Fire stop pillows are soft, fire resisting bags packed tightly into cable tray openings in walls and floors. In heat they swell and close the gap. Because they lift out and go back, they suit openings where cables are added or changed often. They are rated only when installed to the tested pattern, packing density and opening size.
What are fire stop pillows?
Fire stop pillows, also called fire barrier pillows or fire safety pillows, are bags filled with a fire resisting material that swells when heated. They are packed into the opening where a cable tray or a bundle of cables passes through a fire-rated wall, and sometimes a floor. In a fire, the material inside expands, closes the gaps between cables and pillows, and holds back flame and smoke for the period the system was tested for.
Their main advantage is that they are re-openable. A pillow lifts out by hand. A new cable goes through. The pillow goes back. There is no mixing, no wet work and no waiting for anything to cure. That makes them well suited to the openings in a building that change most often.
Why cable tray openings are different
Most fire stopping is installed once and left alone. Cable tray openings are not like that. Data, power, security, fire alarm and building management cables are added and changed throughout the life of a building. In offices, hospitals and data centres, risers can be opened many times a year.
When the opening is sealed with rigid mortar, every change means breaking the mortar, pulling the cable, and making good. In practice, the making good often does not happen properly. The cable goes through a rough hole, and someone stuffs the gap with whatever is to hand. We see this pattern in almost every building we audit, and we describe it in service penetrations and fire stopping.
How pillows are installed properly
Pillows only perform when they are packed the way the test packed them. That usually means:
- Pillows of the right size for the opening, laid flat like bricks, with joints staggered between rows
- Packed tightly, so they are compressed against each other, the cables and the opening edges
- Pillows worked between cables, not just stacked around the outside of the bundle
- Small gaps closed with the compatible sealant or putty named in the system
- The full depth of the opening filled, as the system requires
- A label at the opening and a photograph of the finished seal
Loose pillows with daylight between them are not a fire stop. They are a pile of bags.
| Question | Fire stop pillows | Fire stop mortar |
|---|---|---|
| Re-openable for new cables | Yes, by hand | Only by breaking it out |
| Wet work and cure time | None | Yes |
| Suits large shafts through slabs | Limited, only where tested | Yes, over rebar and mesh |
| Resists casual damage | Lower | Higher |
| Good as a temporary seal during works | Yes | No |
Where mortar is still better
Pillows are not the answer everywhere. For large openings through floor slabs, such as main electrical and plumbing shafts, fire stop mortar laid over rebar and mesh creates a solid, durable floor seal that people and equipment will not disturb. It is the method we describe step by step in shaft closing. For openings that will not be touched for years, mortar is harder wearing.
In practice many risers use both: mortar at the slab, and pillows in the wall openings where the trays leave the riser and change most often.
Pillows during construction and fit-out
Pillows are also useful as a temporary seal. During construction or a large fit-out, cables are pulled over weeks. Leaving the compartment open all that time is a real risk, especially in an occupied building. Packing pillows back into the opening at the end of each working day keeps the compartment closed overnight, then the final seal is completed once cabling is done.
Keeping pillow seals working
The weakness of a re-openable seal is that it depends on people putting it back. Two simple habits help. First, a label at each opening that says it is a fire seal and must be restored. Second, a rule that whoever opens a seal records it, and the seal is checked and photographed once restored. Facility teams that do this keep their risers sealed. Those that do not usually find the pillows sitting on the floor next to the opening.
For joints and small penetrations around the trays, a fire rated sealant may also be needed. See fire rated sealant for how to choose one.
EN 1366-3:2021+A1:2024, penetration seals. ASTM E814, fire tests of penetration firestop systems. National Building Code of India, Part 4 (Bureau of Indian Standards). Test standards and rules are revised, so check the current edition for your project. Last reviewed 27 October 2026 by the Simfy Firesys team.
Where we fit, and where we do not
We install fire safety pillows and cable fire barriers, and fire stop mortar where a rigid seal suits better, with every opening photographed and logged. We do not pull or install cables, and we do not design the electrical system. If your risers keep getting opened and left that way, talk to our team about seals that survive the next cable pull.
