Passive fire protection helps limit the spread of fire and smoke between building compartments. In linear joints and gap seals, a fire retardant PU foam such as FP404 can support this role when it is used within its certified scope. The final fire resistance does not depend on the foam alone, but on the tested system, the correct joint depth and the surrounding construction.
Why passive fire protection depends on the right fire stopping system
A safe building relies on fire compartments that help contain flames and smoke for a defined period. These compartments only work when walls and floors keep their fire resistance after joints, gaps or service openings have been sealed with the correct fire stopping system.
Fire protection systems must suit the exact use case. A linear joint between rigid walls needs a different approach than an opening around a pipe or cable. This is where a product such as FP404 PU foam can play a role, but only when the application matches its certified use for linear joints and gap seals.
Product choice starts with the joint type and the required fire resistance. This helps prevent fire from spreading between compartments for the time stated in the approved system.
Fire stopping in linear joints and gap seals
Linear joints occur where building elements meet, such as a wall connection or a joint between a wall and floor. These areas form part of fire protection compartmentation, so the seal must help maintain the fire resistance of the surrounding construction.
Bostik FP404 Fire Retardant PU Foam is designed for linear joint and gap filling applications in rigid wall and floor constructions. It can be used in walls with a minimum thickness of 70 mm and floors with a minimum thickness of 100 mm, where the certified system allows it. The product is available in gun and hand-held versions, so you can choose the format that suits the application method on site.
On site, fire retardant PU foam offers a familiar application method and can help treat joints quickly. For FP404, the certification covers use as a single product in specific fully filled joints, with fire resistance up to 2 hours. In certified combinations with Bostik Fire Protect sealants, the fire resistance can reach up to 4 hours.
Why certification must guide every fire protection measure
In passive fire protection, a fire rating belongs to a tested application, not to a product alone. The same material can achieve a different result when the joint width, wall thickness or installation method changes.
For FP404, the European Technical Assessment and Declaration of Performance define its intended use for linear joint and gap seals. The product has CE marking based on ETA 20/1118 and has fire resistance classifications under EN 1366-4 1. These documents also state the required wall or floor thickness, permitted joint width and application direction.
This matters on site because a fire rated result cannot be claimed for an untested setup. A shallow joint, a different substrate or a service opening may look simple to seal, yet the certification may not cover it. Before application, the required fire resistance must match the certified system. This keeps fire protection measures clear, traceable and aligned with standards.
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1 EN 1366-4 |
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| European fire resistance test standard for service installations, specifically linear joint seals. It defines a method to assess the fire resistance of joint seals according to their intended end use. |
Application rules for certified fire retardant PU foam use
Correct use of fire retardant PU foam starts with joint depth. FP404 must fill the joint as described in the tested system. Empty spaces inside the joint can reduce the performance of the seal because fire and smoke may find a path through weak points.
The declared fire resistance for FP404 applies to fully filled linear joints within the certified scope. The surrounding wall or floor must also meet the required thickness stated in the certification. In practice, this means the installer must check the construction before the foam is applied.
FP404 can also form part of a certified system with Bostik Fire Protect sealants. In these cases, the sealant position and sealant depth must follow the tested configuration. The foam cannot be treated as a generic filler behind any fire resistant sealant. Each certified build up has its own limits, and those limits define the final fire rated performance.
Where fire retardant PU foam should not be used
Fire retardant PU foam is not suitable for every fire stopping situation. Service penetrations need particular attention, because pipes and cables can change shape or lose material during a fire. Plastic pipes may melt away. Cables or combustible insulation can leave an open path through a fire compartment. In many penetration applications, an intumescent or other certified penetration-seal solution is required, because the product must be tested for the specific pipe, cable or insulation configuration.
FP404 is designed for linear joints and gap seals. It should not be used around plastic pipes, cable bundles or insulated metal pipes where the certified system does not allow it. Metal pipe penetrations are treated as penetration-seal applications and must be matched with a product or system certified for that specific configuration.
Shallow joints also need caution. Fire retardant PU foam needs the certified depth to resist flames as tested. It should not be used as a backing material behind fire resistant sealants unless that exact build up appears in the certification. An application that has not been tested cannot be given a fire rating.
How to choose the right fire protection solution on site
The first check is the type of opening. A linear joint or gap filling can fall within the certified scope of FP404, subject to the substrate and required fire resistance. A pipe or cable penetration needs a product tested for that purpose, such as an intumescent sealant or pipe collar.
The next check is the construction itself. The wall or floor thickness must match the ETA or Declaration of Performance. The same applies to joint width and foam depth when FP404 forms part of a certified system with another Bostik Fire Protect sealant.
For reliable passive fire results, the document should guide the job before any product enters the joint. This helps the team apply the right fire protection solution for the actual site condition.
Building safer fire protection systems with clear limits
Fire retardant PU foam has a clear role in passive fire protection when the application matches the tested system. FP404 can help seal certified linear joints and gap seals with a fast on site method, while the required fire rating remains linked to the approved configuration.
Clear limits make fire protection systems safer. A joint seal, a penetration seal or a sealant combination each has its own certified use. The safest result comes when the product choice follows the fire compartment design and the proof of performance.
For detailed guidance on certified fire protection applications, you can consult the Bostik Academy passive fire protect resources.
Explore the Bostik FireProtect range
FP404 is part of the wider Bostik FireProtect range which includes certified passive fire protection solutions for linear joints, movement joints, openings and service penetrations. For a broader view, read our introduction to Bostik’s FireProtect range and our article on firestopping systems.
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