Compartmentation is a recognised strategy for preventing fire spread and aiding swift evacuation, but little is understood about its true effectiveness. Here, Simon Ellison of fire glass specialist, Pyroguard, explains how fire spreads and why compartmentation isn’t just about controlling the flames.
It is widely accepted that sub-dividing spaces into compartments separated from one-another by walls or floors of fire-resisting construction (used in both residential and commercial buildings), will slow the spread of fire. It is called compartmentation and forms Section 8 of the Building Regulations Approved Document B for Fire Safety.
What is less widely appreciated however, is exactly how compartmentation slows the spread and size of fire, above and beyond acting as a physical barrier to growing flames. Is it simply a case of putting up a wall so the flames take longer to spread to the corridor? To fully understand the role of compartmentation and products such as fire glass, let us first remind ourselves about the behaviour of fire.
The four stages of fire
Fire can have devastating effects, both on people and business, but the first step in truly understanding fire risk is to understand the four stages to any fire: ignition, growth, fully developed and decay.
Remember, for ignition to occur, there must be three components present; heat, oxygen and fuel, the term ‘fuel’ is used to describe anything that will burn. If the combustible material (the fuel) is heated enough to reach its ignition point, and enough oxygen is present, a sustained chemical reaction will take place.
During the growth stage, the ignition flame acts as a further heat source and additional fuel will begin to burn. Convection and radiation will also ignite other surfaces, but we’ll look at this in more detail later. As the size of the fire increases a plume of smoke will develop and reach the ceiling, causing hot gases to collect and transfer heat to other combustible materials in the room. Much of the room’s contents will increase in temperature at the same time, and some of it may ignite independently of the original flames.
A fire is described as ‘fully developed’ when it has consumed much, if not all, of the available fuel. During this phase, oxygen is consumed at a rapid rate, temperatures are at their highest and there will be substantial heat damage.
The natural end to a fire is known as decay, or burnout. Once a fully developed fire has used all available fuel or there is no longer enough oxygen present to sustain the chemical reaction, the temperature decrease and the fire becomes less intense.
How fire spreads
From ignition through to flashover, it can take less than five minutes for fire to spread throughout a building. With that in mind, it is important to remind ourselves that there are three ways in which heat can be transferred in a burning building. Of course, where heat goes, flames will follow, so understanding how heat moves, enables us to better predict how the threat is likely to spread.
Conduction
Conduction is the passage of heat energy through direct contact. In other words, flames directly heat a fuel source, which will catch fire before the heat is spread further (molecule to molecule). Flames then follow the heat. An example would be an oven fire heating a nearby wooden chopping board, which ignites and starts to heat a tea towel hanging from a cupboard door.
Convection
Convection describes how heat energy travels through fluid or gas, which of course includes air. The heated air rises in a plume of hot gas and smoke, which inside a building will quickly reach the ceiling. With nowhere left to rise the hot air will spread horizontally across the ceiling before being forced downwards. The process of convection will cause currents of hot air to spread throughout a burning building, transferring heat into other potential fuels, which could cause further fires to breakout.
Radiation
Radiant heat is the heat we feel when sitting in front of an electric fire. It is carried via electromagnetic waves without using objects or gases as a means of travel. Radiated heat leaves the fire in all directions and often goes unnoticed until it meets an object, which could be a wall in the case of compartmentation. Burning buildings can heat surrounding structures, sometimes passing through windows to ignite objects inside.
How does compartmentation prevent fire spreading?
In the guidance outlined in the Building Regulations Approved Document B for Fire Safety, compartmentation is described in detail as a crucial design consideration for the prevention of fire spread and growth, plus the protection of escape routes and access points for fire service personnel.
When specifying compartmentation walls and considering material types that meet the relevant fire-retardant European classifications, it is important not just to consider the behaviour of the construction material when faced with flames, but also how effective it is at containing hot gases (convection) and radiant temperature increases (radiation).
Many conventional compartmentation wall materials, such as Georgian wired glass and ceramic glass perform to the classified standard when exposed to fire on one side, but do not offer adequate protection from the spread of hot gases and when radiant temperature increases. This protection can save lives, especially in larger buildings where evacuation takes longer.
It is for these reasons higher performance fire glass continues to increase in popularity for the erection of compartmentation walls, with transparent performance classifications and impressive performance credentials for resisting all the dangers of fire spread – not just the flames.
Fire glass is categorised by performance under European and UK standards as either E, EW or EI:
Fire glass classification E: Integrity
Fire glass which is category E certified offers effective and economical protection from flames and smoke, but fails to prevent the transmission of heat during a fire. In other words, it prevents conduction and convection, but not radiation. Remember, heat transfer via radiation can cause ignition in adjoining buildings, so adjoining corridors or offices are at increased risk.
Radiant temperatures during a fire can create additional challenges during evacuation, with frighteningly high levels of heat overwhelming trapped occupants.
Fire glass classification EW: Radiation Control
Compared to E-rated products, fire glass categorised as EW offers some radiant heat control, keeping the amount of radiant heat transfer to below 15 kW/m2 on the unexposed side. Some leading manufacturers like Pyroguard offer UV stable products which are therefore suitable for both interior and exterior compartmentation walls.
Fire glass classification EI: Integrity and Insulation
This category of fire glass is considered premium in its performance against flames, smoke and radiant heat, significantly reducing temperatures on the unexposed face to below the required European and UK standard.
More effective compartmentation for easier evacuation
Practically all compartmentation building materials meet the basic E-grade classification for fire resistance, but there is still a substantial knowledge gap when it comes to the benefits of EW and EI rated products.
Radiant heat is an important factor in the spread of fire and the harm and fatality of building occupants in the event of a serious fire. As a result, the installation and clear identification of safe exit routes is a primary concern for many businesses.
Remember, fire-rated glass is designed to withstand temperatures of around 800°C, whereas the human body can only withstand temperatures of around 120°C for a period of around 30 minutes – a stark difference which clearly outlines the importance of compartmentation using materials that can insulate these high temperatures and protect occupants and fire personnel during the evacuation process.
In addition to the clear fire safety characteristics of high performing glass from Pyroguard, the use of glass as a compartmentation material has a host of other benefits which are valuable in work environments. The laminated qualities of fire glass, for instance, can be beneficial in sound reduction, something which is particularly useful in busy environments. Glass also allows work spaces to utilise natural light, creating brighter rooms without compromising safety.
For more information on Building Regulations guidance for successful compartmentation, download the Approved Document B. For further information on fire glass from Pyroguard, please contact us.
