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Radiation calculator for vertical fire curtains

Radiation calculator

Posted February 7, 2017

We are often asked to help fire engineers substantiate the use of a fire curtain in many different building types. This is done by stepping through the different types of irradiance performance between the products. Let’s explain.

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Why do I need a fire engineer to use a fire curtain?

Fire curtains are predominately used to protect openings in fire walls within buildings. The requirements to protect openings is covered in Part C3 of the National Construction Code (NCC) Volume One. The fundamental minimum requirement for fire barriers to protect openings is an insulation performance, usually at least 30 minutes.

Insulation is defined in the NCC as :

Insulation, “in relation to an FRL, means the ability to maintain a temperature on the surface not exposed to the furnace below the limits specified in AS1530.4.”

The limits set out for insulation in AS1530.4 – 2014 is :

  • the average temperature on the unexposed face of the fire barrier can be no hotter than 140°C above the ambient (room) temperature
  • any location on the unexposed face of the barrier can not be hotter than 180°C above the ambient (room) temperature

As fire curtains are normally made from a glass fibre fabric that is approximately 1 mm thick, it is not possible to achieve the performance requirements for insulation as the temperature on the fire side can exceed 1000°C.

A fire engineer is required to assess the suitability of the fire curtain in the particular application, in the particular building due to the lack of insulation which is required to demonstrate the deemed-to-satisfy (DTS) requirements of the NCC.

How can a fire engineer substantiate the use of a fire curtain?

Insulation allows a building designer to have confidence that fire spread within a building is minimised or eliminated and the safe egress of occupants can be achieved. A fire engineer will evaluate the suitability of the fire curtain, usually with reference to the below criteria:

  1. Is there likely to be fire spread by having combustibles in close proximity to the location of the fire curtain. Combustibles could be
    1. flooring
    2. furniture
    3. stock or other materials that may be stored near the location
    4. fixed building materials; walls, cupboards etc
    5. moveable items that may be expected to be left in the area near the fire curtain
    6. any other items that may be considered to be of risk
  2. Is there required to be egress near the fire curtain. If the answer is yes, the fire engineer may consider
    1. will the temperature near the fire curtain be too hot for people to egress? Is it tenable?
    2. how many people and for how long is it expected people to egress past the fire curtain
    3. is the egress path wide enough, taking into consideration the higher temperatures due to the lack of insulation
  3. Will fire brigade intervention be affected by the incorporation of an insulated fire curtain

The list above is not exhaustive and a fire engineer could consider other points that me be required to consider on a project by project basis.

The scientific measure used by a fire engineer to determine whether fire spread may occur or if people will be safe is irradiance. This is the flux of radiant energy per unit area and is measured in kW/m2. By comparing the different irradiance for the different uninsulated fire curtains a fire engineer can justify which type of fire curtains is going to be most suitable for the particular application.

What is the different performance of fire curtains? Aren’t all fire curtains the same?

There can be a large difference between fire curtains and certainly the way that their performance is reported in the market. We all know of the FRL and this is the common way to distinguish between products. To keep this comparison as simple as possible I will focus on 3 Coopers fire curtain products.

Looks like FireMaster Plus is the lesser performing product. Well that depends on the application.

Irradiance is tested as part of an AS1530.4 test and the result recorded at 365 mm from the fire curtain. Most fire curtains are tested to international test standards which normally record the irradiance at 1,000 mm from the fire curtain. Usually a 3m x 3m fire curtain is tested and the results recorded over the duration of the test.

The irradiance of each of these products being reviewed at 1,000 mm is given in the table below.

Irradiance (kW/m2) @ 1 m for a 3m x 3m fire curtain

Time (mins)FireMasterConcertinaFireMaster Plus

What these results show is that FireMaster Plus is going to reduce the irradiance significantly more than the other two products and will be more suitable for use in areas with combustibles or expected egress in close proximity to the fire curtain.

What affects the irradiance?

We can see from the table above that irradiance increases over time. That makes sense as the standard fire heating curve gets hotter over time.

We also learnt that different fire curtains and the materials within them can perform better or worse with regard to irradiance.

There are three other factors that will provide different irradiance figures. They are; the setback distance, the size of the fire curtain and the fire heating curve.

Setback distance – the further your move away from the fire curtain the lower the irradiance. Likewise the closer you get to the fire curtain the higher it will be.

The size of the fire curtain – by increasing the size of the fire curtain you are effectively increasing the size of your radiator/radiant panel. This means that more heat will pass through the fire curtain and the irradiance will increase.

Fire heating curve – it is possible that a fire engineer may consider a different fire scenario to that of the standard heating curve in AS1530.4. If the fire is expected to be hotter or cooler, this will effect the irradiance through the fire curtain.

What is the magic number that will make it safe for egress and the fire brigade?

Most published guidelines believe that designing a system based on safe egress where irradiance is limited to a maximum of 2.5 kW/m2. Below are links to a number of credible sources for this information.

It is therefore going to be of most interest to a fire engineer to determine at what distance and time is irradiance under 2.5 kW/m2 for the fire curtain being considered. This can be calculated using our calculators that have been provided in this article. Below is a table of the results using the 3m x 3m test fire curtain size.

Setback distance (mm) to have less than 2.5 kW/m2 irradiance

Time (mins)FireMasterConcertinaFireMaster Plus


Not all fire curtains are the same. Understanding the application, risks and developing the performance criteria is important to ensure that the right fire curtain is specified.

In case you missed it at the top of the article, you can download our calculators which will provide you irradiance based on the curtain size being used.

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