The pressure to build upwards and increase housing density, in the search for greater housing affordability, comes with risk and complication. One of them is the threat of fire. For ventilation systems in apartments it poses a specific problem. At the crudest level, if we take the combustion triangle, which says for a fire to exist there must be fuel, oxygen and heat, it becomes easy to see why. Ventilation ducting has the ability to supercharge fires by providing an access route for all three elements to spread from one location to another. It means ventilation in apartment buildings must be carefully assessed for fire risk.
We can make some assumptions on the likelihood of a fire in, say a three storey walk-up building, to occur. It’s hard to find statistics for New Zealand but we know in the United States, over 75% of fires between 2015-2019 were in residential buildings (source: National Fire Protection Agency). There’s no reason to suggest such an overwhelming skew towards residential fires would not be present in NZ to some degree, assuming our behaviour is similar. We can predict the risk of a fire in a residential building is greater than in a Supermarket.
Taking this further, if fire is more likely in a residential building then the logic should follow that in a multi-tenanted apartment building, the risk increases due to the presence of sheer numbers. There are more sources for a possible fire outbreak (i.e. cookers) and people (residents) to accidentally start a fire. We should note this is not fear mongering, nor is it a reason to not build up. Indeed, the statistics from the US show the likelihood of a fire resulting in death is lower in apartments than in single family residential homes. Part of the reason is stricter fire regulations in such buildings.
Our own regulations are comprehensive. Yet ventilation is a possible weak point and a poor design can have disastrous consequences. Indeed, the presence of regulations does not always mean everything works as intended, just as the presence of laws does not guarantee people either understand or abide by them.
The BRANZ Guide to Passive Fire Protection in Buildings provides an excellent overview of the regulations. It identifies the principles of NZ Building Code, explains the terminology commonly used and talks to the main components which have a passive fire protection role.
The term ‘Passive Fire Protection’ refers to the construction elements within a building that are designed to prevent or delay the spread of fire and/or smoke to different parts of the building. This differs from active fire protection, which would include the use of sprinklers and suchlike to actively manage the fire. Passive fire protection has two key aspects:
Resistance to fire. Such as the ability of a fire-separating element (i.e. wall or floor/ceiling) to limit fire spread, including passage of fire products through the elements, or to prevent collapses in the case of a load bearing element (such as a column).
Reaction to fire. Such as the surface burning behaviour of an element or material and the extent to which it promotes rapid flame spread or smoke production.
In a 3 storey walk-up apartment building, firecells are used to help prevent the spread of fire and this is where ventilation design becomes important. It’s a weak point if done poorly and can negate the passive protection properties of a firecell.
Firecells are designed to prevent the spread of fire for a specific time period. They use construction elements which should have a rating for the same time period as that specified by the firecell. The idea is to contain the fire to allow residents enough time to safely exit the building, and fire services time to get to the fire before the defences are breached and the fire spreads. Get it wrong and the consequences can be unimaginably horrific. Grenfell is a case in point. 72 residents of Grenfell Towers died, when a fire in a 4th floor apartment, spread quickly. It took just 16 minutes for flames to reach the 23rd floor, via the external facade cladding which was combustible.
For ventilation, when an apartment building is constructed using concrete slabs for the ‘mid-floors’ (the ceiling of one apartment/the floor of the one above) the design is relatively straightforward. The concrete slab becomes the fire barrier to prevent the vertical transfer of fire. The decorative gib ceiling underneath does not need to be fire rated, so ventilation systems can be fed through the gib ceiling and ducting run between it and the concrete slab, with little problem or complication.
Timber mid-floor structures are more difficult. The timber is not fire rated so the fire protection line becomes the gib ceiling itself, which must be fire rated. Any ventilation grills in this type of ceiling have to be protected. The most common way of doing this is through the use of an intumescent fire damper. These allow the flow of air through fire rated walls and ceilings. If a fire breaks out, the rising temperature will cause the intumescents to expand, closing the gaps in the grill to prevent vertical fire spread. An intumescent will typically expand at a rate of 50:1. This means that a 1mm coating will reach an estimated density of 50mm in the event of a fire.
It is possible to avoid the use of a fire damper by running the ducting through a bulkhead, although doing so isn’t desirable aesthetically. Another alternative is the use of a false ceiling. Some apartments with a timber mid-floor structure will increase the floor separation from 2.7 to 3 metres, allowing for a false ceiling, creating a channel between it and the fire barrier above for services and ducting.
Kitchen rangehoods present another complication. Ideally, they will be located by an external wall so they’ll be ducted to the outside. That’s not always the case in apartments and it means some adaptation is needed. If there’s no false ceiling to contain the ducting from a rangehood, a fire damper is needed. This might not sound like an issue, given they’re used in the laundry and bathroom. However, a rangehood will become covered in grease and that is a problem. Intumescents are quick to expand but they still take time. With all that grease, the risk of flames licking through the grill and igniting it, is just too great. For this reason we don't recommend, nor will we install, a ceiling mounted fire damper from a kitchen rangehood.
One alternative is to use a more expensive spring loaded steel damper. Yet with moving parts needing regular maintenance, possibly every 3 months, we feel they present a risk too. Without any guarantees the regular maintenance will take place, we preclude these from consideration.
Fire spread through walls is different. Here, the requirements will differ depending on the location of the wall. If close to the boundary, with the risk of flames crossing the boundary line, the entire wall will be fire rated. In this case, any extract cowls and ducting passing through the fire zone will need to be dampered.
Walls located away from a boundary are different. To prevent fire spreading up and reentering the building at a higher level (as was the case at Grenfell) a 1500mm wide fire rated spandrel is required. This essentially acts as a fire break. It’s often possible to locate ducting and cowls outside of this spandrel, so the use of a fire damper is not required. In certain situations where it isn't achievable, a quirk of the building code allows for a notch in the spandrel. That is, any ventilation cowl can be compensated for by an equal ‘rise’ in the spandrel above or below it. As long as there is still a clear 1500mm section, it’s ok. However, if windows are not stacked vertically, this can be harder to achieve and may not be possible so a fire damper will be needed.
When it comes to extraction ventilation in apartments, passive fire protection is both simple and complex. The regulations are easy to understand but implication comes with complication. Such as the issue with kitchen rangehoods. The only consistency is the need to do it properly. The consequences of not doing so, and we make no apologies for appearing dramatic here, can be fatal.
If you have an apartment building project and would like some advice on ventilation design then please contact us. We’re here to help.
