Covid 19 and Natural Ventilation

A view of triple bank fabricated natural ventilation weather louvres installed into a factory building with blue sky and clouds in the background

Covid 19 and Natural Ventilation

Providing a good level of ventilation for your employees has become even more important due to the Coronavirus pandemic. The HSE state

“Employers must, by law, ensure an adequate supply of fresh air in the workplace and this has not changed.

Good ventilation can help reduce the risk of spreading coronavirus, so focus on improving general ventilation, preferably through fresh air or mechanical systems.

Where possible, consider ways to maintain and increase the supply of fresh air, for example, by opening windows and doors (unless fire doors).

Also consider if you can improve the circulation of outside air and prevent pockets of stagnant air in occupied spaces. You can do this by using ceiling fans or desk fans for example, provided good ventilation is maintained.

The risk of transmission through the use of ceiling and desk fans is extremely low providing there is good ventilation in the area it is being used, preferably provided by fresh air.”

HSE Website   07-09-20

With this in mind, maintaining a good source of fresh air is paramount to reducing the risk of Covid 19 transmission in the workplace. The CIBSE Guide B2 provides the following table laying out the recommended ventilation rates for air quality. All rates are per person and apply to non-residential buildings. Please note that classroom ventilation has a separate standard which is covered in our guide here.

ClassificationIndoor Air Quality StandardVentilation Range (l/s per person)Default Value (l/s per person)
Table 2.4 Ventilation requirements (reproduced from BS EN 13779-2007)

Building regulations Part F (NBS 2013b) require a minimum ventilation rate of 10l/s per person for office applications. As a rule the fresh air supply for other workplaces should fall between 5 and 8 litres per second per person (please note that there are other factors which apply but for a rule of thumb this should be sufficient).

Maintaining a good level of natural ventilation is beneficial in any workplace but the added risk of Covid 19 transmission has made the provision of airflow within offices an even more pressing issue.

Calculating your ventilation requirements

This guide is intended to provide a basic toolkit to allow you to calculate general ventilator sizes based on the number of occupants in a given building. Please note that other factors such as coefficients of discharge (Cd) and pressure drops (Pd) are not included in this guide but the calculations below should give you a rough estimate of requirements.

Ideally, all natural ventilation systems will have a low level inlet and a high level outlet to enable good crossflow of air within a given space. For example if you required 2m² of inlet vents then the equivalent number of outlet vents should also be installed

Ventilation rates are calculated using the velocity of the incoming air, the free area % of the vents being used and the volume of air required in m³ per second. A basic calculation would be as follows;

  • 200 l/s (0.2m³/s) volume required, (equivalent to 20 people working in an office). Use the table above to calculate how much airflow you require. All calculations are in m³ so please divide your litres per second figure by 1000 to get the equivalent m³.

  • 2m/s free area velocity (wind driven natural ventilation)

  • 48% Louvre Free Area (McKenzie Martin A Series weather louvre)

Volume (m³/s) ÷ Free Area Velocity (m/s) = Free Area required (m²)

0.2m³/s ÷ 2m/s = 0.1m² free area required

Now you have the free area requirement in m² you can calculate the Ventilator area required using the Louvre free area %.

Free Area required (m²) ÷ Louvre Free Area % = Answer

Answer x 100 = Vent Area (m²)

An A Series weather louvre provides 48% free area so

0.1m² ÷ 48 = 0.00208

0.00208 x 100 = 0.208m²

PLEASE NOTE – if your calculator has a % function then dividing by 48% in the first calculation will remove the need to times the result by 100.

Once you have your overall m² measurement you can size your louvres accordingly. The above calculation would require a louvre 0.456m wide x 0.456m high for a total area of 0.208m². These sizes can be modified to suit the situation on site, for example a louvre 1m wide by 0.208m high would provide the same area.

Specifying a Product

The table below gives a selection of our products and the litres per second rates they would provide per m² of louvre. These figures are based on wind driven ventilation with both an inlet and outlet vent at 1m².

ProductFree AreaInlet per m² @ 2m/s Free Area velocity
A Series Weather Louvre48%960 l/s approx.
Maximair 100 Louvre52%1040 l/s approx.
Maximair 75 Louvre43%860 l/s approx.
Maximair 50 Louvre49%980 l/s approx.
Acoustic Louvre 300mm30%600 l/s approx.
Acoustic Louvre 150mm34%680 l/s approx.
Acoustic Louvre 100mm33%660 l/s approx.
Mackridge Ridge Vent50%-70% (by size)1400 l/s approx.
Kenstack Roof Vent50%-70% (by size)1400 l/s approx.
Clearvent Roof Vent70% (fully opened)1400 l/s approx.
Thermac 200048% (fully opened)960 l/s approx.
Operating Box Louvre80% (fully opened)1600 l/s approx.
Fixed Blade Louvre48%960 l/s approx.
Triple Bank Louvre46%920 l/s approx.

McKenzie Martin has a wide range of products developed over our 55 year history which are fully bespoke to suit any requirements you may have on your premises. If you would like any further information on any of our range please feel free to contact us on 0161 723 2234 or via email at