Component 03 of 10 · Sealing system

Intumescent seal

Intumescent fire seal. The heat-activated gasket.

Intumescent material set into the leaf edge or the frame. Under heat it expands to many times its size, closes the perimeter gap against hot gases and helps hold the leaf in the frame. Activation is a property of the material rather than a single number.

Key facts

Function: Expands under heat to seal the gap between leaf and frame
Specification: Per the manufacturer's evidence. Commonly 10 mm, 15 mm or 20 mm wide; higher ratings often call for wider or twin seals
Materials: Commonly hydrated sodium silicate, intercalated graphite or mono-ammonium phosphate, in a PVC or PVC-u housing
Critical tolerances: Continuous runs, tight joints; sized and positioned exactly as the evidence specifies
Standards: No standard for the strip alone. Proven within the fire door's own test evidence
Role in fire: Converts the perimeter operating gap into a sealed joint once the fire takes hold, and holds the leaf to the frame as it tries to deflect
Where it sits: Grooved into the leaf edges or the frame reveal, per the evidence

Compliance

Seal size, type and position are fixed by the fire door manufacturer's test evidence and specific guidance. Swapping to a different profile, even a bigger one, is an unproven modification. Specifying a seal is a job for a competent person working from that evidence.

Did you know

Expansion is not just about volume. A sodium silicate seal expands mainly in one direction and sets rigid, pushing hard against the leaf. Graphite expands further and flows, filling a gap that has gone out of shape as the door distorts.

On the model

Intumescent core: Silicate, graphite or phosphate; expands many times in fire
Outer housing: Usually PVC or PVC-u; colour is not a code

What inspectors check

Work through the checklist with the door closed, then in motion. Every line here is a recordable outcome for a Fire Door Inspector.

Run a finger along every edge; the seal must be continuous
Check the seal type and size against the fire door manufacturer's test evidence and specific guidance
Look for missing sections, and for seals that do not match each other
Check joints at corners butt tight
Confirm seals sit firmly in their grooves
Note heavy paint build-up over the seal face
Identify the material from its markings and datasheet, never from its colour alone

Common defects

Sections of seal missing or cut short
Seal loose or proud of its groove
Wrong size or type for the evidence
Gaps at corners and joints
Seal heavily overpainted or damaged
Seal omitted entirely on one edge

Go deeper

What the material is

Three materials do this job in fire door seals. They are not interchangeable, and a strip that looks identical from the outside can behave differently in a fire. Manufacturers label them inconsistently, so the datasheet decides, not the colour of the strip.

Hydrated sodium silicate: The traditional material, and the earliest to start: typically from around 100 °C. Water held in the compound turns to steam under heat. While it is expanding the material is soft; once the water has gone it sets into a rigid closed-cell foam that hot gases cannot pass. Expansion is mostly in one direction, so it pushes firmly against the leaf. It degrades in air, so it is coated or encapsulated rather than left bare. It gives a medium-pressure seal.
Intercalated graphite: Generally starts between 150 °C and 190 °C. Graphite flakes with a compound between the layers; heat drives the layers apart and the flake exfoliates, expanding further than silicate and flowing into a gap rather than pushing straight out. It forms a compact carbon barrier that keeps pushing towards the exposed edge as the timber chars. Not hygroscopic, so it copes with humidity and unheated buildings. It gives a high-pressure seal.
Mono-ammonium phosphate: The third material in use, less common in door seals than the other two. Typically around 150 °C to 180 °C, and some products carry on expanding at higher temperatures. It expands into an open-cell mass that hardens as the water is driven off, and it gives off ammonia as it goes, which is itself a flame retardant. It gives a low-pressure seal.
Hard char and soft char: You will see both terms used, and used inconsistently: some suppliers call graphite the soft char, others apply hard char to both materials. Treat the labels as marketing and read the behaviour instead.
Colours and coatings: Housings come in a range of colours to suit the door and the frame. Colour is not a code for the material or the rating between manufacturers, so identify a seal from its markings and datasheet, never from its colour alone.
Painted over: Light paint on a housing rarely matters. Heavy paint build-up over the face can restrict expansion, and paint on a combined seal's brush stiffens it. Record heavy overpainting as a defect.

Activation and expansion

Two things have to be true in a fire. The seal has to start expanding early enough, and it has to expand far enough to close the gap actually in front of it.

When it starts: It depends on the compound. Sodium silicate typically starts from around 100 °C, as its water flashes to steam. Graphite-based materials generally run between 150 °C and 190 °C. Mono-ammonium phosphate products are typically around 150 °C to 180 °C, and some carry on expanding well above that. Any single figure quoted for all intumescents is wrong.
How far it goes: Expansion is finite, and it differs by material, by grade and by manufacturer. That is why the evidence fixes a seal to the gap it was tested against rather than leaving it to a rule of thumb.
Why the gap matters: A seal sized for its tested gap has a limited amount of growth in it. A gap that has been allowed to open up beyond that may be more than the seal can bridge before hot gases are already through; a gap that is too tight can leave it no room to expand at all.
Holding the leaf in the frame: Once expanded, the seal also holds the leaf to the frame, resisting the deflection and twist a burning leaf develops, and helps keep the door closed after the closer has done its job. On a fire door tested and specified unlatched, that is much of what keeps it shut; even a latched leaf can twist enough in fire for the seal to matter.
Size and position are not yours to choose: Seal type, dimensions and location were fixed when the fire door was tested. Fitting a larger seal, a different material, or the same seal in a different groove is an untested change.Swapping a graphite seal for a silicate one, or the reverse, is not like for like. They activate at different temperatures and fill a gap in different ways.

Fire Door Anatomy is an introduction to fire doors, not the whole subject. Always defer to the fire door’s certification, its test evidence and a competent inspection.