further enhanced by the exceptionally high levels of insulation present in the building’s fabric. All elements exceed minimum Building Regulations requirements: exterior walls have a U-value of 0.13W/m2 0.10W/m2
floor is 0.06W/m2
are triple glazed with U values of 0.9W/m2 and 1.0W/m2
/°C, respectively. In addition,
careful detailing has eliminated unwanted heat loss from thermal bridges. The scheme is also exceptionally airtight.
Design workshops were used to develop robust details early in design development, which were continued through the construction phase with the contractor, Thomas Vale. With intelligent detailing and careful construction, the air leakage is limited to an incredible 0.53m3
/hr at 50Pa, a 2,000%
improvement on Building Regulations. Knowing they had an airtight, energy- efficient building envelope enabled the team to develop an elegant, simple ventilation strategy. There are two ventilation modes: winter and summer. In winter, a mechanical heat recovery ventilation (MHVR) system supplies 5,400m3
to the classrooms at the rate of 18m3
/h of pre-warmed fresh air /h per
person. This fresh air is then drawn through the classrooms and out to the street through acoustic transfer grilles. From the street, air
is returned to the MHVR unit where up to 80% of its heat energy is removed before it is exhausted. In summer, the MHVR unit is turned off
and natural ventilation alone can keep the building comfortable. In the street, high level windows and grilles open to allow warmed air to escape to outside. The escaping air pulls in fresh air from the classrooms through the transfer grilles. Manually openable windows
and room-height ventilation grilles in the classrooms ensure sufficient ventilation to maintain comfortable conditions. The ventilation grilles are unusual in
/°C, the roof
/°C, and the U-value for the ground /°C. The windows and doors /°C
that they are effectively louvres set into the wall with a door behind that can be opened or closed by the teacher using an electronic actuator to control ventilation. This interconnected system of grilles and louvres allows secure ventilation and enables the system to be used at night to pre-cool the building in hot weather. This solution eliminates the need for mechanical cooling, while providing high levels of fresh air. The exception to the natural ventilation
strategy is the building’s main assembly hall. Because this is occupied intermittently and the occupancy can vary from one class of 30 to all 230 pupils for school assembly, this room is fitted with a CO2
extract fan. When the CO2
sensor and a supply and sensor calls for
fresh air, the fan draws air into the room from the street. When the sensor detects the hall is empty, the fans turn off to stop the system supplying fresh air to an empty room. This simple strategy enables fresh air to
follow the pupils as they move out into the street or to the hall. If the children are not in the classroom, then all the fresh air will still be available when it is drawn into the street. Likewise, if the children are all eating lunch in the hall, then the fresh air in the street will be drawn into the hall. ‘Because the building fabric is so airtight, this solution enabled us to keep the plant size and distribution ductwork to a minimum,’ says Andy Jarvis, a partner at E3 Consulting Engineers. Modelling and analysis of the occupied spaces demonstrated that the scheme complied with the overheating recommendation in Building Bulletin 101:
The wedge-shaped design takes advantage of solar gain in the winter by orientating the main elevation to face south
32
CIBSE Journal May 2013
www.cibsejournal.com
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