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3 Key Functions of Intumescent Materials

Most materials expand when heated, but what makes ‘intumescent’ materials unique is that they expand dramatically upon reaching a certain temperature. This is due to a number of properties – which, in some cases, enable the material to expand many times their original volume.

Because of this, intumescent materials are the ideal solution to sealing the gaps that exist around the edges of fire door leaves.

If properly specified and installed, these seals can lay dormant for years – only activating and expanding when subjected to a significant change in temperature and pressure condition.

In this post, we explore specifically at how this unique fire doorset component can help to save lives, but before we explore that further, we’ll take a look at the three types of materials currently used in seals for fire resisting doorsets.

What Materials Are Intumescent Strips Made From?

Whilst there are a number of materials that exhibit intumescent behaviour, only three are used in seals for fire doors. They are:

Ammonium phosphate – This material has an activation temperature of 180°C and generates virtually no pressure during expansion.

Hydrated sodium silicate – Mainly uniaxial expansion, giving hard foam which exerts considerable pressure during expansion. Activation temperature around 110-120°

Intercalated graphite – This has water layers sandwiched between the layers of carbon. Upon heating, the water is turned to steam, causing a light ‘fluffy’ material.

Whilst all materials have one common characteristic – expansion – they differ greatly in the initiation temperature, the increase in volume and the pressure generated.

For this reason, one product cannot be substituted for another without careful consideration, and what material deemed to be the ‘best’ will vary according to what’s required from the intumescent.

It’s therefore crucial that decisions made about your fire doorset’s intumescent protection are made by the intumescent manufacturer or an appropriately experienced expert.

Now, let’s get into the specific ways in which intumescent strips can save lives.

Fire Door guide CTA

#1 Intumescent Strips and Overpressure

Firstly, we have overpressure. Overpressure wasn’t acknowledged within fire door testing until 1972, following the production of BS476: Part 8, which became even more stringent in 1987. Prior to this, some components within the test environment were rated artificially high, resulting in a more than ‘optimistic’ fire rating.

However, overpressure is not to be neglected. During a fire, gases expand as a result of increased temperature. This, combined with additional products of combustion, causes overpressure to develop within a fire compartment.

Without intumescent seals, it is extremely unlikely for doors to achieve an integrity rating of even 20 minutes.

#2 Intumescent Strips & Smoke Penetration

Within our industry at present, we have found that there is more concern about smoke than there is about fire. After all, the majority of fire-related deaths caused by smoke inhalation is at 89% according to recent statistics.

A standard FD30 fire door without smoke seals will leak smoke at about 200m3 per hour, compared to a FD30S door with a perimeter gap of 3mm, which will leak at 15m3/hour. The difference is significant.

Smoke is arguably more dangerous than the fire itself and this is, in part, due to its unpredictability. To put this into context, the production of smoke and toxicity is related to the elements within the given area. For example, different furnishings and fabrics will emit varying types and concentrations of smoke production.

Additionally, large perimeter door gaps of over 3mm will potentially fill an escape corridor within minutes. This will render escaping occupants effectively blind. Even emergency lighting would be non-effective.

These are just some of the reasons why it is becoming increasingly necessary to specify a doorset that contains supplementary sealing systems to improve and reduce the amount of leakage through the doorset assembly.

Building Regulations highlight the provision of smoke control doors in Approved Document B (England & Wales). When there is a requirement for smoke control, it is indicated by the suffix ‘S’ which is applied to the fire door classification (e.g FD30 becomes FD30S).

#3 Protecting Users From Fire Coming From Different Directions

A fire door’s primary purpose is to protect one area of a building from another (for example, a corridor from adjected rooms, a staircase from linking corridors, and so on).

Yet, as we’re all aware, fires can be unpredictable at best.

For this reason, intumescent protection should be applied in such a way that its benefits are not lost should a fire come from an unlikely direction.

Technically speaking, the intumescent seal should be applied symmetrically about the centre line of the leaf, although this may change depending on items of ironmongery or builder’s hardware for example.

That’s why it’s important to select a manufacturer who will take into consideration the specific combination of door leaf and door frame components, modes of action and configuration.


Without doubt, intumescent material is one of the most crucial components of the fire doorset. Incorrectly specified, you will be significantly compromising the safety of users.

As stated, the intumescent recipe will differ according to the individual doorset’s construction, so it is strongly advised that you purchase all components of a doorset together – as opposed to purchasing component parts.

For any advice or information on intumescent protection, speak to an expert at RW Joinery today.

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