Ventilation has been provided within educational buildings since their creation, however, the strategies deployed to provide ventilation have changed drastically over the last few years driven by changes in legislation and advanced research into the importance of indoor air quality.
Why is ventilation required within the classroom environment?
Ventilation is required to mainly provide fresh air for respiration, to remove stale air and pollutants from the space and to maintain carbon dioxide, temperature and humidity levels within acceptable limits.
Children are more vulnerable when exposed to environmental pollutants, largely due to increased respiration rates per bodyweight, in comparison to adults. This results in the increased spread of infection within the classroom environment. Poor levels of ventilation within buildings has an impact on both the health of the occupants and the building fabric.
A lack of ventilation increases the relative humidity within the room, which can contribute to rot and mould developing within the structure which creates an environment where dust mites can rapidly multiply. Low levels of ventilation also allow a higher concentration of Volatile Organic Compounds (VOCs) from e.g., floor coverings, furniture, painted surfaces and electronics to remain in the stale air. The cumulative effect of the aforementioned by-products created by insufficient ventilation can have an adverse impact on human health as excessive exposure can cause asthma and allergies.
Natural ventilation
Historically natural ventilation, utilising openings within the building fabric to facilitate the supply of fresh air and the extraction of stale air from the classroom environment, was the predominant means of ventilation within education facilities. Natural ventilation, either via single sided, cross ventilation or stack ventilation had been the adopted solution due to its simplicity, low capital expenditure and low operational energy consumption.
The system effectiveness of both buoyancy-driven and wind-driven natural ventilation is influenced by external conditions such as wind speed, temperature and surrounding topography. The building itself can also impact on the effectiveness of natural ventilation based on the orientation, fenestration layout, window pane opening size / locations and internal temperatures.
Natural ventilation does not allow sufficient control over ventilation rates and can have a dramatic impact on internal ambient air temperatures in winter conditions. Another area of concern is the potential for noise and airborne pollution entering from outside through the ventilation openings. The results of studies conducted by universities across the UK on ventilation within classrooms are astounding, with some studies highlighting that up to 40% of the sample classrooms were recorded as exceeding 1500ppm CO2 concentration throughout the course of the day. High levels of CO2 within classrooms can cause drowsiness and reduced cognitive ability, which impairs the engagement and focus of pupils and can ultimately affect academic performance.
BB 101: Ventilation, thermal comfort and indoor air quality 2018
BB101:2018 was a welcome update to the ventilation requirements within education buildings, particularly from an indoor air quality perspective. BB101:2018 demands a higher degree of control over the ventilation provided to the classroom environment, with some key changes over BB101:2006 including:
- Indoor air quality: CO2 concentration (whereby mechanical ventilation has been deployed) to not exceed a daily average of 1000ppm and a maximum CO2 concentration of 1500ppm for no more than 20 consecutive minutes.
- Filtration: M5 filtration in non-polluted areas and a minimum requirement of F7 filtration in polluted areas.
- Draughts: The temperature differential between the external ambient air entering the room and the internal ambient air temperature cannot exceed 2°C within Category 2 zones and 1.5°C within Category 1 Special Educational Needs (SEN) environments.
BB101:2018 raised the bar for indoor air quality within the classroom environment, demanding improvements to the acceptable range of CO2 concentration within the classroom, setting a standard for supply air and room air temperature differentials to mitigate cold draughts and including the requirement for filtration of the incoming air in mechanical systems.
Covid-19 and ventilation
The arrival of Covid-19 in 2020 reinvigorated both the interest and scrutiny over indoor air quality in many education buildings, particularly those that still avail of natural ventilation solutions.
A study in Italy conducted by the ‘Hume Foundation’ across 10,441 classrooms, found that 6 air changes per hour reduced the spread of infection by over 80%. This equates to approximately 300l/s based on a standard 60m2 classroom, compliance with BB101:2018 would achieve this ventilation rate in most cases. Although directly linked to Covid-19 in this case, there are clearly tangible benefits to providing sufficient ventilation to reduce the spread of respiratory diseases.
Covid-19 has also raised the debate on whether recirculation of air within classrooms is acceptable. Some ventilation units on the market directly inject a portion of the room air into the incoming fresh air, to raise the temperature of the incoming air. The Chartered Institution of Building Services Engineers (CIBSE) ‘Covid-19: Ventilation’ document permits the use of heat recovery via thermal wheel or recirculation, providing the unit serves a single room, to mitigate the risk of recirculating airborne pathogens from one room to another. Selecting a ventilation unit with plate heat exchangers is the optimum method to ensure that there is no cross contamination between supply and exhaust air streams during the heat recovery process.
Hepa filtration and germicidal ultraviolet light
The University of Leeds conducted a £1.75m pilot trial based on a sample of 30 primary schools in Bradford to review the addition of Hepa filtration and germicidal ultraviolet light to purify the air within classrooms to reduce the spread of airborne pathogens., was conducted by University of Leeds. This research is applicable to classrooms with natural ventilation, where the standalone filtration units can filter and re-circulate the stale air, however, as seen from the study in Italy, providing sufficient continuous ventilation is a highly effective strategy to severely decrease the spread of airborne diseases.
Hybrid and mechanical ventilation systems
A hybrid ventilation system or mechanical ventilation with heat recovery will both achieve compliance with BB101:2018 and BB93 acoustic requirements. These systems also provide the flexibility and adaptability of providing ventilation at the optimum rate to satisfy various room conditions, i.e. part occupancy or full occupancy, continuously controlling the volume flow rate of the ventilation system based on temperature and CO2 levels within the room (demand based ventilation).
The move towards hybrid and mechanical ventilation systems also features greatly when we consider the shift in the construction industry towards Nearly Zero Energy Buildings (NZEB). The Northern Ireland Building Regulations Technical Booklet Part F2 2022 is driving the construction industry into the new era of ultra-efficient NZEB buildings. This will require design teams to adopt a fabric first approach, reducing U-Values of the building fabric and reducing the air permeability. In doing this we must be cognisant that ‘buildings need to breathe’ in order for humid and stale air to be exhausted.
Increasing the ‘air tightness’ of the building ensures that building owners do not use excessive heating energy to overcome the air permeability through the external envelope of the building. This creates a scenario whereby buildings will no longer breathe through random openings or cracks within the building structure, which places more reliance on the auxiliary ventilation provided to the building. This reduces the feasibility of utilising natural ventilation, as the indoor air quality will be at the mercy and reliance on human intervention to open windows. To ensure sufficient ventilation rates are achieved in airtight buildings, a demand controlled ventilation system should be deployed.
The use of a hybrid or mechanical systems will undoubtedly increase the electrical demand and electrical energy consumption of the building, when compared against natural ventilation solution. However, the addition of heat recovery offsets a significant portion of a buildings heating load. A mechanical ventilation solution would simply not be sustainable without the inclusion of heat recovery. BB101:2018 alludes to the lower capital expenditure of natural ventilation but the potentially astronomical rise in heating energy within the life cycle and maintenance section “systems with low initial capital costs may have unaffordable running costs”.
The increased volume flow rate of air via natural ventilation to achieve both CO2 requirements and maintain thermal comfort would not be possible from either an environmental or economic perspective. A solution that achieves the optimum indoor air quality to enable the occupants to thrive, whilst ensuring that we capture and utilise the waste heat from the classroom, can be achieved through various hybrid and mechanical ventilation systems.
The health of pupils
Given that pupils spend a large portion of their lifespan within the classrooms, it is imperative that we achieve a high level of indoor air quality, create a comfortable indoor climate, finding the balance between air quality and energy efficiency.
At Bennett Freehill we use the most advanced Dynamic Simulation software to analyse a building’s performance, enabling us to achieve the most appropriate design solutions for each individual project.
This bulletin has been prepared by Cormac Freehill, Managing Partner, Bennett Freehill. Please feel free to contact Cormac with any ventilation queries you may have T: 028 9076 0050, E: cormac.freehill@bennettfreehill.com.
