Windcatchers are used to naturally ventilate a major new primary school in Sussex.
Completed in May this year, Surrey County Council’s Leatherhead Trinity School is a 2765 square metre primary learning facility that replaces Woodville Junior school and two local infants schools: All Saints and St Mary’s Church of England. Designed by NPS South East, the £7.5m scheme accommodates 390 pupils and is located on a 1.85 hectare site on the grounds of the former junior school. The brief was for a two-form entry building with a strong identity; the aim being to improve social cohesion and integrated learning across Leatherhead.
Sustainability was a key factor, with natural ventilation, daylighting and the use of thermal mass considered essential prerequisites by the client and architect. Among the main environmental moves adopted by NPS has been the use of Monodraught Windcatchers throughout the scheme. The roof-mounted devices not only provide a reliable means of maintaining comfortable daytime temperatures and secure night-time cooling, they are also highly cost effective – particularly compared to installing and maintaining mechanical ventilation systems.
Situated on a steeply sloping site, the two-storey building is rectangular in shape, with a primary north-south axis running parallel with the ground contours. The structure is built off a cut and fill platform, which helps to minimise its volume in relation to the surrounding suburban housing. The apparent mass of the building is further reduced by visually separating the upper and lower levels in terms of materials and colour. Dark facing-brick forms a protective base at ground level, while a rainscreen cladding system comprising brightly coloured fibre cement boards in cool blues and warm reds, oranges and yellows is employed at first floor level.
The plan is organised around a central, top-lit double-height street running the entire length of the building. As well as being an efficient way of getting from one room to another, project architect Hugh Bennett describes the street as a creative space in its own right, allowing adjacent rooms and activities to spill out into it. This area is also articulated by two semi-circular pods, which are carved out of the ground floor classrooms, and act as informal learning/reading areas.
The main service spaces, including the hall, studio, kitchen and administration area, provide the public face of the building, protecting the more private classrooms on the eastern side of the plan. The teaching spaces for the youngest children (reception and key stage one) are grouped together on the south-eastern corner of the building and have their own outside play area.
The year-five and year-six classrooms on the first floor are connected externally via a steel walkway. A bridge-link spanning from the middle of the walkway provides access to an outdoor amphitheatre and sports facilities. The school also houses a 150 square metre dedicated SEN, a language unit and offices for Surrey Arts.
The decision to naturally ventilate the building was taken early on and combined with the use of medium-dense concrete block internal walls to maximise thermal mass. While this strategy proved relatively straightforward for much of the scheme, both the street and ground floor classrooms presented problems in terms of ensuring adequate levels of fresh air. Issues of safety and security meant that the rooflight over the street – formed with translucent insulated cladding panels – had to be specified without opening lights. The deep plan, single-aspect nature of the classrooms meant that opening windows alone could not be relied on to ventilate the rear of the spaces.
The team’s response was to use Monodraught Windcatchers. The roof-mounted GRP units are designed to catch the wind from any direction – using a system of external louvres connected to quadrants and internal turning vanes – bringing a controlled rate of flow in while simultaneously expelling warm air through the same route as a form of displacement ventilation.
Windcatchers were used to naturally ventilate seven classrooms and one studio space on the ground floor, as well as the 60 metre long by 6.8 metre high internal street. Project architect Hugh Bennett says the system’s inherent simplicity and reliability combined with its ability to provide secure night time cooling in the summer were important factors in the specification process. Other advantages offered by the units are that they can be designed and sized to meet the exact needs of a space, without relying on other elements, such as opening vents and rooflights. They also contain few moving parts (none externally) and therefore pose minimal long-term maintenance needs/costs.
The architect worked closely with Monodraught and used TAS thermal modelling software to analyse the individual needs of the spaces and formulate a detailed natural ventilation strategy. The starting point for the design was DFES Building Bulletin 87 (guidelines for environmental design in schools), BB101 (ventilation of school buildings) and Part F (ventilation) of the Building Regulations. Limiting carbon dioxide build-up and controlling heat gain was achieved by targeting ventilation rates in the order of 12 litres of fresh air per second per person (12l/s/p) as opposed to 8l/s/p recommended in BB101.
The manufacturer produced a detailed proposal indicating the preferred size and specification of the units for each area of the building (a total of 15 Windcatchers were specified).
In terms of sizing, the approach adopted by Monodraught is to recommend a system that will provide the required air change rate at a wind speed of 2-3 metres per second. The Met Office suggests that the average wind speed in the UK is 4-4.5m/s throughout the year, so sizing below this gives a good assurance that the desired airflow will be achieved even at low wind speeds.
During periods of little or no wind, air movement is still created by stack effect, ie the warmer, less dense air is vented at roof level and replaced by cooler denser air from outside. The two-storey configuration of the school, combined with the double-height nature of the street, meant that it was particularly suited to this type of ventilation.
The air change rate for each individual space is based on factors including room size, occupancy, solar gain, artificial lighting and electrical equipment. The street has a predicted heat gain of 30kW necessitating 10 air changes per hour (ac/hr). The teaching spaces are typically rated at 5-6kW requiring 15-17ac/hr. The classrooms are ventilated using single 1200x700mm rectangular units, while the studio is fitted with two 800mm square units. The street is served by six 1500x800mm rectangular units. All the Windcatchers were specified in grey to match the fenestration and aluminium standing seam roof.
Installation and commissioning
The location of the ventilators serving the ground floor classrooms corresponds to a series of masonry ventilation shafts, which run down the rear walls of the first floor teaching spaces. The shafts for the Windcatchers positioned over the street are formed from stud partitioning with 100mm thick acoustic insulation. Following preparation of the roof openings (with upstands and kerbs) and ventilation shafts by the roofing subcontractor, Monodraught installed the units. The Wind-catchers are fixed to the top of the upstands using 75x75mm steel angles. Fitted to the base of each unit is a GRP weathering skirt, concealing the kerb flashing.
At the base of each ventilation shaft is a motorised damper control assembly with a powder-coated stainless steel diffuser grille. The distance from the base of a typical classroom Windcatcher to the ceiling diffuser is approximately six metres. Monodraught says its systems can accommodate riser lengths of up to ten metres – equivalent to three floors. The Windcatchers are automatically controlled using the manufacturer’s iNVent (intelligent natural ventilation control) system. They are connected to the system by temperature sensors positioned on the internal walls at a height of 1.5 metres from the floor. Readings taken from the sensors determine the degree by which the dampers are opened or closed.
Under summer daytime operation the dampers start opening at 16°C and then continue to open in increments of 20 per cent with each additional degree rise in temperature. At 10pm the dampers open fully for night-time cooling. This has the effect of purging the building of warm, stale air, leaving it cool and fresh for the start of the next day. The dampers are programmed to automatically close during this process if the internal temperature falls below a specified set point (usually 18°C).
During winter the dampers can be programmed to stay closed, or left five per cent open to provide trickle ventilation. A manual override located on the sensor allows staff to fine-tune comfort levels and/or respond to unseasonal weather conditions. The override is linked to a timer, which re-sets the system to its programmed position after 20 minutes.
Architect: qs, m&e, pm: NPS South East; landscape architect: Wynne Williams Associates; structure: TA Millard; contractor: Mansell; client, clerk of works: Surrey County Council.
Selected subcontractors and suppliers
Passive stack vents: Monodraught; m&e: MSL; opaque rooflight panels: Stoakes Systems; rainscreen cladding: Marley Eternit; standing seam roofing: Rigidal.
AT201/September 09 p66