A primary school in Exeter won’t win any architectural awards, but is earning gold stars in zero-carbon and Passivhaus design. Thomas Lane swots up on how to deliver a low-energy building on a budget

If this school were sitting a zero-carbon design exam it would get a very good mark. Take a look at its answer paper. Use of proven low energy design methodologies: tick. Use of robust construction techniques: tick. Use of renewable technologies with very generous government grants: tick. Potential to form a standardised design for use in other schools: tick. Future-proofed against climate change: tick.

This star pupil is Montgomery Primary School in Exeter. Its Passivhaus design cuts energy use to an absolute minimum. The energy that is still needed for heating, lighting and powering appliances such as computers comes from an enormous 1,300m2 array of PV panels that are integrated into the roof of the building. But what really distinguishes this school from other Passivhaus designs is that it is the first in the UK to be built by a big contractor, and is also the first to use precast concrete panels rather than timber or conventional masonry.

Contractor Bam and the rest of the design team had to go through a steep learning curve that included drawing on Bam’s experience of Passivhaus construction in Germany and Holland. But the effort has paid off: Bam reckons the experience helped it win the job of building the Co-op’s ultra-green HQ in Manchester, worth £100m. Perhaps more importantly for the future of Passivhaus design, the technique of using insulated precast concrete panels to build the structure could easily be adopted for other schools, which fits with the recommendations in the James Review. So how did the team do it?

The budget option

This project started with the last government’s aspiration to make all schools zero carbon by 2016. The Zero Carbon Task Force was handing out grants to enable pilot schools to be built in each region of the UK so Exeter University, designer NPS and Devon council put in a successful bid for funding. The council received a grant of £1.1m, which it topped up to £1.5m over and above the primary capital programme cost of £7.3m.

The Passivhaus and PV combination was selected for two reasons. Firstly the team had to achieve zero carbon without relying on external energy sources such as biomass. “There is no wind on site or trees for biomass so you end up using PV,” explains David Coley, senior research fellow at the University of Exeter. However, the cost of PV meant the school had to be as low energy as possible to keep within the budget. “If we had just spent the funding on renewables we would never have achieved zero carbon,” explains Coley’s colleague Stephen Simm who is working with NPS on the design. “Improving energy efficiency is a better route and the advantage of Passivhaus is there is a lot of research and experience behind it which show it works.”

Bam took advantage of the Passivhaus experience of its European colleagues; the Dutch arm of the company has been building Passivhaus projects for 16 years and has consolidated its experience into a huge book that covers everything from design through to installation and validation. The UK team visited Bam projects in Holland and the German team came to the UK to talk about their experience. A key message was the importance of ensuring the design conformed with the strict requirements of the Passivhaus planning package design tool [PHPP]. “It was clear from our experience in Europe that we needed to address the modelling at an early stage,” explains John Willcox, construction director for Bam Western.

As part of this process, before the contract was awarded the team got precast concrete panel maker Buchan and M&E specialist NG Bailey involved in the design, and asked Passivhaus consultant Warm to do the modelling, with Bam acting as a preconstruction advisor. The modelling highlighted in advance the issue of thermal bridging: attaching heavy PV panels to the roof without compromising the insulation would necessitate a specially-made insulated connecting bracket. Another potential problem was heat loss through doors into
the playground. The solution was a minimal number of doors, with lobbies, and electro-mechanical operation to keep heat in.

Meeting Passivhaus standards

The second lesson learned from abroad was how to ensure the project met the onerous Passivhaus airtightness standards. Bam employed an airtightness specialist whose role included making sure the workmanship was up to scratch. Issues included tackling thermal bypass, a phenomenon where convection currents within insulated cavities reduce the effectiveness of the insulation. Any gap greater than 2mm could be a problem, so polystyrene insulation fixed at the junction of the wall and slab had to be secured firmly against the edge of the slab and sealed to prevent thermal bypass.

On-site prototyping revealed that expanding foam was the best solution.

Insulated precast concrete panels were chosen for several reasons. A central aspiration was to protect the school against climate change, by getting thermal mass into the building to help even out temperature swings. Theoretically this should help reduce energy use too as heat gain from IT and pupils is stored and less energy is needed to heat up the school in the mornings. The panels would also be quick to erect and easy to make airtight. “We could have gone for a precast concrete frame but that would have meant a separate cladding system and more interfaces and workmanship issues,” explains James Turner, Bam’s senior site manager. “It was also a scaffoldless system which meant fewer people on site.” Chris Rea, NPS’ QS and client representative, adds that another benefit was the ability to repeat the project easily elsewhere in the county.

The panels feature a 150mm structural inner leaf joined to a 75mm outer skin. Sandwiched in the middle is 150mm of PIR insulation, which gives a Passivhaus-friendly U-value of 0.15W/m2K. The high quality concrete finish can be left as it is externally and simply painted on the inside, which is cheaper and helps maximise heat transfer between the concrete and interior. The really clever bit is the jointing system. The panels feature a rebate at the ends with wire hoops protruding into the gap. Steel rods are used to link the hoops from each panel together and grout is poured into the gap to lock everything together. This provides a high quality structural and airtight joint.

What will the neighbours think?

The two-storey external structure was erected in just nine weeks. The speed of construction was great for Bam but meant careful management of neighbours’ expectations. “People who live in houses backing on to the site would go to work then come back in the evening and find a classroom there,” says Turner.

With the panels erected, the next step was installing the windows. But what is the best way? Positioning the windows so they sit on the same plane as the insulation is good for thermal performance but bad for airtightness as air could get into the cavity. After much debate, modelling and prototyping with a mock-up panel and window the team settled on fixing an EDPM cloak around the window edge to stop warm air escaping. The window is fitted into the aperture between the insulation, which minimises heat loss. A special adhesive paste is used to fix the cloak to the panel’s inner leaf, ensuring an airtight seal. This is covered with a decorative wooden strip. The integrity of the seal was locally tested with smoke pencils.

A strategy was also needed to ensure the windows and other elements were installed correctly. “It’s one thing getting it right once but it needs to be done consistently on every window,” say Willcox. This was tackled by holding an on-site workshop with the installer to agree the installation process. This was drawn up into a method statement and defect avoidance plan that the installer can refer to for each window. Turner says it is important to get away from an office-based box-ticking mentality for this to work. “It’s similar to how we tackle health and safety, it’s a behavioural issue,” he says. “We aren’t going to dog installers about how long it takes, they’ve got to get it right.”

Up on the roof, airtightness has been addressed using an old fashioned torch-on roofing felt. The roof consists of concrete planks topped by the roofing felt followed by the insulation. Bam decided to go with the torch-on felt because its Dutch colleagues have been successfully using it for years. “It’s a more reliable solution than a single-ply membrane as you can see the bitumen bleeding out from the joints so you know it’s airtight,” explains Turner. The lapped joints were a potential problem where the rooflights are fitted as this creates gaps where air could escape. A solution was suggested by the roofing contractor and consists of an aluminium frame that sits on the felt with the gaps filled with mastic, creating a flat surface for the rooflights. Service penetrations have been tackled by locating fireproof batts around the pipes, which are sealed with a liquid fireproofing material. This has been done twice, with a waterproof seal used externally: “We’ve effectively got three seals,” says Turner.

The south side of the roof slopes down at a steep 35º angle, which is perfect for the PV panels. Steel beams were attached to the special thermally broken brackets and extend beyond the edge of the building and over the roof. This structure supports the PV array. There are also banks of panels sitting on frames on the flat roof behind this array. Although these are predicted to produce more than enough power for all the school’s needs they are potentially the one aspect of the project that marks it down. Given the government U-turn on the definition of zero carbon, it is unlikely schools will ever be required to produce all the energy needed for computers and appliances. And the changes to the feed-in tariff now mean installations of this size no longer make financial sense. But in this particular instance the team has got the PV panels up in time to benefit from the generous, pre-Review feed-in tariff rate, so gold star for being quick off the mark.

Marks out of 10?

Has this robust, rigorous approach paid off for Bam? The first pressure test was carried out on the ground floor in late July and the result was 0.5 air changes per hour, beating the Passivhaus requirement of 0.6 airchanges per hour. That translates to 0.88m3/hr/m2@50pa when using the UK measurement standard, over 11 times better than Building Regulations requirements. “It’s testament to our prototyping and testing-to-destruction approach and embedding a culture of ’don’t walk by that detail’,” says Turner. “This is what we’ve learned from our European colleagues.” Turner is confident that the whole building will pass the full air pressure test as the same approach has been adopted on the upper floors.

The way things are going, the school should get its Passivhaus certification. But how well will it perform once staff and pupils take over in September? Building’s recent post-occupancy evaluations (see issues 1 July and 5 August) show there is a huge gap between the predicted energy performance of low-carbon buildings and the reality. The project team are well aware of this and will use Soft Landings to ensure the school performs as designed - a process that ensures buildings perform as designed after handover. This means making sure everything works at handover and includes the project team staying involved with the project for three years afterwards. Turner says a six week period is being allowed to ensure commissioning services work optimally. The project team will also meet every quarter for five years after handover to tackle performance issues as they arise.

But energy performance is ultimately down to the users. The idea is that the thermal mass of the concrete will help the building maintain a constant temperature. Heat gains from pupils and equipment will be retained, with temperatures boosted as necessary in the mornings and after weekends, but this balance could be disrupted by users by leaving doors and windows open. The team has taken teaching staff and the caretaker around the building and is educating children about energy issues. This rounded approach to energy use is commendable and not only gives the school excellent marks now but ensures it has a bright future too.

What makes Montgomery School zero carbon

The first step in hitting the zero target is to minimise energy use, hence the adoption of the tried and tested Passivhaus standard. This sets minimum standards for envelope U-values and stipulates heating demand must not exceed 15kWh/m2 per annum. Getting the building shape right is crucial. “If you have a rubbish shape then you have to make up the heat losses elsewhere, which is expensive,” explains Sally Godber, partner at low energy consultant Warm. Montgomery is a simple rectangle, which keeps the external wall area down and minimises thermal bridging, which Godber says the sandwich panel construction helps with. Thermal modelling indicates the primary energy demand will be 14.2kWh/m2 each year.

The second challenge is minimising primary energy demand, which Passivhaus sets at 120kWh/m2 each year. This includes the energy used by the ventilation system, particularly difficult in schools. “In schools you have high occupancy rates in small areas and they move around,” she says. Natural ventilation is used at the school during the summer, except for difficult to ventilate areas. In the winter, air, which is warmed electrically in the ventilation system, is introduced into classrooms first then pulled into corridors and breakout areas to minimise energy use. Electrical heating was chosen as it is simpler than a wet heating system. The heating will not come on automatically when the building is occupied; users switch on the heating for 15 minutes at a time. It is hoped that manual control, coupled with the knowledge electricity is expensive, will cut energy use.
Godber says the Passivhaus planning package gives some credit to the increased thermal mass of this building but the real benefit is the heavyweight construction should help even out temperatures.

That primary energy target also includes the power consumed by computers and the kitchen. Godber says this makes hitting the target challenging as UK schools have more IT and on-site catering than their continental cousins. She doesn’t know if the project will hit this target. “We don’t know what equipment the client is going to buy until late in the day.” She says calculations show the scheme has a “good chance”, showing energy use of 160-170,000kWhr each year. How the equipment is used will make a big difference.
Does this matter when there is a 1,300m2 PV array on the roof? Passivhaus doesn’t give any credit for this as the standard is about reducing energy use in the first place. But it will provide enough power for the school and offset winter electricity use by exporting power over the summer and is the final element in making the school zero carbon.

Project team

client Devon council
architect/QS/sustainability consultant NPS South West
structural engineer Robson Liddle
M&E engineer John Packer Associates
design team advisor The University of Exeter
Passivhaus consultant Warm
project manager WT Hills
contractor Bam
airtightness testing HRS Services
M&E NG Bailey
precast panels Buchan Concrete
groundworks D20
steelwork Taunton Fabrications
roofing DFR Roofing
carpentry Marshalls Carpentry
polished concrete floors Concept Flooring
doors and windows Solaglas
rooflights Metal Solutions
lifts Schindler