Fire and smoke seals: Where Vanquish flexible seals fit in a doorset

A fire door is an essential part of a buildings passive fire protection system. To allow the door to function, a 2-4mm gap is required around the top and sides between the leaf and frame. This gap is the first place heat, flames and smoke will try to pass through. Close that gap with the right fire and smoke seals and the door will perform as tested. 

As a UK manufacturer of intumescent seals and passive fire protection products, Vanquish delivers certified engineered passive fire protection solutions, including FlexiFire® and Interdens® intumescent hardware protection, as well as flexible fire and smoke seals. This guide explains what each type of seal does, where Vanquish flexible seals sit within a door and how they support overall fire door safety. 

Why do fire and smoke seals matter in a fire door?

A certified fire door is tested as a complete system. The leaf, frame, hinges, glazing and seals all contribute to its rating, so omitting or swapping any one component can invalidate the test evidence and lead to premature integrity failure during a fire. 

Fire seals are necessary to ensure a fire door performs as stated. Fire doors must resist fire for a specific length of time (e.g. 30 or 60 minutes). Without them, the perimeter gap lets heat and smoke bypass the door entirely and will lead to premature integrity failure. Fire seals are tested in full scale fire tests within doors to either BS 476:22 or EN1634-1. 

Smoke is the leading cause of fatalities in fire related incidents in buildings. Ambient temperature smoke can leak through the gap between the leaf and frame and impair vision of people trying to escape a fire and can cause asphyxiation. This is especially dangerous in HMOs containing multiple inhabitants that may be asleep. Smoke seals are often incorporated into fire seals to reduce the spread of ambient temperature smoke throughout a building. Ambient smoke seals are tested to BS 476:31.1 or EN 1634-3. 

It is essential that fire resisting doors are tested and specified to meet building regulations in the UK. Approved Document B (part of building regulations) provides detailed instructions on how buildings should be constructed to prevent internal fire and smoke spread and specifies where and to what integrity ratings fire doors should fitted. 

Fire seals and smoke seals: Two distinct purposes

The two types of seals serve distinct purposes. Each one responds to a different stage of a fire, which is why many doors need a combination of the two. 

Intumescent fire seals under high temperatures

Fire seals, commonly called intumescent seals, contain intumescent material that stays dormant in everyday use. In the event of a fire, temperatures around 200°C cause the material to expand and close the gap between the door edge and the frame. The char it forms blocks the spread of flames and hot gases, helping the door hold its integrity for the rated period. 

Smoke seals and cold smoke migration

In the early stages of a fire, smoke is relatively cool. Intumescent seals have not yet reacted at these temperatures, so cold smoke can pass through an unsealed gap and fill escape routes long before flames arrive. A smoke seal uses a flexible fin or brush pile that makes contact with the leaf/frame when the door closes, reducing the rate at which smoke is transmitted from the outset. 

Benefits of Flexible Intumescent Seals Over Traditional Rigid Box Seals

Flexible intumescent seals offer several potential advantages over traditional rigid box seals when used in fire door assemblies. Depending on the product design and application, these can include reduced machining requirements, improved material efficiency, greater flexibility in supply lengths and opportunities to integrate fire, smoke and acoustic sealing functions into a single product. 

1. Reduced machining requirements

Flexible intumescent seals can be manufactured in relatively thin profiles while providing the fire-sealing performance required for their intended application. In many cases less material may need to be removed from the door leaf or frame to accommodate it.  

2. Design flexibility for integrated smoke sealing

Flexible intumescent materials offer manufacturers considerable freedom when developing seal profiles, including products that combine an intumescent core with a flexible cold-smoke seal. 

The smoke fins must withstand repeated opening and closing cycles while maintaining effective contact with the opposing surface. Poor material compatibility, inadequate bonding or unsuitable geometry can compromise durability. There are known issues with flexible fins detaching from rigid box seals through continuous use. This is less of a concern with flexible seals as the topper is manufactured from the same material as the fin in a single manufacturing process. 

3. Material efficiency and reduced offcuts

Flexible seals supplied in continuous lengths can be cut to suit individual door dimensions, helping installers manage stock and reduce offcuts. 

This can be particularly useful when manufacturing doors in non-standard sizes or when several different seal lengths are required. Continuous supply can also simplify stock management by reducing reliance on multiple pre-cut lengths. 

It is preferable to have a continuous seal on each edge of the frame/leaf for fire and smoke sealing. Achieving this with standard 2.1m rigid box lengths can lead to wastage as the seals are trimmed to size. A seal supplied on a long reel (e.g. 50-100m) reduces wastage and means a continuous seal can be easily achieved.  

4. More efficient packaging and transport

The ability to coil flexible seals can provide packaging and logistics advantages over transporting equivalent products as long rigid lengths. 

Coiled seals can reduce package dimensions, improve handling and make storage more convenient. It may also reduce transport costs. 

5. Combining fire, smoke and acoustic sealing functions

Flexible seal designs offer opportunities to integrate multiple sealing functions into one product. 

Not only can intumescent seals include flexible fins for cold smoke leakage, but some profiles can be designed to contribute to acoustic performance by reducing sound transmission through the gap between the door and frame. This may be in the form of a bubble or compression-type seal. 

Integrating these functions can reduce the need for separate sealing components, simplify installation and help achieve a more compact overall seal arrangement. 

Choosing the right seal

Flexible and rigid intumescent seals both have a role in fire door design. The most appropriate solution depends on the door’s construction, required fire resistance, operating gaps, smoke-control requirements and the available supporting evidence. 

When comparing products, manufacturers and specifiers should consider not only the dimensions and material cost of the seal, but also installation requirements, material wastage, packaging, durability and the performance of the complete door assembly. 

Ultimately, the objective is to select a seal that provides the required performance while offering a practical, reliable and cost-effective solution for manufacture and installation. 

Talk to Vanquish about fire and smoke seals for your next project

Vanquish flexible seals are manufactured in the UK in a wide range of sizes and coil lengths, with or without self-adhesive tape, to suit individual requirements. contact the Vanquish technical team for expert advice.