Richmond Hill school, built to Passivhaus standards, is setting a new benchmark in airtightness. Elaine Knutt reports.
The airtight SIPS panels, combined with specialist sealing tape, delivered extremely high air-tightness
Richmond Hill School isn’t quite an argument for practice makes perfect, but it certainly demonstrates that practice improves air tightness. As the third Passivhaus certified project by contractor Interserve, it has achieved an incredibly low level of heat leakage, shrinking its future energy budget to a fraction of the average for the national schools estate.
Having persuaded client Leeds City Council to adopt Passivhaus accreditation at Richmond Hill rather than pursue a renewables route to sustainability, Interserve clearly felt it had something to prove. As a result, the school has achieved 0.25 air changes per hour, compared with the Passivhaus Institute target of 0.6 ACH. That in itself is 10 times tougher than Part L 2010. “Compared with this, most buildings leak like a colander,” comments Rob Soulsby, contracts managing surveyor with Interserve.
The two-storey, 630-pupil school was designed by a contractor-led team including architect Space Group, BGP as structural engineer and Hoare Lea on building services. The school is designed around a learning street to facilitate flexible teaching, and also features a multi-purpose community space to serve the local community.
As well as airtightness, the Passivhaus goal was to deliver high levels of occupier comfort, with excellent ventilation, opening windows, and good daylighting.
The ambitious air-tightness target was achieved through rigorous attention to detail. Throughout the project, one member of staff was dedicated to record checking and documenting the construction strategy. Key subcontractors were also brought into the design and build project at an early stage, to coordinate the design and detailing while ensuring Passivhaus certification.
The envelope is built from SIPS wall and roof panels — manufactured offsite by McVeigh — coordinated around a steel frame. The thick airtight SIPS panels have a standard polyurethane insulation core sandwiched between two layers of oriented strand board (OSB), with the joint between each panel sealed with specialist tape. Interserve then added a further layer of Kingspan insulation to the interior OSB face, battening the insulation to the SIPS. The combination achieves a low U-value of 0.1W/m2K.
One of the trickiest aspects of the build was sealing the gaps between the triple glazed part-steel, part-timber window assemblies, and the surrounding SIPS panels. A new type of 3-inch wide sealing tape was used where timber met timber, steel or concrete. “With Passivhaus, you can’t gunge it all up
with expanding foam and hope it will be all right. We’ve used other products in the past, but with the tapes we got [levels] about a third better. Without them, we wouldn’t have achieved the air tightness we did,” says Soulsby.
The other tricky part of detailing came in the lantern roof, where the joints between the roof and wall panels were obscured by steelwork. The solution that preserved air-tightness was to wrap an EDPM membrane around the steel.
Meanwhile, cold bridging from the piled foundations to the steel structural frame was considered a major problem, so the design features 500mm thick slabs of foam glass insulation. To further reduce cold bridging, the columns were in many cases recessed into the slab.
After achieving such a high thermal performance in the envelope, the design team turned to energy demand. The lighting strategy includes “daylight linking” of the classroom lighting, which means that the brightness of lights closest to the windows automatically adjusts to the daylight levels outside. Lighting levels throughout the building are regulated with passive infrared sensors, while the external lighting is controlled by photocells and time clocks.
The design team also advised the client on product selection to ensure that the electronic equipment used throughout the school had low-energy requirements and did not compromise the Passivhaus primary energy target. The school is projected to have a primary energy demand of less than 120kWh/m2/year, and a specific heat demand of just 15kWh/m2/year.
One of Soulsby’s final tasks on the project is to gather comparable statistics from other schools in the area for the client. But with national averages for schools running at 300-400kWh/m2/year, Leeds is hardly likely to receive any unwelcome surprises.