Feature

Thermal mass in schools

Exposed structures could play a crucial role in combating overheating in classrooms. Nick Jones finds out how to do it right

As the planet grows hotter, keeping classroom temperatures within acceptable limits will become harder and harder. In 2017, the government imposed tougher overheating targets for school buildings, in an update to its Building Bulletin 101 guidelines on ventilation, thermal comfort and indoor air quality. The Schools Design Group at the Chartered Institute of Building Services Engineers (CIBSE) recently modelled the performance of 11 newly designed schools, both in our current climate and under future projections, using weather files to represent scenarios of 2°C and 4°C above pre-industrial levels.

While all met the BB101 standard today, a number of classrooms failed under the 2°C scenario, and the majority breached the target at 4°C – a temperature rise that could occur as soon as 2065, according to the UN’s Intergovernmental Panel on Climate Change. In seven of the schools, 100% of classrooms failed under the 4°C scenario.

One answer to the problem of overheating would be to install more mechanical cooling – but this is neither sustainable in terms of mitigating climate change or future-proofing schools’ running costs. Instead, we need to find low-energy methods of keeping classrooms comfortable – a quest that is set to become a fundamental part of school design.

One potential strategy is to use the thermal mass of a building to even out diurnal temperature variations, by absorbing excess heat during the day. This can be very effective: CIBSE’s research found that many of the best-performing schools built in recent years were thermally massive structures. But not all thermally massive schools are created equal, and some performed considerably less well, typically those without adequate ventilation. So it is vitally important that designers understand how to leverage thermal mass to its full potential – and that they pass this on to those operating the building to enable them to benefit from low-energy cooling for many years to come.

Insights: Concrete and masonry school structures

School buildings need to be robust, resilient and low maintenance, and concrete and masonry provide these benefits. Of particular importance is the inherent fire resistance of these materials – each year more than 2,000 schools in the UK suffer fires large enough to need action by the local fire and rescue service.

Concrete and masonry not only provide fire protection for the occupants, but to the structure itself, particularly as this does not usually rely on the ongoing maintenance of additional linings to meet minimum standards. Their non combustibility limits the extent of damage and minimises repairs and disruption, so that the school, or part of the school, can be reopened quickly.

A concrete structure can be cast in situ, precast or a hybrid of the two. The spans required in school buildings are normally around 8m x 8m, with typical room sizes ranging from 57m2 for a junior classroom to 90m2 for a drama studio. Concrete flat slabs, either normally reinforced or post-tensioned, are well suited to this flexible grid.

They can be supported on walls or columns, depending on the requirements of the project. Using concrete or masonry walls means that the heavily trafficked corridors are enclosed in a robust material, while columns offer greater long-term flexibility. Both have been used to good effect in recently built schools.

Concrete can also be left as a low-maintenance finish, a visually pleasing backdrop to school life.

Jenny Burridge is head of structural engineering at The Concrete Centre

Night purging

A low-energy cooling strategy using thermal mass has two essential components. One is a high proportion of exposed heavyweight structure, to absorb heat during the day. The other is a natural or mixed-mode ventilation system that cools the building at night, thereby “resetting” the structure so that it is ready to repeat the cycle the next day.

In order for a night-purging strategy to be successful, the building’s users have to have absolute confidence in it, warns Jeremy Climas, head of education at Max Fordham, which has designed a series of high-performing naturally ventilated schools. “There are thousands of buildings where, for example, the design team allowed for natural ventilation at night but the people who operate the building don’t use it like that. It’s not enough to design something that you think should work, you have to have client buy-in.”

Understandably, one of schools’ chief concerns is security. “If night ventilation relies on opening windows then you can be pretty sure it’s not going to happen,” says Climas, “because nobody wants to leave windows open in an unoccupied building overnight.” A safer alternative is a grilled or louvred opening, often behind a closable panel. If the school is in a noisy location, these units also need to incorporate acoustic buffers to insulate against external noise. This is particularly important in city locations and for rooms close to play areas.

In order to create a cross-draught in naturally ventilated classrooms, there needs to be both an opening through the facade and a means of exhausting air. As rooms rarely have two external facing sides, air usually has to be vented into a single-sided corridor or atrium via a fire-rated, acoustic-attenuated bulkhead against the corridor-side wall of the classroom, which allows air to escape while preventing noise from entering.

External louvres, meanwhile, should be located as close as possible to the exposed soffit, although the layout also depends on factors such as window size and shape and the orientation of the classroom. The extra depth of side panels can be an advantage for placing acoustic buffers, says Giovanni Bonfanti, director at Walters & Cohen, which has just completed the naturally ventilated extension to St Paul’s School in London (see box, below). They can also provide an architectural feature – such as the vertical timber louvres on the facade of Walters & Cohen’s Reigate Grammar School.

The upshot is that, for an essentially simple process, natural ventilation can feel complicated and may need a bespoke solution. “We always say to clients that a naturally ventilated building isn’t necessarily a lot cheaper,” says Bonfanti. “It’s a simple solution, but there’s a lot to build.”

In the reality of today’s public-sector school procurement, compromises often have to be made, particularly where layouts include doublesided corridors with no obvious means of cross-ventilation. At Penoyre & Prasad’s BREEAM Excellent Bobby Moore Academy in east London, mechanical ventilation and heat recovery (MVHR) units were used, partly for acoustic reasons – the building is on the flightpath to City Airport – and partly because of the energy savings that MVHR systems provide. The units, positioned at high level behind fixed louvres, incorporate an ultra-lowenergy fan that can either draw air in from outside or push it out. It also enables night-time purging to take place securely without opening windows.

The BREEAM Outstanding-rated Ashmount Primary School in north London, also by Penoyre & Prasad, offers a variation on the same theme. This three-storey building uses a ‘e-stack’ low energy ventilation system located at the back of the classroom. In winter, the fan draws air down a chimney stack and premixes it with warm air before distributing it into the classrooms. In summer, the fan operates in the opposite direction, exhausting warm air drawn out of the classroom from windows opened in the external wall. “In terms of night purging, both systems work in a very safe way and worked well for an inner-city site,” says Rafael Marks, associate partner at Penoyre & Prasad.

Automation can play a key role – at Bobby Moore Academy, the louvres are connected to a building management system (BMS), which opens them at night, as well as to air monitors, which can trigger daytime opening in response to temperature and carbon dioxide levels. Teachers do still have an element of a manual control, over an openable window at the lower level of the louvre panel.

St Paul’s School second phase, by Walters & Cohen, completed 2020

Walters & Cohen has just completed the second phase of its energy-efficient, concrete-framed teaching complex at St Paul’s School in London. Exploiting the structure’s thermal mass was one of the main drivers from early in the design process.

“The MEP engineer, Max Fordham, was targeting reductions on energyuse within the building, and we are always keen to embrace natural ventilation wherever possible, so a high thermal mass was a good way of bringing the two together,” says Walters & Cohen associate director Tim Rowley.

All of the rooms with natural ventilation have a large, low-level manually operable panel, and a smaller, high-level automated panel. This high-level panel, linked to the BMS, opens when the room is too warm, has too much CO2, and at night. Air is drawn across the classrooms and into the deep-plan building’s circulation area, from where it is exhausted via a series of lightwells.

The acoustic strategy involved the use of fabric hanging-baffles suspended within the classrooms, augmented with some acoustic treatment on the walls. “The use of carpet was crucial as well in order to avoid sound ‘bouncing’ between the floor and ceiling,” Rowley adds.

“We are designing state schools right now which will use exactly the same principle. We might not have the budget for the concrete to be finished quite so finely, but the greater challenge, particularly for schools in an urban context, is dealing with external factors such as noise and air pollution when trying to implement natural ventilation. Using the thermal mass for night-time cooling doesn’t have this constraint so we are always keen to utilise it.”

Structure

The other key element of a successful thermal mass strategy is, of course, an exposed heavyweight structure. Concrete is ideal because its high specific heat capacity and density allow it to absorb a lot of heat, and these are combined with a moderate thermal conductivity, so absorption takes place steadily over the course of the day. From a school’s point of view, concrete also offers a range of other advantages, from durability and low-maintenance to adaptability.

“I would say 90% of the schools I’ve done have had a concrete frame,” says Marks, “for thermal mass reasons but also because they give other benefits such as acoustic and fire properties. A flat slab gives you the flexibility for different services and also longer-term adaptability.” 

In multistorey schools, where high ceilings are often unfeasible, a thermally massive soffit can be vital for moderating the classroom environment, he adds. “All the energy that’s generated from kids obviously needs to be absorbed.” Marks also points out that precast-concrete planks can be added to other structural systems as a means of incorporating thermal mass – such as at Penoyre & Prasad’s recently completed Anna Freud Centre, a multistorey centre for children and young people’s mental health, which has a hybrid structure with timber.

But exposed concrete also presents challenges, particularly since the BB93 acoustic standard for schools has become stricter on reverberation times. The government’s baseline designs state that acoustic panels should cover about 40% of the classroom ceiling area, which could obstruct much of the thermal mass of a soffit.

Solutions include suspending acoustic panels and incorporating them with light fittings to reduce unnecessary clutter – generally suspended systems need about 600mm clearance, so work well in room with a floor-to-ceiling height of 3.2m or more. “It’s a fairly standard detail now,” says Marks, “and the added benefit is that the space feels lighter and taller.”

Bonfanti suggests vertical rather than horizontal baffles, which have less impact on the soffit. “In some cases they are even more effective, because both sides of the acoustic material are exposed to the sound reverberation.” Carpet is an option for reducing the reverberation between soffit and floor (see St Paul’s School box) – but this means forgoing the useful thermal mass of the exposed screed.

There are various techniques to make the concrete structure as materially efficient and low-carbon as possible. At Sevenoaks School Science & Technology Centre in Kent, Tim Ronalds Architects used ribbed precast-concrete soffits in some areas, to minimise the concrete used. “You don’t need as much of it and it increases the surface area so the thermal mass works even better,” says Climas.

Another innovative approach at Sevenoaks School was the use of groundwater-chilled pipes cast into the concrete to super-charge the slabs’ cooling potential in high-occupancy, high-energy areas such as labs. Although this was a private-sector project, with a corresponding budget, Climas suggests that this solution could also be pre-installed in the state sector as a means of future-proofing new buildings. “You can cast it in now, because it’s just some plastic piping – it’s not a particularly expensive thing to do. The expensive bit is putting in a chiller or heat pump, but once the pipes are in, that can be installed in the future.”

It is also important to remember that solutions may need to be classroom-specific – what works in a normal teaching space might not in a workshop or theatre studio. At Bobby Moore Academy, for example, larger classrooms have two MHVR panel units, and excess air can also be drawn out of the back of the classroom into the 4m-wide corridors, from where it extracts via three louvred rooflights.

Thermally massive buildings that are well-designed and well-operated are already providing relief in the more frequent heatwaves the UK is experiencing. Climas recently returned to one of his first Max Fordham projects, The City Academy in Hackney, on a scorching hot summer day. “The school had continued doing the night ventilation the way they were supposed to, and they managed to get all of the heat out of the concrete overnight. It was wonderfully cool, 6 or 7˚C lower than outside.”

Photos Hélène Binet; Tim Crocker, Ben Tynegate