A fire-rated wall can hold back flames for an hour and still let the fire through. The route is the services. A steel pipe or cable tray that passes through the wall carries heat from the fire side to the safe side, and if the pipe gets hot enough, whatever is touching it on the far side ignites. Understanding heat transfer is the difference between fire stopping that looks sealed and fire stopping that actually works.
The three ways heat moves
Heat leaves a fire by three routes. All three matter in a building, but one of them is the reason passive fire protection exists.
Conduction is heat moving through a solid. Metal conducts well, which is why a steel pipe on the fire side of a wall gets hot along its whole length, including the part in the next room. Concrete and plasterboard conduct poorly, which is why they make good fire separations.
Convection is heat carried by moving air and gas. Hot smoke rises, spreads across ceilings, travels through ceiling voids and ducts, and finds any gap it can. This is how fire moves through an unsealed penetration or an open riser.
Radiation is heat travelling as energy across open space, the way you feel a bonfire on your face from metres away. Radiant heat from a fully developed fire can ignite materials that never touch a flame.
Fire separations are designed to block all three. Conduction is the one that gets through when a penetration is not properly treated.
Why a metal pipe is the weak point
Picture a 100 mm copper or steel pipe passing through a 60-minute fire-rated wall. The wall is intact, the gap around the pipe is sealed with fire-rated mastic, and there is no path for flame or smoke. Fifteen minutes into a fire, the pipe on the fire side is at several hundred degrees. Because metal conducts, the pipe on the safe side climbs towards the same temperature. Anything against it on that side, a timber nogging, insulation, cable sheathing, stored goods, can reach ignition temperature without the fire ever crossing the wall.
This is exactly what the third number in a Fire Resistance Rating measures. A rating of 60/60/60 means 60 minutes of structural adequacy, 60 minutes of integrity (no flames or hot gas through), and 60 minutes of insulation, meaning the unexposed face and anything passing through it stays below the temperature that would ignite materials on the far side. A penetration seal that stops flame but lets the pipe conduct heat can pass integrity and fail insulation. It is still a failed system. We explain the three numbers in Understanding a Fire Resistance Rating (FRR).
What each fire stopping component does about heat
Fire-rated sealants and mastics close the gap between the service and the wall so flame and smoke cannot pass. Intumescent types expand when heated and hold the seal as materials around them distort. They handle integrity. On their own they do little about heat conducted along a metal service.
Insulation is what deals with conduction. A length of mineral wool or rock wool lagging is fitted to the pipe or cable tray on one or both sides of the separation, to a distance set by the tested system. It slows the flow of heat along the service and keeps the surface temperature on the safe side below ignition point for the rated period. The extent of insulation is not a guess. It is specified by the test report for that combination of service, size and wall type.
Fire collars and wraps are for plastic pipes, which fail differently. Plastic does not conduct much heat. It melts and leaves an open hole. The intumescent material in a collar expands as the pipe softens, crushes it closed and fills the opening. Plastic pipes need collars. Metal pipes need insulation. Using the wrong one on the wrong pipe is one of the most common defects we see.
Fire batts and boards close larger openings carrying mixed services. They give integrity across the opening, and the individual metal services passing through them still need insulation to their own tested requirement.
Why the answer changes from building to building
The extent of insulation required depends on the building’s fire design, not just the wall rating. Two buildings with identical 60/60/60 walls can have different requirements for the same pipe.
The main variable is sprinklers. In a sprinklered building, the fire design can assume the fire is controlled early and the separation is never exposed to a fully developed fire for the full rated period. The fire engineer may specify a lower insulation requirement, or none, on some services. In an unsprinklered building, the separation has to do all the work, and insulation on metal services is usually essential.
Other factors include what the separation protects (an exit stair is treated more conservatively than a storeroom), what is on the far side of the wall, and whether the tested system used for the seal already includes insulation in its scope.
This is why we quote from the fire design and consent drawings on a new build, and from a site inspection on an existing one. A product that is compliant in one building can be a defect in another.
What this means at inspection
An IQP checking fire separations for a Building Warrant of Fitness is looking at exactly these points. Metal services through a rated wall with no insulation, or with insulation that stops short of the tested length, are among the defects that hold up a Form 12A. So are collars fitted to metal pipes and mastic-only seals on large cable bundles.
The fix is rarely expensive on its own. It becomes expensive when a building has dozens of them above the ceilings and nobody has a record of which system was used where. That is why every seal we install is tagged with the system, rating, installer and date, and photographed for the register.
Frequently asked questions
Can fire pass through a wall without a hole in it?
Yes, by conduction. A metal pipe or cable tray passing through a fire-rated wall carries heat to the far side. If that surface gets hot enough, materials touching it ignite without any flame crossing the wall.
What is the insulation rating in an FRR?
The third number. It is how many minutes the unexposed side of a fire-rated element stays below the temperature that would ignite materials on that side. It is the rating most affected by heat conducted along metal services.
Do metal pipes need fire collars?
No. Collars are for plastic pipes, which melt. Metal pipes keep their shape but conduct heat, so they need the gap sealed with fire-rated mastic and, where the fire design requires it, insulation lagging to the tested length.
How far does the insulation on a pipe need to extend?
As far as the tested system says. It varies with the service type and size, the wall or floor type and the rating. There is no standard distance that covers every case.
Do sprinklered buildings need insulation on service penetrations?
Sometimes less, sometimes none, depending on the fire design. The fire engineer’s design sets the requirement. It is not something the installer can decide on site.
Who checks this?
On a new build, the installer’s PS3 and the council inspection. On an existing building with a compliance schedule, the IQP during the annual BWOF inspection.
Talk to a passive fire specialist
If an inspector has flagged uninsulated services, or you have a new build where the fire design calls for insulated penetrations, get in touch or call 021 288 1701. We install to tested systems and supply the documentation to prove it. See service penetration fire stopping for what we seal and how.