CASE STUDY | BEORMUND PRIMARY SCHOOL To minimise operational carbon, the school is designed to Passivhaus standards for both fabric thermal efficiency and airtightness, even though it will not be Passivhaus certified. Its not full Passivhaus because of budget concerns about the cost of targeting certification, Sperring explains. To minimise embodied carbon, the building will be constructed predominantly from wood. Theres no point having a building that is super energy efficient in operation if we have to use loads of energy to build it, says Sperring. The walls and upper floor will be assembled from highly insulated modular timber SIP cassettes supported by a predominately glulam frame. The prefabricated timber SIP cassettes will help reduce build time, reduce site traffic during construction in a residential area, minimise waste, improve build quality and, because they are assembled from wood, make the school feel less institutionalised. It is a really good solution because it delivers super-low U values and, inherently, the large pre-assembled modules will help the building achieve Passivhaus-level airtightness, plus you will see elements of the wood structure in the interior, so it feels warm, says Sperring. The challenge for Cundall was to develop a building services solution that would work with the minimal thermal mass inherent in the wooden cassette units. Sperring says the building services design is developed to comply with Building Bulletin 101: ventilation, thermal comfort and indoor air quality and Building Bulletin 93: acoustic design of schools guidance, which Cundall has enhanced with the addition of Passivhaus requirements. BB101 requires an adaptive approach to thermal comfort based on CIBSE TM52 metrics. Everything we do around thermal comfort has to be modelled using a digital model and an appropriate CIBSE Weather File for the location, and, for the DfE [Department for Education], we also have to look at future weather data for the 2020s and two options for the 2080s, to future-poof the school against the effects of global warming, Sperring explains. We did Passivhaus modelling, dynamic thermal simulation modelling, and CIBSE TM54 modelling for operational energy in use. The schools status as an SEMH school means that it has to perform better than a typical school, with temperatures in the classrooms limited to a maximum of 25C The schools pitched roof is orientated so teaching spaces face south 36 April 2023 www.cibsejournal.com Dynamic thermal simulation modelling and TM52 thermal comfort studies helped optimise the buildings thermal envelope and establish the maximum glazed area. We wanted as much daylight as possible for wellness without spaces overheating, so we undertook a lot of modelling to check thermal comfort and glazed areas with Cullinan Studio the final design hits the sweet-spot, says Sperring. Entrances to classrooms are also naturally lit via skylights and lightwells, to flood the corridors with daylight so these appear as bright as the teaching spaces they connect. Daylighting is supplemented by LED lighting throughout. In the teaching spaces, daylight sensors help keep lighting levels constant, but with provision for teacher override. Absence detection turns off the lights when spaces become unoccupied. Air quality is another key aspect of pupil comfort, so the design is based on the Passivhaus approach of constant ventilation to classrooms. Two central air handling units (AHUs) serve the teaching spaces, while a separate AHU serves the first-floor offices and staff spaces. All AHUs incorporate thermal wheel heat recovery except for the kitchen unit that uses a run-around-coil. Air is ducted to the classrooms along the corridor. Duct sizes are optimised to keep air velocities low and minimise fan-power, and to reduce air turbulence noise. Noise control is further enhanced with sound attenuation where ducts enter the classrooms. This is to stop noise transferring between the classroom and the corridor, so children can sit and work with a teacher outside the classroom if necessary. Inside, diffusers disperse the air. Were pushing air in at a high level so it will have mixed with the warmer air within the classroom, tempering it before it reaches the pupils, says Sperring. Air returns to the AHUs along the corridor, via an attenuated air-transfer grille located at high level in the classroom.