Keep the noise down
Can natural ventilation in urban schools satisfy the requirements of the Department for Education? Carl Sutterby describes the results of modelling a number of solutions.
For schools in urban areas, disturbance created by noise entering classrooms through open windows is a major issue. Natural ventilation depends on being able to open and close windows as necessary to ensure sufficient air changes to maintain optimum indoor climate quality. It is therefore vital that natural-ventilation solutions mitigate noise entering a room through an open window.
WindowMaster worked closely with the design team of Rokeby School, which was rebuilt on a new site in Canning Town, along with M&E consultants Hoare Lea and architects Scott Brownrigg to deliver a bespoke scheme based on the WindowMaster NV Advance control system.
Modelling studies were undertaken to determine the optimum times during the day for opening and closing the windows. The studies showed that opening windows between lessons and at class changeovers removed the need to open them during lesson time when external noise could be a problem.
Normal CO2 level and temperature requirements were met with this pattern of opening and closing windows programmed into the system.
Because the classrooms have one external façade requiring single-sided ventilation, the school uses a hybrid ventilation system. Natural ventilation is provided through facade windows, roof windows and clerestory glazing on upper floors and natural ventilation with mechanical assistance on ground floors.
This flexibility allowed the designers to use natural ventilation in areas where noise could have restricted the use of a passive solution.
The modelling for Rokeby took into account three scenarios.
The first scenario allowed for one 15-minute pulse in the morning, a one-hour purge at lunchtime and another 15-minute pulse during the course of the afternoon.
The second operating scenario added two 5-minute pulses in the morning and again in the afternoon.
Scenario three doubled the number of pulses provided during both the morning and the afternoon to four.
Scenarios one and two were modelled with a mechanical exhaust at 8 l/s per person and then 12 l/s per person.
Temperature, CO2 and ambient noise can all adversely impact the concentration of a building’s occupants. Difficulty in concentrating affects a pupil’s ability to learn.
‘Building bulletin 101 — ventilation of school buildings’, which provides a guide to the legal requirements for ventilation and the control of overheating in school buildings, specifies a maximum concentration of 5000 parts per million (ppm) of CO2 in classroom air.
The modelling carried out at Rokeby School was undertaken in a maths room with 30 pupils and two adults. It showed that CO2 concentrations can be maintained at significantly lower rates than the legal maximum.
Mechanical ventilation rates of 8 l/s per person in both scenario one and two kept CO2 concentrations well below the BB101 maximum of 5000 ppm. In scenario one the maximum concentration was 4000 ppm while the maximum for scenario two was 3200 ppm. In both scenarios, concentrations dropped well below the daily average of 1500 ppm to as low as 500 ppm during the lunchtime purge.
Increasing the mechanical exhaust to 12 l/s/p further reduced these figures, particularly in scenario two. CO2 levels did not peak above 2500 ppm and remained at below the daily average for longer periods.
Concentrations of CO2 were also significantly lower in scenario three which added an extra five minute pulse during the hours of occupation in the morning and afternoon. Mechanical ventilation rates were 8 l/s person in this scenario. CO2 Concentrations were much lower than the BB101 maximum. At their highest they were 2600 ppm at the end of the day.
The BB101 average CO2 concentration is 1500 ppm. Modelling showed that scenario one and two with mechanical ventilation rates of 8 l/s per person maintained air quality at just above the average at 1600 ppm but still significantly lower than the maximum. Increasing the mechanical ventilation to 12 l/s per person resulted in scenario two being able to maintain CO2 levels below both the maximum and average CO2 requirements of BB101, so there is still a correlation between effective natural ventilation and CO2 reduction. In fact, scenario two with increased mechanical exhaust was the most effective solution.
The modelling also demonstrated that scenario three maintained the CO2 levels below the requirements of BB101 for both maximum and average levels, with a mechanical ventilation rate of 8 l/s per person.
Maintaining comfortable temperatures, keeping air fresh by reducing CO2 levels to maintain a good internal air quality and controlling the opening of windows to reduce the impact of ambient noise can be achieved using a single control system.
Studies have found that although CO2 levels can be higher in naturally ventilated rooms, the feeling of wellbeing and the ability to concentrate for longer periods is improved. Therefore, a combination of natural and mechanical ventilation systems can provide the best results and aid educational performance in schools, like Rokeby, that install such systems.
Carl Sutterby is with WindowMaster.