An intumescent strip is a band of material fitted into a groove in the door leaf or the frame that expands many times its thickness when it gets hot. Its job is to fill the gap between leaf and frame as the timber shrinks and chars, keeping flame and hot gases out of the escape side. It does nothing at all at room temperature, which is why smoke sealing is a separate function.
How it behaves
The material is inert until it reaches its activation temperature, at which point it foams and expands to many times its original volume. That expansion is what closes the gap at the moment the door is being distorted by heat. Different products activate at different temperatures and expand by different amounts, which is why the certification specifies the product rather than a generic strip.
Integrity and smoke are two jobs
The intumescent handles hot gases and flame once the fire is developed. Cold smoke in the first minutes passes straight through the same gap, because nothing has got hot yet. Where smoke control is required, a brush or fin seal is combined with the intumescent, usually as a single combined strip. A door specified without the smoke seal where the guidance asked for one has an unaddressed early stage risk.
Installation errors that disable them
Fitting a strip into a groove it does not suit, so it stands proud or sits loose. Leaving gaps at corners or at hardware, which become the path. Painting over the strip, which restricts expansion and can bond it in place. Cutting the strip short at the head so the corner is unprotected. Replacing a combined seal with a plain intumescent during redecoration, which removes the smoke function silently.
Leaf or frame
Some certifications place the strip in the leaf, some in the frame, and a few permit either. Doubling up because more seems safer is not neutral: it changes the gap the door closes against and can stop the door latching. The certification decides.
Questions people ask about intumescent door strips
Do intumescent strips need replacing over time?
They do not have a service life in normal conditions, but they are replaced when damaged, painted, contaminated or missing, which in occupied buildings is often.
What is the difference between a plain and a combined seal?
A plain seal is intumescent only, for integrity. A combined seal adds a brush or fin for cold smoke, and is what an S rated door carries.
Can seals be added to an ordinary door to upgrade it?
Not on their own. Upgrading an existing door is an engineered exercise with test evidence for the whole assembly, not a strip fitted into a routed groove.