Inside Willmott Dixon’s Passivhaus secondary school
The building for Harris Academy Sutton in South London was designed to set a new benchmark for sustainability. As the first secondary school in the UK to be built to the Passivhaus standard and the largest Passivhaus school in the country, Willmott Dixon says the building sets the sector standard for exceptionally low energy use and user comfort.
The four-storey, six-form entry school accommodates 1,275 pupils and 95 staff. Constructed for the London Borough of Sutton by Willmott Dixon, the contractor was appointed following its successful delivery of the UK’s largest non-residential Passivhaus scheme, the George Davies Centre for the University of Leicester.
The school forms part of Sutton’s masterplan for the London Cancer Hub – a major project to create a world-leading life-science campus for research, treatment, education and enterprise. Harris Foundation were appointed as the Free School education provider.
Developed in Germany in the early 1990s, Passivhaus is an international standard for designing and constructing buildings that deliver super-high energy efficiency and impressive occupier comfort. Crucially, to achieve certification, everything must be built and completed exactly as designed and to top quality standards, ensuring that building performance in use mirrors the design model and targets.
Sutton Council has had a forward-thinking approach to sustainability for decades and its ambition was for a school with minimal operational carbon, certainty of energy savings and an excellent indoor environment. The council opted for a Passivhaus building because of its ability to meet these objectives in a more focused way than a BREEAM scheme.
For the school, this translates into energy consumption that’s typically 80% lower than a standard new building, giving tremendous savings on operating costs and carbon emissions.
Internally, better air quality, ideal thermal temperature and the right amount of natural light provide an optimum building to study and work in. Throughout the year, mechanical ventilation (with heat recovery) provides fresh air, with the quantity locally adjusted in response to CO2 level sensors in all the rooms. In the winter, fresh warm air heats rooms and spaces. They never get stuffy, which tends to cause sleepiness, and there are no uncomfortable draughts or cold spots. In the summer, the building stays fresh and cool. As a result, pupils and staff feel alert and comfortable all year round, which positively impacts learning ability, wellbeing and teaching quality.
The Passivhaus difference
Passivhaus has been a relatively unknown standard in the UK, but with sustainability never more critical to people and planet, it is now moving from niche to norm. When Willmott Dixon delivered the Passivhaus standard with George Davies Centre at University of Leicester in 2015, it was the largest non-residential building to achieve the coveted standard at the time.
This is another huge step forward for Passivhaus in the UK.
Visually, buildings are no different to others of a very high quality. The difference is how the building performs – namely, very low energy costs and carbon emissions, excellent air quality and a constant comfortable temperature.
At the heart of the standard are a number of key principles.
The Passivhaus ‘fabric first’ approach to design and construction is about carefully optimising the fabric of the building itself so that energy efficiency is achieved passively.
It centres on measures such as extra thick insulation in walls, floors and roof; triple glazing on windows and doors; and an exceptionally airtight building envelope – approximately 14 times more airtight than building regulations require. Through exceptional insulation and air tightness, heat leakage through the windows, walls, floor and roof is prevented.
Other fabric elements include careful orientation and sizing of windows together with selective shading to optimise solar gains in winter and prevent overheating in summer.
Excellent air quality and thermal comfort
Another mainstay of Passivhaus is a highly efficient mechanical ventilation and heat recovery system (MVHR), which controls and optimises internal air quality and temperature. The system automatically brings in and filters fresh air from the outside, removes stale air from inside and supplies cool or warm clean air at an even, comfortable temperature to all areas of the building.
All ‘free’ heat, such as from daylight, people and equipment, is recovered and used to pre-heats incoming cooler fresh air (in winter mode). This means that the heating systems should only be required on the coldest of days, keeping energy consumption incredibly low. Although radiators are fitted in some of the rooms, they are incredibly small – around 40cm x 45cm for a classroom of about 30 pupils. As there are no cold draughts coming in, they don’t need to be conventionally placed under the window and are typically out of the way behind doors.
The required boiler size equates to a domestic one in a standard four-bedroom house. Given that the building is around seventy to ninety times bigger, it’s exceptionally small, meeting all the school’s needs with minimal energy consumption.
The same construction materials and components are used as conventional buildings, however, in order to meet the stringent Passivhaus requirements, components often need to be top quality (with full test data) and installation standards have to be exemplary.
As part of certification, everything undergoes a rigorous and continual quality verification process on site. This is to ensure that everything is built and completed exactly as designed so that there is no energy performance gap – the difference between efficiency levels on paper and what’s achieved in use.
Passivhaus buildings have a reputation for operating precisely as predicted at the design stage, and don’t experience an energy performance gap. Research shows that in other types of buildings in the UK, the gap is 2-5 times more than design targets, so in this respect, Passivhaus really stands out.
Passivhaus is tightly linked with building performance outcomes and is known for delivering on its promise of low operating costs, certainty of energy savings and a really great environment for occupants.
BREEAM has benefits in terms of addressing sustainability issues over a wide range of themes and is very focused on specification and project processes, but less on outcomes. Quality of installation on site is not checked under the scheme and building performance is not necessarily assessed in use.
Passivhaus, however, doesn’t preclude incorporating BREEAM elements or other sustainable features into a project.
For the school, additional sustainability aspects include use of carbon neutral or carbon positive cross-laminated timber internally for the storeys above first floor level. There is also the flexibility for classrooms to be made bigger or smaller in line with changing needs, as many of the classrooms are defined by non-structural partitions.
Externally, the building was mainly clad with relatively natural and untreated timber plus copper and some aluminium making it environmentally-friendly, low maintenance, durable and recyclable. Measures were also incorporated to ensure that surface water is fully absorbed on site to prevent strain on the local drainage system.
Achieving Passivhaus certification is no mean feat. Design and construction are complex, and considerable expertise and commitment are vital.
One of the cornerstones of successfully delivering a project of this kind is higher than usual collaboration between everyone – from client and architect to contractor and supply chain partners.
From an early stage, Willmott Dixon worked with Architype, expert Passivhaus architect, as a single partnership team with a common goal. This one-team ethos enabled construction expertise to inform design and vice versa. This meant that challenges could be solved effectively to make sure of the best and most cost-efficient delivery.
Close collaborative working continued throughout the entire build period. This included regular site meetings and brainstorming sessions to find solutions to design details that were difficult to put into practice, and joint involvement in ensuring quality on site.
“For all of us, this was very much a co-operative, partnering process,” said Adam Whiteley, senior project manager, London Borough of Sutton. “As the project evolved, everyone demonstrated these values in a very positive way.”
Designing a Passivhaus building is a rigorous process. It needs to be considered in far more depth than a conventional building from a very early stage and involves detailed analysis and complex calculations. Detailed design quality tends to be far more demanding than for most buildings, with all work checked by an independent Passivhaus assessor.
Areas that need very close attention include building orientation and window sizes, which need to be absolutely right to optimise solar gains, natural light and views. Depending on orientation, window shading also needs to be carefully designed so that overheating is prevented without blocking daylight. Most rooms have manually opening windows, with a simple indicator light to show whether opening windows is beneficial or energy costly.
The relationship between the building’s surface area and its volume need to be analysed and considered so that the building’s layout minimises heat losses. Triple glazing, thicker insulation and airtightness need to be planned and factored in. And all heat generated from building use needs to be quantified and taken into account in the design – ranging from numbers of occupiers and school equipment to food preparation in the kitchen.
Passivhaus buildings have more high-performance elements and the specification for mechanical and electrical components requires better products. In addition, contractors need to be extremely diligent to ensure the top quality workmanship and installation that the standard requires.
With quality critical to achieving certification, Willmott Dixon engaged with the supply chain at a very early stage to communicate its significance, and continually reinforced the importance of getting things absolutely right. Graham Thompson at Willmott Dixon explains: “A key part of this was a Passivhaus-specific induction for the entire supply chain. Every person was made aware of the concept, why getting everything right was so important and why stringent quality control was essential.”
To support this, an open, ‘no blame’ culture was established, with everyone encouraged to speak up if they thought something was being done wrong. These messages were strengthened by toolbox talks throughout the project and by having posters on constant display.
High levels of insulation are integral to Passivhaus. Where insulation boards abut there must be no gap, additionally the tolerance (air gap) behind and between can be no bigger than 3mm so quality needs to be outstanding. For example, the school’s ground floor, set on concrete slabs, and CLT roof both required perfectly abutted insulation boards that met these extremely tight gap tolerances, through multiple layers with staggered joints. On the timber-framed second and third storeys, insulation was pumped into the wall void to completely fill it.
Achieving airtightness is a huge challenge on a Passivhaus building and it can’t be left until the end of a project. Every single interface needs to be completed efficiently and to high quality standards, every inch of the airtightness membrane needs to be checked, and checks and tests must to be carried out continually.
Rigorous control of any type of drilling or penetration through the building envelope is essential. On the project, penetration schedules were set, with every item carefully inspected to make sure it was correctly sealed and signed off before being covered over. As part of the process, two Willmott Dixon airtightness champions were appointed. They continually checked every area and made sure that penetrations were fully and correctly sealed, and took photos of everything for compliance submissions to the Passivhaus accreditation body.
On a Passivhaus building, the requirement is for very low air leakage rates, with no more than 0.6 air changes per hour. For conventional buildings, the air change requirement under building regulations is around 10.0 m3/h.m2 and for a BREEAM building, it’s 4.0. Willmott Dixon achieved 0.30 air changes per hour on the school (equivalent to 0.7 m3/h.m2), which for a building of over 10,000 square metres is remarkable.
“The airtightness champions were instrumental in achieving the building’s low air changes per hour”, said Christian Dimbleby, Architype Architects. “They carried out constant checks all around the building for about two years, which really shows their commitment”.
Given what’s involved in construction of a Passivhaus building – design, labour and materials – costs are slightly higher than for a conventional building. Currently, the uplift in cost is equivalent, or less, to building to BREEAM Outstanding, but when the savings on running costs are factored in, it is significantly cheaper in the long run.
On average, there is an additional 5-10% capital cost for Passivhaus, however, this figure is expected to decrease as building regulations tighten and the volume of Passivhaus building in the UK increases.
Template for future schools
For Sutton Council, the new £38m school is a major asset in every respect. It is being perceived as a gateway for the London Cancer Hub, which is set to deliver economic regeneration through education, employment and training.
For pupils and staff, the superb air quality and optimum temperature provide a healthy, productive environment, supporting better learning and wellbeing.
As part of the London Cancer Hub, the school integrates pupils into the campus’s wider remit of scientific research and treatment, encouraging careers within life sciences.
Local people are also benefiting, with some facilities available for community use, such as sports facilities, assembly halls and other spaces.
Completed in July 2019, with the first intake of pupils in September 2019, this remarkable school sets standards and ambition for sustainability and quality and is an inspiring beacon for future secondary schools to follow.
- The UK’s first Passivhaus secondary school
- The country’s largest Passivhaus school
- Very low operating costs
- Very low carbon footprint
- 80% less energy use than a conventional building
- Ideal internal temperature all year round
- Superb air quality for better learning, health and wellbeing
- Exceptional airtightness – around 14 times lower than building regulations
- 0.30 air changes per hour – half the Passivhaus 0.6 requirement
- 40% project spend with local business
“Willmott Dixon airtightness champions were instrumental in achieving the building’s low air changes per hour. They carried out continual checks all around the building for about two years, which really shows their commitment,” Christian Dimbleby, architect, Architype.
“For all of us, this was very much a co-operative, partnering process. As the project evolved, everyone demonstrated these values in a very positive way.” Adam Whiteley, senior project manager, London Borough of Sutton