What Do Intumescent Seals Do? A Practical Guide

Protest ES Ltd
18 minutes ago
4 min read

TL;DR – What This Article Covers

This article explains what intumescent seals do, why they are critical to fire door performance, and how failures or incorrect installation can directly trigger fire door remedial works.

What intumescent seals are and how they work in a fire
Why they are essential to fire door compliance
How intumescent seals differ from smoke seals
Common ways seals fail or become ineffective
When damaged or missing seals require remedial action
What facilities managers should check and record

Intumescent seals are small components with a big job. They are often overlooked because they do nothing under normal conditions. During a fire, they become one of the most important parts of a fire door.

For facilities managers and responsible persons, understanding what intumescent seals do and how they fail is essential to managing fire door compliance and avoiding unnecessary risk.

What Are Intumescent Seals?

Intumescent seals are strips of fire-reactive material fitted into the edges of fire door leaves or frames.

Under normal temperatures, they remain dormant. When exposed to heat from a fire, they expand rapidly.

This expansion is not incidental. It is a tested and essential part of how a fire door performs.

What Do Intumescent Seals Actually Do in a Fire?

When a fire breaks out, temperatures rise quickly. As heat builds, intumescent seals react by expanding many times their original size.

As they expand, they fill the gaps between the door leaf and the frame. This creates a tight seal that helps to:

Prevent flames passing through gaps
Reduce the spread of hot gases
Maintain the fire resistance rating of the door
Protect escape routes and compartments

Without this expansion, fire and heat would pass through the natural gaps around the door far earlier.

Why Intumescent Seals Are Critical to Fire Door Performance

Fire doors are tested as complete assemblies. That includes the door leaf, frame, ironmongery, and seals.

If intumescent seals are missing, damaged, painted over, or incorrectly fitted, the door can no longer perform as tested.

This is why issues with seals are one of the most common triggers for fire door remedial works following inspection.

Intumescent Seals vs Smoke Seals: What’s the Difference?

Intumescent seals and smoke seals are often combined, but they serve different purposes.

Intumescent seals react to heat and expand during a fire. Smoke seals are designed to limit the movement of cold smoke at lower temperatures, often in the early stages of a fire.

Some seals combine both functions in a single strip. Others are installed separately. What matters is that the seals fitted match the door’s certification and intended performance.

Incorrect combinations can compromise compliance.

Where Intumescent Seals Are Typically Installed

Intumescent seals are usually installed:

In the door leaf edge
In the door frame
Occasionally in both, depending on certification

They must be continuous, correctly positioned, and appropriate for the fire rating of the door.

Interruptions, incorrect lengths, or poor fitting all reduce effectiveness.

Common Problems Found With Intumescent Seals

During fire door inspections, seal-related defects are extremely common.

These often include seals that are missing, damaged, painted over, incorrectly sized, or replaced with non-compliant alternatives. In some cases, seals are present but incompatible with the door set’s certification.

Any of these issues can invalidate the door’s fire performance and require remedial action.

Why Painted or Damaged Seals Are a Serious Issue

Painted-over seals may appear intact, but paint can prevent them from expanding properly.

Similarly, seals that are split, brittle, or partially missing cannot expand evenly during a fire. This creates weak points where fire and heat can pass through.

Because these defects directly affect performance, they cannot be treated as cosmetic issues.

When Do Intumescent Seal Issues Require Remedial Works?

Issues with intumescent seals almost always require remedial works where performance is compromised.

This includes:

Missing sections
Incorrect seal type
Damage or deterioration
Incompatible replacements

Replacing or reinstating seals must be done correctly, using products that match the door’s tested specification. This is a core part of compliant fire door remedial works.

Intumescent Seals and Wider Passive Fire Protection

Fire doors form part of a wider passive fire protection system. If intumescent seals fail, the effectiveness of compartmentation and escape route protection is reduced.

This is why seal condition is assessed not just during inspections, but also during wider fire safety reviews and fire risk assessments.

What Facilities Managers Should Be Checking

Facilities managers do not need to remove seals or carry out technical testing. However, they should ensure that:

Seals are present and continuous
There is no visible damage or paint build-up
Seal type matches the door specification
Inspection findings are acted upon promptly

Where defects are identified, competent remedial action should follow.

How Protest ES Ltd Supports Fire Door Compliance

Protest ES Ltd supports duty holders by identifying seal-related defects through inspection and delivering compliant fire door remedial works that restore tested performance.

Our approach ensures: