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What is the difference between intumescent strips and smoke seals?


Written by Jonny Millard

June 16, 2021

What is the difference between intumescent strips and smoke seals?

What is the primary purpose of a fire door seal?

Fire door seals are installed between the frame of the door and the door. In an emergency, they fill any gaps that would otherwise enable smoke and fire to escape. They are an essential component of any fire door and must be properly built and installed to provide sufficient protection.

They must be compatible with the door leaf and be made of the correct materials stated in the certificate data sheet. 

How do fire door seals work?

There must be a gap between the door frame and the leaf in every door installation so that the door may open and close freely, this gap on timber doors is usually 2-4mm. However, in the event of a fire, the gap might pose issues because it would enable heat and poisonous smoke to escape, limiting the fire door’s ability to restrict property damage and the risk of harm to people. That is why a seal in a fire door installation is so important: where they are installed they must allow for regular opening and closing of the door without any binding issues. However, they must be ready to expand in the event of fire or stop cold smoke spreading through a building. 

Intumescent seals on fire door are designed to expand when they heat up. Thus, if there is a fire, the rise in temperatures will activate the material inside the PVC casing. This permits the seal to fill the gap between a door and its frame, preventing fire from spreading. Seals are a vital aspect of a fire door’s ability to compartmentalise a fire for 30 minutes to two hours, stopping the spread of the fire to through the building and reducing smoke and flame damage to property, people, internal and external structures. Therefore they are a life saving component.

Types of fire seal

Intumescent Strips

It is a chemically formulated product that aids in preventing fire spread from one area to another. When exposed to severe heat (as produced in a fire), the strip reacts to the heat and expands. The expanded strip then plugs any gaps between the door and the frame, preventing heat from spreading from another area of the building to another.

The main job of an intumescent strip is to inhibit the spread of heat and fire; though it will also prevent some smoke transfer, this is not its main function. It will trap heat and buy time for escape while also protecting other areas of the building that aren’t currently on fire. 

Intumescent strips expand when they are exposed to extreme heat. There are different expansion ratios and materials used to achieve the desired effect. 

If your fire door does not have intumescent seals then you will need to install some. Because a fire door will fail much sooner without any intumescent strips installed. However, it is important that you check the certificate of approval for a 30 or 60 minutes door. You need to ensure that the seals you install are compliant. Furthermore, if the door does not have any seals installed or a groove cut then you will need to use a competent person to carry out this work. A competent fire door installer/maintainer will know to check the certificate data sheet. 

Some doors need 10mm seals, some need 15mm seals. Some may specify a particular brand or type of intumescent material. 

For example there are two main chemical compounds used for intumescent strips: 

  1. Graphite
  2. Sodium Silicate (brand named Palusol)

As a general rule of thumb intumescent strips that use graphite are very dark grey inside. Whereas sodium silicate fire seals are white inside. 

Any intumescent seals that you install should be sourced from reputable manufacturers, have third-party certification and proof of performance. An example of a third-party certification scheme is Certifire. 

 It is important to recognise that there is no such thing as a 30 minutes or 60 minutes fire seal. They are tested as part of the door assembly and help to provide the required level of fire resistance depending on the type, size, location and configuration of the door. 

Watch the video below to watch an intumescent compound expand:

Smoke Seals

A smoke seal is similar to an intumescent strip, but it is more successful in preventing the spread of smoke. It functions in a similar way. However they do not expand. They usually have a brush or a fin that stops the fire. Therefore allowing vital time for escape from a building or an area while also preventing the spread of poisonous smoke gases.

The principal function of a smoke seal is to trap potentially harmful smoke; it is installed between the fire door and its frame or in the rebate.

Smoke seals must be installed on flat entrance doors. 

A fire door that is designed to stop fire and smoke for thirty minutes has a rating of FD30s. The suffix ‘s’ means that the door also has smoke stopping properties. 

Smoke inhalation is kills more people than the fire itself in the event of a fire. The installation of smoke seals will prevent smoke inhalation and reduce the risk of exiting a building suffering from lung damage and maybe passing out, putting their lives in jeopardy. Therefore, it is important that smoke seals are installed when necessary. 

Smoke seals cannot be damaged or painted over. If they are then they will fail an inspection

Smoke seals are only designed to stop smoke at an ambient temperature. Once the door is exposed to extreme heat the smoke seals will only be able to continue working for a few minutes until they are no longer effective at stopping smoke transfer. 

One of the problems with combined intumescent strips and smoke seals is that the brush often binds on the fire door when it self-closes. This can stop the door from closing properly. A fire door that is not closed is useless in a fire. Therefore it is important that if combined intumescent and smoke seals are used that you take action to avoid binding from occurring. For example you could set the frame to door leaf gaps at the maximum amount.

When smoke seals are installed on a fire door IFSA recommends that the door has a leakage rate not exceeding 3m3/m/hour for all edges (head, jambs, base and, where applicable, meeting edges) when tested at 25Pa under BS 476-31.1. Or meets the additional Sa classification when tested to BS EN 1634-3 (with the gap between the bottom of the door and sill remaining as proposed to be installed, not sealed with impermeable material).

See the video below from Lorient on how smoke compartmentation works:

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