Solving the virus VS fire spread conundrum when upgrading ventilation

Fire Spread

Rupert Coggon of Tenmat is saying we are currently facing two major, life critical issues, which will influence how buildings will be ventilated in the future. Whilst the reduction of indoor virus spreading is at the top of everyone’s minds, the risk of rapid vertical spreading of fire is not far behind it, and neither issue can be resolved without affecting the other.

“Unfortunately, almost all of the required actions designed to increase ventilation rates to minimise viral spread can dramatically increase the risk of fire spread through a building” says Rupert

Ventilation and spread of viruses

It is almost unanimously agreed that indoor ventilation has a dramatic effect on the spread of a virus. Government statistics state that the indoor virus spread can be reduced by 70% when ventilation is increased, many reports suggest that the 70% figure is modest. Needless to say, there has already been much written about this topic and it can be concluded that indoor ventilation will be a major weapon in the fight against current and future virus spread.  Ventilation volumes need to be significantly increased and controlled compared to previous recommendations to reduce indoor viral spread.

Mechanical ventilation

Whilst in the summer months, increased ventilation can be as simple as opening windows, this is not a viable controlled or long-term solution. For effective virus control the ventilation needs to be uniform across the room space which is difficult to achieve by opening windows. Correctly designed mechanical ventilation can produce the required uniform air flows required for optimum virus control. A number of studies have already looked into actual flow rates required and how the flow is distributed around different designs of internal spaces, this requires careful design and positioning of both the supply and extract ventilation points

Mechanical ventilation with heat recovery

It is widely accepted that the recirculation of air simply aids the transfer of viral particles and therefore the extracted air needs to be removed from the building. As winter approaches then the extraction of increased amounts of heated air from a building and introduction of unheated air, will dramatically increase the amount of energy required to heat the building, even though current recommendations state that increased ventilation should take priority over thermal comfort.

An obvious solution is to use heat recovery units, whereby the ducting from extraction and supply points are brought to a common heat recovery unit which is often positioned in the loft space or some other appropriate area.

More ventilation without recirculation means more extract and supply points.

To effectively increase and uniformly distribute ventilation, the number of extract and supply points will need to be increased. Extract and supply points often consist of air valves or recessed ceiling fans which penetrate the ceiling membrane of a fire rated floor/ceiling construction.

What does this all mean in terms of potential fire spread and fire protection?

Unfortunately, almost all of the required actions designed to increase ventilation rates to minimise viral spread can have a dramatic detrimental effect on the fire protection of a building.

Increased numbers of air valves or ceiling mounted extraction fans in ceilings, where the ceiling forms part of the fire compartmentation, represent an increased number of potential pathways for both fire and smoke to enter the floor/ceiling cavity which will cause both increased vertical spread of fire and potential premature collapse of the floor itself.

This situation is made potentially worse by the increased use of light weight timber joists which are prone to rapid collapse if directly exposed to fire. The only protection these joists have is from the plasterboard layer below which is likely to have a far greater number of large penetrations for the air valves and fans.

Rupert Coggon
Rupert Coggon

The plasterboard layer of a floor/ceiling construction represents the major defence of that construction against fire. Extensive fire tests have shown that once the plasterboard membrane is breached in a 60-minute fire rated floor/ceiling construction, then total collapse can occur within 15 minutes when the joists are made from solid timber and only 4-6 minutes when joists are of light weight construction. Therefore the maintenance of the plasterboard layer for as long as possible is key to ensuring that the construction performs correctly in a fire scenario.

Premature collapse of a floor/ceiling construction not only allows a fire to spread rapidly but is also highly dangerous for both occupants and fire fighters who may have to enter a burning building.

Has any company fire tested multiple ventilation products in?

When compared to other typical penetrations in both walls and ceilings, ventilation penetrations tend to be of a large area, often up to 200mm diameter for an air valve, and therefore represent a significantly greater danger to the passage of fire than most other service penetrations.

Now that it will be expected that these large penetrations are to be used in far greater numbers and often several within the same floor/ceiling construction, then they will require protecting with suitably test fire protection products. Any fire protection product must have been full scale fire tested and in a representative floor/ceiling construction including multiple penetrations within the same ceiling in order to be full representative of how the products will be installed

As an option; TENMAT manufacture a range of passive fire protection solutions which allow the increased use of both ceiling fans and air valves in fire rated timber floor/ceiling construction.

Extensive and onerous fire testing proves that the number of ventilation penetrations can be increased without having a detrimental effect on the fire protection of the timber floor/ceiling construction, but only when used with correctly tested fire protection solutions.

The increased use of heat recovery units means that there will be an increased amount of ventilation ductwork which is likely to be passing both vertically and horizontally around a building and penetrating fire compartment barriers as it does so, this creates an ideal pathway for fire to move rapidly around the building and through the fire compartment walls and floors, unless it is efficiently fire protected.

Again, products have been developed which are designed to cover all common duct sizes and will offer fully tested fire protection when either singular or multiple ducts pass through a fire compartment wall. Ducts can be protected when they are either stacked or side by side.

It is clear that ventilation rates must be increased and that this will be a key factor in the battle against virus transmission. The design and installation of upgraded ventilation systems will be a high priority in the coming months. However, it is also clear that the fire protection of both existing and new buildings should not be compromised.

The only way of doing this is to ensure that the fire protection of all of the ventilation penetrations have been fully tested in a way which represents how they are now being used. Products previously tested to meet the low ventilation rates and single units installed per ceiling are unlikely to perform when used in multiples and therefore great care needs to be taken when selecting these life critical products.

Rupert Coggon is Business Manager at Tenmat

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