Birchensale School 1

The extension to Birchensale Middle School in Redditch is one of six school buildings being procured by Worcester County Council. While all have different architectural teams, they follow a similar briefing and procurement process.

ECD Architects invited the PROBE team to attend design team meetings to make observations on progress and suggest ways in which PROBE data could aid them in their decisions.

In this context the researchers act purely as passive commentators and information providers. It is not the PROBE team's responsibility to formally advise or make value judgements, but to identify potential pressure points, relate instances of problems and solutions from other buildings, and monitor the usefulness of that information to the design team.

Design details

The PROBE involvement took place over two design team meetings. ECD Architects and services consultant Whitby Bird & Partners had already established the requirements of the project, which is to double accommodation for 8-12 year olds from 300 to 600 within a budget of £1.5 million. Just over 80% of the budget will cover a new extension, while the building services – both new and refurbished – will account for 12%. This allows for 1784 m² of new-build and 539 m² of improvement to the existing fabric, to give a new total building area of 4336 m².

The existing building is a 1970-vintage SCOLA system-built construction in two blocks – a single-storey ancillary unit with hall, offices and boiler house, and a two-storey teaching block with classrooms and specialist teaching areas.

The blocks are connected with a single-storey link, which also provides the reception and main entrance.

Originally there were four blocks of four classrooms within two two-storey blocks, linked by a single-storey block of specialist teaching areas. These have since been rearranged to accommodate additional specialist teaching areas, some having to double as form rooms.

The client initially wanted to increase the number of classrooms to six per educational year. The brief has recently been amended to five and a half classrooms per year ie the building will be accommodated with classrooms for a five class entry school, but with specialist accommodation for a sixth form entry school.

The building will also house a music room and two practice rooms, one 'learning resources' room, toilet facilities for 600 children, an assembly hall/dining room, a gym, and a staff suite (with work area, social area, seating, kitchen and social area, and staff toilets). Staff facilities include offices, medical rooms, a reception office, staff training room, a store and stock cupboard, and premises management facilities.

Key design objectives

After consultation with school staff, ECD identified the design and planning of the four year-groups as the key to its strategy. This sets the criteria for the structural planning, the room sizes, circulation spaces and, in spaces like the laboratories, the choice of furniture.

To achieve this, ECD had three options:

• the addition of a new classroom block (the easiest to achieve, but not the best functionally)
• to build two new wings either side of the existing teaching block
• to extend the building along one elevation

The design team recognised that wrapping the existing building with two new wings would be both aesthetically pleasing and would serve to mitigate the poor performance of the existing fabric. However, it also created problems with natural ventilation and daylight. As the pros and cons were juggled, it became obvious that the strategy should be driven primarily by the pedagogic objectives and rather less so by the physical, environmental and engineering criteria. Hence the only option was to extend the existing building along the south elevation.

Figure 1, shows the ground floor plan of the existing building (with areas to be refurbished highlighted) and the new-build along the south elevation. Birchensale is a tad unusual in catering for children in the 8-12 age range. This means the facilities must be designed to span two stages of educational development, from general teaching to semi-specialist education.

Circulation is another major issue. Figure 1 shows that the original building was designed with a racetrack type corridor – fortunately generous in width – and classrooms with a reasonable ratio of external elevation to plan depth.

The architects have to deal with three issues: first they must ensure that the circulation works in relation to the new year-groups (and budget limitations mean that only the existing circulation routes can be extended). Second, the services engineers must provide environmental solutions for new land-locked spaces, and natural ventilation for new perimeter zones. Third, some areas are to be planned to accommodate possible future use by the public.

Servicing issues

Birchensale Middle School presents a paradox: schools are generally simple structures with minimal servicing, largely for reasons of cost and precedent. But as a consequence, the environmental challenges are immense.

Consulting engineer Whitby Bird & Partners had two specific targets: a daylight factor of 4% in the classrooms and a temperature limit of 28°C, with a permissible exceedence of 10 days in the summer term. These are the preferred criteria laid down by the Department for Education and Employment (DfEE) in Building Bulletin 87.

Whitby Bird has undertaken daylight modelling to establish daylight factors and thermal modelling to analyse the potential for overheating. A Tas simulation was run for the thermal analysis, and the daylight modelling was carried out using Lightscape. The following is a short summary of the issues and results.

For the thermal analysis Whitby Bird modelled the school inclusive of the planned extension, simulating the building over the year to identify the incidence of overheating in classrooms. Two types of window system were modelled, along with different glazed areas, varying degrees of solar shading, and several rooflight scenarios. U-values for the extension were taken as 0.23 W/m²K for walls, 0.24 W/m²K for the roof.

As readers will appreciate, for a classroom space to appear daylit the minimum average daylight factor should be above 2.5%. Anything below 2% is likely to require electric lighting during daytime. The architects were keen to expose the timber roof trusses as a cost-effective means of improving the aesthetics while removing the need for a suspended ceiling. Unfortunately, the required daylight factor could only be achieved with a plasterboard ceiling.

Whitby Bird & Partners discovered that the new classrooms are exposed to a lot of sunlight, and so the solar gain is high. Subsequent modelling showed that a 40% glazing ratio was the maximum that could be used before the classrooms began to overheat. Indeed, all the rooms were found to overheat unless August was omitted from the simulations.

The most effective glazing system was a three-part, 1.4 m high window with the top and bottom 0.5 m sections openable, plus shading to 45O on the lower window sections. With this, the balance between solar gain and daylight penetration resulted in a 3.4% daylight factor on the working plane – acceptable under Building Bulletin 87. The landlocked laboratories and the art room in the existing block are rather less exposed, but even here openable rooflights were deemed necessary to improve ventilation and daylighting. Whitby Bird also concluded than an openable rooflight running along the back of the new classrooms would be beneficial.

PROBE input: the brief

The brief was already well advanced by the time the PROBE team became involved, and therefore it was too late to include its input. Nevertheless it is helpful to write a brief so that the project targets are understandable to all. This should also cover cradle to grave monitoring, ease of maintenance and usability, comfort, health and aesthetics.

The brief should certainly not be couched in jargon, but sometimes it is difficult to appreciate when the use of a certain term can imply more than was intended by its author. An example here is the use of the word laboratory.

Laboratories imply fume cupboards, specialist gases and environmental risk – all sorts of baggage which might lead the client to expect more from the space than can be provided; or worse, for teachers to suffer more complexity than they can manage. In the PROBE team's view, it would be safer to use the expression 'science room', followed by a definition.

Usability and manageability should be key briefing points. School budgets tend to allocate over 80% of resources to staff, leading – among other things – to invidious trade-offs between maintenance, redecoration, energy and the staff themselves. Simple, low cost buildings are often easier to run and maintain.

The PROBE team questioned the need for the building energy management system to be controlled from Worcester County Council, and not delegated to the school itself. The balance between remote expertise and local inability to dictate events is not always well managed.

Comfort and control issues

In a naturally ventilated building, the design of the window elements and the quality of their engineering are crucial. Here, the PROBE team questioned why the interior blinds were being specified as a separate fit-out item and not integral with the window system.

The design team expressed a preference for mid-pane blinds, which do seem to offer the best returns in a three-element window. However, this was perceived to be a high capital cost item which the school may not be able to afford.

Usual practice is to leave the choice of glare control mechanism to the fit-out contractor, who will have a limited choice and more often than not will buy in simple venetian blinds. These blinds are often put in either late or only after the occupants start complaining. Then they are installed in haste.

This often leads to the choice of cheap (and easily broken) blinds, or worse, the wrong type which foul window handles or other forms of opening mechanism. Blinds can also prevent easy access to hopper windows, even when the blinds are retracted, and can rattle in air paths, thereby disturbing both pupils and teachers.

Mid-pane blinds can resolve such problems, with one important proviso. Leaving aside the issue of perceived higher unit cost (and the perception of extra design effort for no extra fee), this option introduces the need to cross, move or otherwise disrupt conventional boundaries of professional responsibility.

One way out of that is to design the facade elements as a package that can be dismantled contractually, with clauses to protect the integrity of the solution. That way the client is more likely to get freestanding and coherent packages which fit and perform well together

Architectural engineering

Careful consideration was given to the question of roofing an open courtyard to provide a protected space. Experience has shown such internal atriums are often poorly used if unheated, and an additional energy burden if heated.

Enclosing the space would also severely compromise the ventilation of the surrounding rooms. Public spaces like penthouse courtyards can also end up being 'colonised', which restricts free access and remedial actions to solve the aforementioned problems. The design team has heeded this message and has retained an open courtyard.

While the SCOLA-inspired circulation routes in the original building have worked well, there are questions over their effectiveness as ventilation routes. For example, it is the convention for corridor doors to be propped open to improve natural ventilation paths, with adjustable transfer grilles above the classroom doors. This immediately raises a number of design issues, such as the ventilation/acoustic trade-off, and the means by which ventilation flaps operate.

Schools historically have one less adjusting pole than the number of classrooms, hand winders can also end up overwound or trapped by furniture1. Electrical motors ultimately fail. So while ventilation flaps can be very simple, the entire system needs to be manageable, intuitive, foolproof, robust and easy to use.

The same is true of the well motivated desire by the architect Brendan O'Neill to provide special zoned wall switches with sun, cloud and night symbols to encourage appropriate switching. The PROBE team concluded that this will have to be tightly written in the specification to survive any cost-cutting exercise.

Future proofing

While prevailing guidance is dogmatic on structures like classroom clusters, group sizes, and teachers needs, these are being challenged by the demands of 'multi-model' switching. Schools are also expected to be more intensively used and to cater for increased hours of operation.

The architect has anticipated such demands in the design of some of the classrooms, removing walls to create a more appropriate structural grid. Mobile furniture is also being considered in place of fixed benches for the science rooms.

Designers clearly need official guidance that recognises both the change in building use, and the growing evidence that a mix of servicing strategies – mechanical and natural – can achieve energy efficiencies lower than those possible through natural ventilation alone.

The responsibility for ensuring Birchensale Middle School is 'long-life, loose-fit' seems to lie more with the authors and publishers of Building Bulletin 87 than it does with Birchensale's design team. Schools will probably be used more intensively, possibly year-round, thereby straining natural ventilation strategies.

No targets have yet been established for thermal and electrical energy use and the Probe Team would like to see the application of the 'energy tree' assessment technique of CIBSE TM22. The results would then assist cradle-to-grave benchmarking.

In the Probe team's recent experience many naturally ventilated buildings have suffered problems of overheating, despite having been the subject of modelling exercises. It would recommend that the design team reviews the modelling in this case to ensure that the assumptions are sound and that the results are robust.

Overall, the design of Birchensale Middle School looks compact and simple, and meets the client's needs. Time will tell if the design has been specific enough in the crucial areas, like year groups. It will also be interesting to see how the window/ventilation/blinds issue is resolved.