The design and construction of an exemplar Zero-Carbon Primary School

In 2008 the Department for Children, Schools and Families (DCSF) identified funding for pilot/exemplar projects under the Zero Carbon Task Force (ZCTF). The projects were focused on increasing the knowledge and understanding of energy use in school buildings moving towards the ambition of delivering zero carbon in-use school buildings from 2018 in accordance with Government guidelines. Montgomery Primary School in Exeter, Devon, is a new 420 pupil primary school (plus nursery) which became one of these pilot/exemplar projects, designed to replace the existing facilities which had become outdated and no longer fit for use.

It is acknowledged that the simplest way to produce a Zero-Carbon design would be to build a typical school replacing the gas boiler with biomass for heating and buying electricity via a green tariff for power and lighting. We consider such an approach unsustainable and without value as it relies on continued use of precious resources. Montgomery has been designed to use the minimum amount of resources, including fuels, based on Passivhaus standards utilising a modular design approach with off-site pre-fabrication. All of the energy required for heating, lighting and power will be generated on-site. The design team has the view that passive-design is not about applying ‘green bling’ to a standard product, it is an integrated design process from first principles - a holistic approach - focusing on reducing energy demands as a first principle.

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Our design philosophy was to dramatically reduce the requirement for space heating and cooling, whilst also creating excellent indoor comfort levels.  This is primarily achieved by adopting a fabric first approach to the design, specifying high levels of insulation to the thermal envelope over the top of a high thermally massive structure complete with exceptional levels of air-tightness. Photovoltaic panels were chosen as the most appropriate on site renewable energy source to meet the Zero-Carbon in use target, since the site did not lend itself to wind or water-based power generation.

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The pre-cast concrete panel solution was not critical in the desire to achieve a Passivhaus building.  Where concrete becomes important is that it provides high thermal mass which will prove essential in the step-up from Passivhaus to Zero Carbon. The thermal inertia of concrete allows it to absorb and store surplus heat or cold, and release these back to the air as part of a designed thermal strategy. Concrete also displays airtightness, safety and security benefits that are related to its massiveness and density, provided the jointing methodology is also cementitious in nature.

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“Where concrete becomes important is that it provides high thermal mass which will prove essential in the step-up from Passivhaus to Zero Carbon.”

Achieving the required airtightness levels was all about avoiding gaps: The uncontrolled leakage of air in-or-out of a building causes localised discomfort due to draughts, increased energy consumption, and increased risks of condensation within the building fabric which may eventually reduce the performance and lifespan of the building. The BRE promote the maxim ‘Build tight, ventilate right’, where the principle is that accidental air leakage should be avoided, and managed air flows should be designed in. This is where Passivhaus standards excel in promoting the use of a mechanical ventilation system with heat recovery, thereby ensuring that fresh, tempered air is delivered where it is needed, when it is needed, without losing valuable energy in the form of heat.

“All insulation has been tightly butted and sealed with tapes and joined with low expanding rate foam.”

The majority of heat will be provided by the occupants and if they fail to maintain sensible practices, such as shutting windows and doors and controlling the plant, they will notice a drop in temperature. It is hoped that by understanding how the building works will encourage a change of behaviour and attitude to energy and sustainability.

“It is hoped that by understanding how the building works will encourage a change of behaviour and attitude to energy and sustainability.”

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Structural junctions and the interfaces of different building elements can be problematic because of the requirement to provide a continuous insulation wrap around and under the building. Air movement through or around the insulation bypasses its effectiveness and reduces its performance. All insulation has been tightly butted and sealed with tapes and joined with low expanding rate foam. Junctions at tops and bottom of insulation are also sealed on a thin bead thereby eliminating thermal bypass. In addition, eliminating the need to breach the insulation layer must be of prime consideration as even modern products struggle to compete thermally with the insulation they are bridging.

The School was awarded a Quality Approved Passivhaus certificate on the 1st February 2012. It is the first Passivhaus school in the UK and is on target to what we believe will be the first ‘true’ Zero-Carbon in-use school in Europe. Exeter University will be monitoring the building’s energy performance over five years to ensure the Zero-Carbon ambition is met and maintained.

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Arthur Tatchell

Architectural Director

NPS Group (Exeter)

‘The design and construction of an exemplar Zero-Carbon Primary School’. [online] Available at: < http://www.publicarchitecture.co.uk/knowledge-base/files/exemplarzerocarbonschool.pdf> [Accessed 30 January 2015].