What is an intumescent strip?

An intumescent strip is a narrow strip of material fitted into a groove in the edge of a fire door or in the door frame rebate. The strip contains intumescent compounds (often graphite or sodium silicate based) that expand dramatically - typically to 3-10 times their original volume - when heated to around 150-200 degrees C. In a fire, this expansion seals the gap between the door and frame, preventing hot gases and flames from passing through the gap and maintaining the fire resistance of the door assembly for the rated period.

What is the difference between an intumescent strip and a smoke seal?

Intumescent strips seal against fire and hot gases - they only activate under heat. They do not seal against cold smoke (smoke below the activation temperature). Cold smoke seals (typically brush pile, rubber, or foam strips) seal the door gap at normal temperatures, preventing smoke from passing through the gap before the fire reaches the door. An FD30S or FD60S fire door requires both intumescent seals (for fire) and cold smoke seals (for smoke). The two functions are sometimes combined in an intumescent and smoke seal strip (called a combination seal), which has an intumescent element and an integral brush or foam smoke seal.

Where are intumescent strips fitted on a fire door?

Intumescent strips are fitted to the top and both side edges of the door leaf, or in the corresponding rebate in the door frame - the location depends on the tested assembly specification. Some fire door sets have the strip in the door edge; others in the frame; some in both. The strip is typically fitted in a routed groove of exact dimensions matching the strip size. The number of strips and their exact position must match the tested assembly - altering the specification can invalidate the fire door rating. On double doors, intumescent seals are also required between the meeting stiles.

Can you retrofit intumescent strips to an existing door?

Retrofitting intumescent strips to an existing solid-core door is possible but must be done carefully and cannot guarantee compliance with a certified fire door assembly. The most reliable approach is to fit a complete certified fire door set. However, where a solid-core timber door of adequate weight and construction is already in place, a specialist can rout grooves in the door edges and frame, fit appropriate certified intumescent and smoke seals, add a certified self-closer and three certified hinges, and test the installation with a door gap gauge to confirm the gaps are within tolerance. This is common in existing housing stock where full door set replacement is cost-prohibitive.

Fire Safety noun

Intumescent Strip

/ ɪnˈtjuːməsənt strɪp /

Also known as: intumescent seal, fire seal, cold smoke seal, combination seal

Definition

An intumescent strip is a material fitted in the edge of a fire door or its frame that expands dramatically (typically 3-10 times its volume) when heated to around 150-200 degrees C, sealing the gap between door and frame to prevent fire and hot gases from passing through. Intumescent strips are mandatory on all fire doors. Smoke-rated doors (FD30S/FD60S) additionally require cold smoke seals (brush or foam) that seal the gap at normal temperatures. Combination seals incorporate both functions in one strip.

In practice

Intumescent strips are fitted into routed grooves in the door edge or frame rebate - the groove dimensions must match the strip size exactly as specified in the tested assembly. The strip must run continuously along the top and both sides of the door; the threshold is typically addressed with a drop-seal or intumescent threshold strip. The gap between door and frame on all sides must be within the tolerances of the tested assembly (usually 3-4mm) for the intumescent strip to function correctly - too large a gap and the expanded material cannot bridge it; too small a gap and the door may be difficult to operate.

Intumescent materials are also used in other fire protection applications beyond doors: intumescent pipe collars wrap around plastic waste pipes where they pass through fire-resisting floors or walls - when the plastic pipe burns away in a fire, the intumescent collar expands to close the hole. Intumescent mastic is used to seal service penetrations through fire compartment walls. Intumescent coatings on structural steel provide fire resistance by expanding to form an insulating char that slows the heating of the steel member.

Building Regulations

MandatoryRequired on all fire doors - a fire door without seals is not compliant

Smoke-ratedFD30S/FD60S also requires cold smoke seal

PenetrationsIntumescent pipe collars required for plastic pipes through fire walls/floors

CertificationMust be certified for the specific fire door assembly tested

Approved Document B requires fire doors to comply with BS EN 1634-1, which tests the complete door assembly including seals. An intumescent strip must be from the same tested assembly as the door and frame - mixing components from different tested assemblies is not permitted without evidence that the combination achieves the required performance. Intumescent pipe collars for penetrations through fire-resisting construction must be tested to BS EN 1366-3 (fire resistance of service installations). Building Control inspectors will look for intumescent seals and cold smoke seals as part of the fire safety inspection of a loft conversion or garage conversion.

Full Building Regulations guidance

Intumescent Strip

Just so you know...