We’ve just delivered another Passivhaus first; a secondary school that meets the ultra-high energy efficient standard, with Harris Academy in Sutton following the George Davies Centre at University of Leicester.

We spoke with construction manager Graham Thompson to find out more.

Graham, can you tell us about Harris Academy that makes it unique?

Harris Academy Sutton is the UK’s first secondary school aimed at meeting the Passivhaus standard. The four-storey, six-form entry school for the London Borough of Sutton accommodates 1,275 pupils and 95 staff.

It's part of Sutton’s masterplan to create the London Cancer Hub – a major project to provide a world-leading life-science campus for research, treatment, education and enterprise. The school is the first element.

What are the benefits of Passivhaus for the school?

The key benefits are ultra low energy use and a superb indoor environment.

Energy consumption at the school will typically be 80% lower than a standard building, giving tremendous savings on operating costs and carbon emissions. Internally, students and teaching staff will enjoy better air quality; an ideal temperature and the right amount of natural light provide an optimum building to study and work in.

How does Passivhaus differ from a traditional school?

Visually, a Passivhaus school is no different to a traditional school that's designed and built to 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.

What was the main driver for Sutton in choosing Passivhaus?

Sutton's ambition was for a school with minimal operational carbon emissions, 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 other standards, and wanted to use our expertise given our Passivhaus track-record at Leicester.

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Is Passivhaus more expensive than typical construction methods? Do the benefits outweigh the initial investment?

The design, labour and materials costs involved in delivering a Passivhaus building 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, Passivhaus is significantly cheaper in the long run.

On average, there is an additional 5-10% capital cost, however, this figure is expected to decrease as building regulations tighten and the volume of Passivhaus construction in the UK increases.

How did we work with the architect to deliver this project?

From a very early stage, Willmott Dixon worked with Architype, expert Passivhaus architects, as a single partnership team. A one-team ethos enabled construction to inform design and vice versa and meant that challenges could solved more productively and effectively to ensure 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.

What is a performance gap and how is it measured?

An energy performance gap is the difference between the energy efficiency levels that a building is designed to achieve and what it actually achieves in use.

Passivhaus buildings have a reputation for operating precisely as predicted at the design stage, with no 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. harris Academy Sutton main picture 2 mid.jpgImportantly, knowing how the building will perform in use gives owners and operators considerable certainty of what their energy costs will be.

What does air tightness mean and what do you have to do to achieve this vs a normal construction method?

Airtightness means eliminating all unplanned gaps on the building's external envelope in order to prevent heat losses.

Achieving airtightness is a process rather than a construction method. It requires extraordinary attention to detail and rigorous control, and checks and tests need to be carried out constantly throughout the project. Every single interface needs to be completed efficiently and to high quality standards. Every inch of the airtightness membrane needs to be checked. Every item has to be carefully inspected post building envelope penetration to make sure it is correctly sealed and signed off before being covered over.

Passivhaus buildings are about 10 times more airtight than building regulations require. The stringent requirement is for very low air leakage rates, with no more than 0.6 air changes per hour. On a regular building, the air change requirement under building regulations is around 10 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.

Air quality – why is it different from a normal building?

Passivhaus buildings incorporate 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 a building.

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What impact does this have on the building’s end users – in this case, students?

In some conventional buildings, pupils and staff could have to contend with sleepiness in the winter months due to the rise in CO2 levels when windows are closed.

Here, the MVHR ensures fresh air all year round. As a result, pupils and staff always feel alert and comfortable, which positively impacts learning ability, wellbeing and teaching quality.

What is it about Passivhaus that makes it so efficient?

Passivhaus energy efficiency derives from a ‘fabric first’ approach to design and construction. The fabric of the building is optimised to prevent heat leakage, which means that energy efficiency is achieved passively by the building itself – hence the term Passivhaus.

The approach centres on measures such as super thick insulation in walls, floors and roof; triple glazing on windows and doors; and an exceptionally airtight building envelope - about 10 times more airtight than building regulations require. Other elements include orientation of windows for maximum solar gains and strategic shading to prevent overheating.

Everything undergoes a rigorous and continual quality verification process on site, which has to be evidenced as part of independent certification so that quality is assured.

What will energy bills be for this Passivhaus building versus a normal building?

Typically, heating energy use will be 90% lower than the Chartered Institution of Building Services Engineers (CIBSE) benchmark and electrical energy 70% lower.

This delivers an overall 80% saving on energy bills compared to a conventional building.

Is a Passivhaus building heated in the same way as a normal building?

All 'free' heat generated in the building - such as from people and equipment - is recovered and utilised by the MVHR for warming incoming air. This means that conventional heating is not required.

Within the new school, ventilation ducts deliver heated air around the building. Although electric radiators are fitted in some of the rooms, they are incredibly small - around 50cm x 50cm for a classroom of about 30 pupils. As there are no cold drafts coming in, they don't need to be placed under the window and are typically out of the way behind doors.

The required boiler size for the school equates to a domestic one in a standard four-bedroom house. Given that the building is around seven times bigger, it's exceptionally small, meeting all the school's needs with minimal energy.

Will pupils and teachers know it is Passivhaus? Should they care?

The school was completed in July 2019 but had opened in temporary accommodation on the site in 2018.

Both pupils and staff were involved in the project as it evolved to give them insight into the building's energy efficiency. The new school's long-term environmental sustainability credentials resonated particularly well with them given the climate crisis that we are facing.

Why aren’t more people choosing the Passivhaus method?

Developed in Germany in the early 1990s, Passivhaus is still relatively unknown in the UK. There are concerns about the higher upfront costs of design and construction - though over time, these are offset by the lower running costs. Capability is also an issue. Successfully delivering Passivhaus requires a lot of expertise and there are relatively few designers and contractors that have this.

However, with sustainability never more critical to people and planet, Passivhaus and its proven benefits has been increasingly in the spotlight and the standard is now transitioning from niche to norm.

Will Passivhaus buildings last as long as conventional building methods?

Construction materials used are no different to conventional buildings and the building will last for exactly the same period. In order to meet Passivhaus requirements, however, components often need to be top quality and installation standards have to be exemplary.

Explain some of the technical details of constructing a Passivhaus building

Very high levels of insulation are integral to Passivhaus, and U-values of 0.13W/m2/k needed to be achieved for the school's building envelope – roof, walls and ground floor slab.

Where insulation boards abut, the gap can be no bigger than 3mm so quality needs to be outstanding. The ground floor, set on concrete slabs, and roof both required perfectly abutted insulation boards that met these extremely tight gap tolerances. On the timber-framed second and third storeys, insulation was pumped into the void to completely fill it.

What are the differences in designing a Passivhaus building compared to a conventional building?

Designing a Passivhaus building is a rigorous process and needs to be considered in far more depth than a conventional building from a very early stage.

It involves detailed analysis and complex calculations that address a wide range of factors such as building orientation and window sizes, the relationship between the building's surface area and its volume, and the amount of heat that will be generated internally during building use.

Design quality tends to be far higher than for most buildings, with all work checked by an independent Passivhaus assessor.

What are the differences in constructing a Passivhaus building compared to a conventional building?

Passivhaus buildings have more high-performance elements such as triple glazed windows and deeper insulation, and the specification for mechanical and electrical components requires better products.

The standard also requires contractors to be absolutely meticulous to ensure ultra-high quality workmanship and installation.

Insulation, for example, needs close and diligent attention, and gap tolerances between insulation boards are very tight.

Achieving airtightness is also a huge challenge when constructing a Passivhaus, and unlike conventional buildings, it can't be left to the end. Right from the start, airtightness needs to be integral to every item and every piece of work on and around the building envelope. It also has to be continually assessed and checked throughout the project.

What is the payback period between the additional capex costs and the reduced operational running costs?

There is no defined payback period as this will vary from project to project.

How does Passivhaus differ over BREEAM?

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, it is very focused on specification and project processes, and 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 sustainable features into a project.

What did you learn you'd pass on to others building Passivhaus?

It's vital to clearly communicate and reinforce the importance of high quality and of getting things right to everyone working on the project. We involved the supply chain at a very early stage and provided a Passivhaus-specific pre-construction induction for every supply chain member. Everyone was made aware of the concept, why things needed to be absolutely right and the need for stringent quality control. We also established an open, 'no blame' culture, with everyone encouraged to speak up if they thought something was being done wrong. We reinforced these messages with posters on the walls and toolbox talks throughout the project.

Scrupulous care of airtightness and tolerances is also critical and needs to include rigorous control of any type of drilling or penetration through the building envelope. Setting penetration schedules is key, with every item needing to be carefully inspected to make sure it's correctly sealed and signed off before being covered over.

As part of the process, we appointed two Willmott Dixon airtightness champions. They carried out continual checks all around the building for about two years and took photos of everything for compliance submissions to the Passivhaus accreditation body.

Thanks Graham!