Students learn from the largest UK commercial Passivhaus

The Trust took participants of the Passivhaus Student Competition, sponsored by Tarmac, to the University of Leicester’s Centre for Medicine. The building was certified in January 2016, and at approximately 13,000 m² it is the largest non-domestic Passivhaus in the UK. It accommodates approximately 2,400 staff and students, unifying several departments and uses under one roof. The site visit aimed to inspire students with the large sizable building breaking several Passivhaus stereotypes, providing lessons on how the design was achievable while maintaining and ensuring building performance.

After a welcome address from Tarmac, PHT Patron member Willmott Dixon & standard member Associated Architects presented the building design concepts, construction challenges and lessons learned. A copy of the presentation slides can be viewed here. After hearing about the £42 million scheme, students were taken on a guided tour to experience the spaces first-hand.

University of Leicester Centre for Medicine (LCfM) Credit: Martine Hamilton-Knight

Client Aspirations & Designs

The client, University of Leicester, originally wanted a ‘zero carbon’ building. However, it was explained that Passivhaus was an alternative where the long-term reduced running costs outweighed the additional capital costs; This coupled with occupant comfort benefits and a more defined target, had the client convinced that Passivhaus was the right target for them.

The design subverted the conventional Passivhaus approach that maximises efficient form factors, by introducing towers, large atriums, minimal protrusions on any of the facades (avoiding southern overhangs normally used to reduce unwanted summer solar gains) which follow similar floor to ceiling window rhythms and external finishes. This was driven by a key concept that all rooms should be deemed as equal to successfully bring together the departments of Schools of Medicine, Health Sciences and Psychology into one building. The compromise for the non-compact form was made by meeting much more stringent airtightness and thermal performance targets to make the PHPP figures work. The airtightness target, usually 0.6ACH to meet Passivhaus, was halved to 0.3ACH. Intermediate tests were very important to check they were on target and conducted in every room.

Leicester Center for Medicine LCfM key section, Associated Architects

At feasibility stage, the designs investigated different materials such as Cross Laminated Timber (CLT), but the client was keen to work with contractors that they had a good relationship with and had used before. A factor not often considered at a theoretical design stage, and a good reminder for students that buildings are built for people, by people.

The Architects originally proposed a concrete panel cladding system but both contractors who tendered for the job were concerned with the weight of lifting concrete sandwich panels at high levels, and successfully joining and sealing the panels to meet airtightness requirements. A masonry cavity was adopted at lower levels, and brick slip cladding on upper levels. The façade was tested with a full-scale factory mock-up.

The exposed concrete frame provides a large thermal mass which helps regulate internal temperatures, and allowed for a thermally activated building structure (TABS). It also provides a good level of acoustic performance and fire resistance.

LCfM Atrium maximises daylight to ground floor. Image credit: Passivhaus Trust

Image below LCfM external facade highlighting window patterns Image credit: Martine Hamilton-Knight

Summer Comfort

LCfM windows. Credit: Martine Hamilton-Knight With floor to ceiling glazing a lot of planning went into creating a robust summer comfort strategy. Khasha Mohammadian from Willmott Dixon emphasises that PHPP was a great design tool to test scenarios. The building aims for 0% overheating - well below the Passivhaus target of up to 10% overheating per annum, although the recommendation is to aim for no more than 5%. During the tour the following were pointed out:

Openable vent panels with external fixed louvre bars to maintain security during night purge ventilation.
Openable vents at high level on vertical upstands of Lamilux PR60 Energysave Passivhaus certified roof-lights within atria
Ceramic fritting on glazing below desk level to reduce solar gain
Fixed shading louvres to colonnade on south elevation
Automatic external shading roller blinds to glazing on east, south and west elevations
External fabric blinds to rooflights to limit solar gain
Pre-cooling of incoming air to Air Handling Unit (AHU) within basement via the Ground Air Heat Exchange (GAHE) during the summer
Embedded cooling pipework within the concrete slab (with exposed soffit) to the high occupancy teaching spaces result in a thermally activated building structure (TABS)

Key Stats

TFA: 9863m²
Form Factor: 1.5
Primary Energy: 116kWh/m²/a
Heating Demand: 15 kWh/m²/a
Heat Load: 10.8W/m²
Airchanges/ Hour: 0.34ACH @50pascals
Cost: £2300/m²

LCfM flexible consultation rooms with moveable walls

Lessons Learned

Soft landings

It’s always interesting to see how occupants interact with the building. A 3-year soft landings process is in place which helps iron out issues and involves close collaboration between the building estates team, design team and building users. A lot has already been leaned, there was a learning curve for occupants to understand the building and know how to properly use rooms.

Leicester Centre for Medicine, Lecture Theatre 1

LCfM Main Lecture Theatre 1

A building this size, with so many different uses and varying occupant sizes, is smart - It is controlled via a combination of state of the art automated building monitoring system (BMS) and manual operation. Occupants are encouraged to open windows before playing with the smart sensors which are available in every room. The extensive BMS system allows a significant amount of data to be collected and the University remains committed to analysing and interpreting the data to feed into future projects. Over 90 sub-meters’ record electricity, heating/cooling and water consumption at 30 minute intervals. The first year’s running costs are less than £3/m^2.During the Christmas holiday, with only the occasional member of the estates team checking the building, the temperatures never dropped below 16 °C.

The entrance is a double lobby. The receptionist was constantly reminding people who were leaving the building which door to use. Some more visual clues within the architecture would make this space work more smoothly, such as part frosted glazing on the wall/ door that should remain closed (wheelchair access only.) or attention grabbing exit doors.

Leicester Center for Medicine double lobby

Contractor challenges

There was a lack of on-site experience when tendering for the job back in 2013. Mainstream contractors tend not to be familiar with the demands of a Passivhaus, especially at this scale, and it was a challenge to get so many sub-contractors to do things differently on such a large site. Training and education meant the build took more time and money and Willmott Dixon took responsibility and liability. Check out the Trusts technical report: Quality Assurance on large and complex Passivhaus buildings for guidance.

A two-stage design and build process was undertaken. Willmott Dixon emphasises the importance of an iterative design process – back from architect to contractor to engineer – Repeated details help and should be as simple and cheap as possible to minimise risk.

The building was originally designed in 2010, and back then it was difficult to source Passivhaus products that would fit the desired aesthetic. Large certified air handling units were not readily available and bespoke units were created and tested by BSRIA.Standard curtain wall systems, common in commercial buildings, struggled to meet Passivhaus requirements. A labour-intensive solution that really pushed the specialised curtain walling system with brick slips, incorporated vent panels and external shading blinds, to the limit. Some of the brick slips were being replaced during our visit, but located outside of the airtightness layer, therefore causing no issues to performance.

Leicester Center for Medicine Green Wall

If they were to do it again

Jon Chadwick suggests they would not bury the 1.6km of piping for the ground air heat exchange directly under the building within the piling and underground services. This caused many onsite headaches.
A prefab solution for high level parts of the building would have greatly helped quality assurance, as most of the onerous is moved from onsite workmanship to controlled factory conditions.
Specialist sub-contractors would be appointed earlier in the process with a more collaborative and open design approach. Costs and risks would be more open, and design development, particularly detailed construction details, would be allocated more time to fully resolve.

Many thanks to the University of Leicester for hosting the visit and Associated Architects and Willmott Dixon for sharing their experiences. More images are available on the Passivhaus Trust Flickr.

Participants of the competition will be submitting their entries at the end of their academic year. The Trust and a juding panel will be deciding winners in June 2017, with winners awarded at the 2017 UK Passivhaus Conference.