Modular school building achieves Passivhaus Plus

St Edward's School in Hampshire now has a new Passivhaus Plus certified modular building, which is the first SEND Passivhaus Plus school building and, after Hackbridge primary school, is the second Passivhaus Plus school in the UK.

St Edward’s is a SEND school providing specialist compensatory education, social care and therapeutic services to local authorities for pupils experiencing social, emotional and mental health difficulties. The new 631 m² teaching block includes six classrooms, group learning areas and a sensory room. Many of the original 19th century Melchet Court school buildings are Grade II listed, which influenced the aesthetic requirements for the new teaching block.

At St Edward’s, we are committed to the stewardship of our environment and therefore carbon neutrality is a target that has a moral imperative for all of us and that is why we are delighted that the new build will be Passivhaus.

Sally Webb, Director of Development, St Edward’s School

Key stats

Occupancy: 6 classrooms, group learning areas and a sensory room.
Completion date: July 2022
Construction: Modular steel frame
GIFA: 631 m²
TFA: 542 m²
Certified: 2022

St Edward's School Passivhaus Plus building. Image credit: Darwin Group

Construction

Modular off-site steel frame construction was the chosen construction methodology due to the speed of construction, evidenced by the construction period of less than one year, as well as a way to control quality on site, and Darwin Group, the architect/ contractor chosen for the project, specialises in this form of construction.

Building performance

To achieve Passivhaus Plus status a building must not only not only drastically reduces energy use, but it also produces as much energy as occupants consume. There is a 26kWp PV photovoltaic array on the roof of the building which generates electricity for the building. The array was positioned vertically to minimise its visual impact. The MVHR (mechanical ventilation with heat recovery) plan is concealed at ground level so that it did not cause overshading and meant that most of the roof could be dedicated to the PV panels. Heating to the building comes via an air source heat pump and hot water is delivered by direct electric.

Overall U-values

Wall 0.105 W/m²K

Rainscreen, plywood, mineral wool insulation in timber frame, plasterboard over plywood

St Edward's School Passivhaus Plus building under construction. Image credit: ZE Passivhaus

Floor 0.157 W/m²K

Isoquick foundation, 200 mm insulation

Roof 0.107 W/m²K

Insulated roof trusses, mineral wool insulation, plasterboard

Further information can be found at the International Passivhaus Database listing

The client was keen to pursue Passivhaus certification as they wanted a building which not only efficient in running costs but also provided a comfortable environment for students.

David Brownstein, Principal Engineer, Cundall

Designed energy performance

Airtightness (≤ 0.6 ACH @ 50 Pa)	0.56 @ 50 Pa
Space Heating Demand (≤ 15 kWh/m².a)	10 kWh/m².a
Heating Load (≤ 10 W/m²)	8 W/m²
Primary Energy Demand (≤ 120 kWh/m².a)	109 kWh/m².a
Primary Energy Renewable (PER) Demand (≤ 60kWh/m².a)	43 kWh/m².a

Challenges

Glazing

The historic Grade II listed site proved to be challenging as it placed restrictions on the look and feel of the building. The proportions and style of the glazing from the adjacent Georgian buildings had to be matched in the new development. Ordinarily, the team would have sought to optimise the sill heights and head heights of the windows to balance daylighting and solar gain, perhaps with overhangs to shield from the higher summer sun. However, in this case the team used the mature trees to the south of the building as part of the shading strategy, since they shade the building in summer and lose their leaves in the winter.

The building has openable windows, that can be opened at any time. The client did not want to rely on windows being open constantly during hotter weather as it could be distracting for pupils. Instead a reversible heat pump was included within the mechanical ventilation with heat recovery system to gently cool the supply air. This makes a big difference in comfort for only a small increase in energy demand and meant that opening windows are only needed during the hottest weather.

Modular steel frame

During the design stage, a few aspects of the construction detailing needed to be amended by the Passivhaus consultants, so that the project could achieve Passivhaus certification.

The conventional foundation solution (steel modules laid on pad foundations) would have created a large thermal bridge and so the Passivhaus Consultants Cundall proposed an insulated raft slab to which the modules could be fixed directly, enabling a continuous envelope around the whole building.
The thermal bridges introduced by the joins in the modules were frequent so several iterations of thermal bridge analysis were undertaken to come up with the optimal detailing. As a result, the thermal bridges accounted for only 4% of overall heat loss.
Airtightness detailing was also a challenge. Most of the envelope was manufactured off-site which meant that the airtight layer had to be installed internally to the steel frame. This required a lot of complex detailing, including a liquid airtight membrane sprayed to the internal sheathing boards, which accommodated this quite well and helped the building pass the airtightness blower door test.