Response to the NSW Department of Education School Audits

COVID-19 is airborne.

It always has been, but there have been delays in recognising this fact. This has caused a huge impediment to understanding the importance of clean air and the role it plays in reducing transmission indoors. The US Center for Disease Control states “that airborne virus can be inhaled even when one is more than six feet away from an infected individual.”

In Australia, the impact of indoor transmission is reverberating through schools, where 1892 children aged 0-9 years contracted Covid in the first five weeks after NSW schools reopened for Term 4. In Victoria, the picture is much the same with the vast majority of clusters occurring in educational environments. 

Recognising that Covid is airborne, the NSW Department of education attempted to address ventilation concerns via a commissioned  report by Engineering Consultancy firm Steensen Varming. This report supports their position that natural ventilation alone is sufficient in reducing the risks of airborne transmission in schools. 

The report, however, makes a multitude of assumptions, which when tested means that there are critical errors in the reported results. 

The fatal errors in the report stem from:

1. Assuming what a “typical” classroom layout is

2. Using the wrong “target ventilation flow rate” (based on adult requirements - not those of children - which actually means classrooms are under ventilated)

3. Misuse of variables in Ventilation calculations - painting a better picture of ventilation that could actually be achieved in the classroom.
   

Together, these inaccuracies mean that children have been forced to return to school in unsafe classrooms.

For those interested in the detail of the errors discovered, please see below.

The first assumption of the consultancy report to critique is the model of a “typical” classroom. 

A typical classroom is deemed to have a floor area of 65m2.  In order for a classroom of this size to meet the National Construction Code - buildings that rely on natural ventilation must have openings that add up to 5% of the floor area.  In this case 3.25m2. 

The consultancy conveniently found that the “typical” classroom had openings (windows and doors) that add up to a total area of 3.25 m2.   However, just because a room has openings in size of 3.25m2 - it doesn’t mean that it is necessarily well ventilated.

Professor Geoff Hanmer, Honorary Professional Fellow, Faculty of Design, Architecture and Building, University of Technology Sydney, outlines the necessary difference between calculating the total opening area for NCC compliance as opposed to calculating the ventilating area in his recent article for The Conversation.

Take a 1 m2 awning window for example. It only opens a small amount, but this is ignored when adding up the opening areas for NCC compliance. If a classroom has two of these windows, plus a 1.7 m2 standard door, the result will exceed the 5% NCC requirement for a 65 m2 room (1 + 1 + 1.7 = 3.7 m2).

However, if the window swings open, at say an angle of 20 degrees, and this fact is taken into account, then its effective area for ventilation calculations will be about 0.3 m2, not 1 m2. So the total opening area for that classroom would really be 2.3 m2 (0.3 + 0.3 + 1.7).

In its calculations, the consultancy uses the larger NCC-compliant area, not the effective opening area. This has resulted in significant errors in the report’s findings and does not offer a true reflection of classroom double hung sash, or horizontally sliding windows, which make up the majority of windows used in NSW classrooms.

Additionally, in its ventilation calculations - “typical classrooms” are configured as follows:

1. For Buoyancy - Two openings, one above the other, on the same wall   

   1. Assuming that a window is at least 1m from the floor, and between the 2 windows is at least 0.5 m of wall, and 0.5m of wall above and that the windows are 1m x 1m - the classroom height would need to be at least 4m in height.

2. For Buoyancy  - one opening only

   1. The one window must have an opening area of 2.8m2  To achieve this your one window would need to be (for example) 2.8m wide by 1m tall - a window that you would never find in any classroom - let alone a typical one.

3. For Wind Driven - smallest window opening required

The classroom would need a 1.55 m2 unobstructed window - again a minimum of 1.55m wide and say 1m tall.  Again, due to safety, you would never have a window of this size, let alone in a typical classroom.

Regarding window safety, the government requires that windows cannot open more than 12.5cm to avoid children falling out / getting stuck.  And hence, in the ventilation scenario requiring the smallest opening - that is wind driven scenario - you would need 12.4m width or height worth of windows multiplied by the 12.5cm to achieve the 1.55m2 requirement.

The second error in the consultancy report is in the target ventilation flow rate used. 

The consultancy has set a target ventilation flow rate (i.e. the amount of fresh air coming in) of 260 L/s using a required flow rate of 10 L/s per person and average room occupancy of 26. 

This flow rate is typical for an adult or person over 16 years of age, however, the recommended flow rate for a person under 16 years, i.e. the majority of people in a classroom, is 12 L/s and therefore the subsequent target flow rate should be 312 L/s. 

In setting a target of only 260 L/s, the consultancy is setting too low a benchmark for its “typical” classroom to meet.   This is not even taking into account that most classrooms will house more than a teacher and 25 students.  

Thirdly, the consultancy ventilation calculations, when dissected, reveal a multitude of incorrect assumptions.  

Natural ventilation is driven by buoyancy (which is produced by temperature differences) and wind. In order to calculate buoyancy-driven ventilation, the consultancy has modelled two buoyancy scenarios: one with two openings, and another with just one.

Two openings, one above the other, on the same wall

According to the consultancy, in order to achieve the target flow rate of 260 L/s, the total area of the two windows needs to be 2.296 m2. This is smaller than the assumed total opening area (3.25 m2) so the consultancy concludes that a “typical” classroom will easily be sufficiently ventilated in this scenario.

But herein lies a miscalculation. The assumed total opening area the consultancy used includes the door, however, the buoyancy calculation is for two openings of the same size, one above the other. So, before you can check if the “typical” classroom has large enough windows, you have to subtract the area of the door from 3.25 m2. When you do this, you’ll find that the “typical” classroom doesn’t have large enough openings at all.

Now, if the classroom has windows that don’t completely open (as is usually the case for safety), then you must use the effective opening area, not the assumed area for NCC compliance. When you do this, the shortfall is significantly worse. If the two windows open to an angle of 30 degrees, then their effective area is one third the required size.

Then, if you set the target ventilation flow rate to 312 L/s instead of 260 L/s, the outcome will be worse again. These calculations are not shown because the insufficiency of the assumed window area has been demonstrated well enough using the smaller flow rate.

Just one opening

In order to achieve the consultancy’s target flow rate, the window must be very large: 2.8 m2. Just as before, the consultancy has concluded that its “typical” classroom opening area, 3.25 m2, easily exceeds this.

But this is wrong too! As with the first scenario, you have to subtract the door from 3.25 m2 before comparing it to the required area. When you do this, you’ll find that the “typical” classroom falls well short of meeting the target.

If the window opens to an angle of 30 degrees, then you have to use its effective area, which is less than one third what is required, and it would be even worse if the target had been set to the higher, more appropriate, 312 L/s.

Leaving aside the incorrect calculations for a moment, can you imagine if 2.8 m2 openings really existed in schools? Students would jump through them to get to class, bypassing the doors, and they would surely leap out of the room to evade detention. Such an unsafe opening would never exist without an awning or sliding sash in the way; it’s surprising that the consultancy didn’t put its figures through a reality check before releasing them.

Now, let us examine what the consultancy has done for wind-driven ventilation. Unlike the buoyancy calculations which give you a required opening area, the wind formula gives you a predicted ventilation flow rate. So the consultancy predicts a flow rate of 487.5 L/s for its “typical” classroom, which easily exceeds its target of 260 L/s.

But, as with the buoyancy scenarios, this is wrong! The wind ventilation formula requires you to use the area of the smallest opening, but the consultancy has used the total opening area, 3.25 m2. 

In order to perform the calculation correctly, you could assume that one of the openings is a door (1.7 m2), and subtract this from 3.25 m2 to obtain the area of a window (1.55 m2). The window is smaller than the door, so you must choose this. Then you will obtain a predicted flow rate of 232.5 L/s, which is lower than the target of 260 L/s.

You would not want a real classroom to have a 1.55 m2 unobstructed window, as this would be unsafe. You would more likely have an awning or sliding sash. If a sliding sash exposes 50% of the opening, then the predicted flow rate becomes 116 L/s, which is less than half of the 260 L/s target, and about 63% less than the more appropriate 312 L/s target.

Overall, the consultancy’s “typical” classroom won’t meet its own ventilation target in any scenario.

Based on the ventilation requirements, and the inadequacy of openings in classrooms, it is clear that schools need additional mitigations in place in order to ensure our kids and teachers are breathing in clean air. To this end, Covid Safe Schools is advocating for:

1. Mandatory masks for everyone

2. Indoor air quality to meet Australian standards

3. Publish timely advice of outbreaks

4. Provide optional learning from home until vaccination is completed

5. Provide work-from-home for teachers who do not want to teach face to face until safe

Join us for pushing for these basic safety measures.

Other issues

Advice on fans

The consultancy advises schools to use fans in deep-plan rooms, to get air circulating in dead spots. However, this risks moving virus aerosols from an infected student towards others. It is possible for fans to be used safely, but the advice should be much clearer about when and where their use would be appropriate. It’s not advisable at all to encourage fan use without proper education.

Oversimplification of environment

The consultancy has simplified its model of a “typical” classroom in an “average” environment to the extent that it may not apply to many classrooms at all. It has ignored factors that affect both the driving force and the amount of ventilation achievable: wind direction, building orientation and sheltering from nearby trees and built structures. It has also used only one outdoor temperature and wind speed, although there are several climate zones in NSW that should have been modelled separately.