The UK’s largest non-domestic Passivhaus-certified build ever has brought with it unprecedented construction conundrums.
Project: Centre for Medicine
Client: University of Leicester
Main contractor: Willmott Dixon
M&E subcontractor: NG Bailey
Sustainability consultant: Couch Perry Wilkes
Start date: November 2013
Completion date: December 2015
What do you think of when you hear the word ‘Passivhaus’?
The image that probably springs to mind is an airtight, extremely well-insulated house with very low energy bills.
But the project team behind the University of Leicester’s new Centre for Medicine is trying to change all that.
Willmott Dixon, supported by subcontractor NG Bailey and consultant Couch Perry Wilkes, has built the largest non-domestic Passivhaus-certified building in the UK.
Achieving this feat came with a number of hard lessons along the way, but the building is now up and running ahead of the first full cohort of students due this September.
All under one sustainable roof
The project’s roots date back 2007, when the University of Leicester identified the need to move its College of Medicine into a new building alongside the departments of psychology and health sciences.
Previously separated, the client felt it was more conducive to effective learning and academic research to have the departments together under one roof.
“This is the first time I’ve had to explain to ground workers how to cut neatly around a pile cap with insulation”
James Elliment, Willmott Dixon
Around the same time, the university had also made an undertaking to reduce its carbon emissions by 60 per cent by 2020.
“Obviously one of the easiest ways to do that is to build a big new shiny building and make it as green as possible,” explains Dave Vernon, a project manager in the university’s Estates Department. “That’s how we arrived at the Passivhaus certification.”
A number of options were considered to deliver a low-energy facility, including targeting a zero-carbon building, but the Passivhaus route was chosen to avoid the need for substantial bolt-on technologies such as solar panels.
The structure consists of teaching areas on the ground and first floors, with a number of floors above dedicated mainly to office space for academic research. It also contains several dry labs for research, as well as a couple of lecture theatres. In all, the facilities cover 12,836 sq m.
University of Leicester School of Medicine NG Bailey Willmott Dixon 4
To get the Passivhaus certificate, the team had to limit energy usage to 15 kWH per sq m per annum, as well as limiting primary energy demand (effectively everything that is plugged in after completion, like computers and printers) to less than 120 kWh per sq m per annum.
And on the airtightness side, the team had to achieve 0.6 air changes per hour (roughly equivalent to an air permeability rating of 1.8).
To hit these exacting standards, the team carried out thermal and energy modelling on the designs, continually checking it and running it past an external consultant, Warm, to refine and improve it.
1.6 km pipework maze
Construction on the £42m project began in November 2013 and finished just before Christmas last year, with Willmott Dixon securing the design-and-build contract through a single-stage tender.
A major challenge emerged during the groundworks phase. The building includes a ground-air heat exchanger, which is typically located to the side of a building. In this instance, however, limited space meant it had to be sited directly underneath the structure.
“It was one of the most difficult parts of the job and it slowed us down,” says Willmott Dixon operations manager James Elliment. “There is 1.6 km of pipework down there and it had to be threaded between the piles and the pile caps. It was a difficult process getting it all in.
“This is the first time I’ve had to explain to ground workers how to cut neatly around a pile cap with insulation. You wouldn’t normally insulate under a pile cap, and when you do you don’t normally see it cut so neatly.”
“This building has pushed the boundaries of Passivhaus because it’s so large. The industry is lagging a bit behind”
James Elliment, Willmott Dixon
Making the building airtight also proved to be a significant challenge. “We did some sample testing, which created a lot of debate as well,” Mr Elliment says.
“You want to try to test as early in the build as possible to make sure you’re heading in the right direction. But you’re relying on temporary air seals, so it’s difficult to know if it’s leaking out of the facade or the temporary air seals.”
Initial tests following the main build programme achieved an air permeability rating of 1.9 cu m/hr/sq m. “I thought that was fantastic and that it wouldn’t be a problem to get it down – but it took six more tests to do it,” Mr Elliment says. In the end, they achieved a level of 0.3 air changes per hour, below the required 0.6.
All manner of features
The building includes a number of elements designed to maximise its efficiency and ensure it requires as little artificial heating and cooling as possible.
The blocks that contain the office space, above the main two-storey building plinth, have been orientated to maximise solar gains and feature external concealed toughened blinds, which keep out excess heat in the summer and are managed by a weather sensor on the roof.
The occupants of each room are still able to move blinds, turn lights on or off and open windows manually, with the electronic elements controlled by a KNX room user control system installed by NG Bailey.
University of Leicester School of Medicine NG Bailey Willmott Dixon_basement plant room
The building also includes two atriums letting in plenty of natural light and doubling as a fresh air storage area. Fresh air supplied to the rooms percolates through the building and gathers in the upstands above the atriums.
“In the winter we can extract heat energy from the top of these atriums, with returning air paths within perforated tiles, exchange that through heat recovery devices and redistribute it back through the building,” explains Carl Stanley, an associate director at consultant Couch Perry Wilkes. “Whereas in the summer there are automatic rooflight openings, so we can discharge the unwanted heat.”
The building’s plant, contained in the basement and in two rooftop plant rooms, has also had to be specially designed by NG Bailey.
Passivhaus is of course usually used for domestic buildings, meaning any plant used on a building of this size has to be scaled up – and Passivhaus certification requires that all elements of plant have been tested and certified to certain standards.
“We’ve demonstrated that we’ve built a sustainable building; now it’s about making sure it’s used in a sustainable manner”
Rob Hutchinson, NG Bailey
“There isn’t much out there of the size we needed,” says NG Bailey senior project manager Rob Hutchinson. “Particularly on the air-handling units, which were bespoke, we had to get them individually tested by BSRIA to satisfy the Passivhaus requirements.”
The air-handling units had to be as efficient as possible to achieve the certification, and the BSRIA testing had to be done to avoid penalties for using non-Passivhaus-certified equipment.
“This building has pushed the boundaries of Passivhaus because it’s so large,” Mr Elliment says. “The industry is lagging a bit behind. The air-handling units are a great example of where the manufacturing side – the kit out there – isn’t quite where it needs to be.
“There is a lot out there for domestic [projects] – you can buy a door with a sticker on it to tick the box. But on this size of building, the doors we need might not have that sticker, so we have to get them tested.”
Body heat will prove the pudding
The building was completed late last year and academics have already moved in. Energy usage testing is under way, but it will be hard to get a full picture until the building is fully occupied with the new batch of students in September.
“The data we’ve got so far is misleading because there’s been a phased move-in,” Mr Vernon says. “This building needs to be fully occupied to operate efficiently, because you need the body heat generated by a full complement of people.”
University of Leicester School of Medicine NG Bailey Willmott Dixon 7
The team has begun a three-year soft landings process to help ensure the performance gap between the designs and the as-built reality is as small as it can be – but everyone recognises that this is perhaps an even bigger challenge than the complex construction itself. Willmott Dixon is also contracted to target a DEC A rating after three years.
“It’s going to be difficult, because if users don’t use it as intended we won’t be able to do that. We have to take a collaborative approach,” Mr Hutchinson says. NG Bailey is closely monitoring energy usage in the different parts of the building to feed back to the client which areas might require greater focus.
The model predicts that the building will use a sixth of the energy compared with CIBSE norms – but that will have to be reduced by a further 25 per cent to get the team to the required DEC A rating.
It’s not going to be easy – but with a building that’s designed to be as efficient as possible, the team will be working hard to make sure its occupants get the most out of it.
Or, as Mr Hutchinson puts it: “We’ve demonstrated that we’ve built a sustainable building; now it’s about making sure it’s used in a sustainable manner.”