is an inevitable trade off with issues such as cost and maintenance. These are critical architectural elements for natural
ventilation and part of the rich vocabulary of European chimneys and middle-eastern wind towers. If there is a lesson to be learnt it is to provide ease of maintenance access, less critical for cleaning as the air is exhausted at these points but necessary to maintain the mesh for the prevention of vermin ingress.
4.2.6 Operational issues. Information on the operation of the theatre is