What is a cold smoke seal?
A cold smoke seal is a brush or fin seal around a fire door that resists the passage of smoke before there is enough heat to activate an intumescent strip. It is often combined with one in a single product.
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<a href="https://fireoptix.co.uk/glossary/cold-smoke-seal">What is a cold smoke seal?</a> — diagram by
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</figure>Smoke moves ahead of fire and is what makes an escape route unusable first, so a door that only responds once it is hot has left a gap in the protection at exactly the time people are still moving.
Combined strips carry both elements in one section, which is why a single visible insert may be doing two different jobs. Where smoke control is required, the door's specification says so — it is not implied by the fire rating alone.
Brush seals are physically delicate. They get crushed, flattened and pulled out, and unlike an intumescent core the damage is visible to anyone who looks at the door edge.
In practice
A care home corridor during a routine door inspection.
The inspector records the lounge door as FD30S with the intumescent intact and the label legible, and ticks the fire element satisfactory. Along the hinge edge the brush has been crushed flat by a wheelchair footplate, and has been for months.
Nothing on that door has failed its fire test. What has gone is the part that was doing the work first: smoke reaches the corridor long before there is enough heat to swell the intumescent.
The S in the designation is why it was specified, and a sheet that scores the door as a pass has reported on the half that was still working.
What goes wrong in practice
- Damaged or missing brush sections left because the fire element looks intact.
- A door required to control smoke fitted with an intumescent-only strip.
- Seals painted over, which stiffens a brush as effectively as it blocks intumescent.