A laser-sharp focus on airtightness and centralised air handling plant were key to meeting the low carbon standard at Two Rivers Primary School in Keynsham, Somerset
Two Rivers Primary School is the UK’s first Passivhaus Plus-certified school to be built using a traditional steel frame, concrete floor slabs and brickwork external walls. It is also only the second ever school in the country to achieve the demanding standard.
Efforts to cut energy demand hinge on an ultra-airtight external envelope, supported by an intelligent heat recovery ventilation system.
According to Simon Hatcher, director of HPA, this meant treating the building as 'a thermos flask' and pushing the entire envelope, including the warm installation line and the triple glazed windows, outside of the steel frame. 'We had to work the details that degree harder and get buy-in from all the trades to ensure that the envelope was never punctured once in place,' said Hatcher.
The £6.75 million project was designed by Hatcher Prichard Architects (HPA) for the Department for Education (DfE), with ISG as main contractor and Max Fordham as Passivhaus consultant.
A building certified to Passivhaus Plus not only drastically reduces energy use, it also produces at least as much energy as occupants consume. The standard requires buildings to exploit a minimum 60 kWh/m2/yr renewable energy generation and cut renewable primary energy demand to a maximum 45 kWh/m2/yr.
A Passive House Planning Package model of Two Rivers Primary School, by Max Fordham, predicts 87 kWh/m²/yr renewable energy generation, all from solar photovoltaics, and 59kWh/m²/yr renewable primary energy demand. The latter higher energy demand figure was permitted under the standard because the building exceeds the minimum renewable energy generation target, said HPA.
DfE schools typically have mechanical air handling units installed in every classroom, but modelling revealed that this would result in too much heat loss to achieve Passivhaus Plus.
Instead, a single large centralised air handling unit was installed to promote the natural movement of air throughout the school via a carefully curated system of ducts, guiding it towards a reservoir where heat is removed and the air is extracted at a single location.
'The unit draws in ventilation air and channels it through ducts to classrooms,' said Hatcher. 'The corridors act as a plenum taking air to the main hall and back into the air handling unit.' Intelligent switching on the server for the unit enables the equipment to be turned off overnight when it’s not in use.
The MEP design ensures a base level temperature of 20o Celsius, air source heat pumps channel warm water into under-floor heating and residual heat is generated by the body temperature of the school’s 420 pupils.
The combined impact of Covid-19 and Brexit created a 'perfect storm' that threatened to frustrate progress on the project. As part of the certification process, Passivhaus certifier Warm signed off all the details and product specifications in advance of construction. However, 'over and again', either the items that had been certified or the manpower were not available, so the project team had to go back through the loop of amending items to what was available and get new sign offs, said Hatcher.
Asked about key lessons from the project, Hatcher highlights the transferable nature of Passivhaus requirements for air tightness and better detailing to other buildings.
'We are working closely with the contractor to incorporate the enhanced design and detailing of the project into all future schemes in line with our net zero strategy,' said Hatcher. 'It certainly can't hurt, and from this point forward regulation is only going to become more stringent,' he concludes.