The short answer
A fire batt is a dense stone wool board, coated on both faces with an ablative coating, cut to fill a large opening in a fire-resisting wall or floor where several services pass through. It is sealed at every edge and around every service with an intumescent or ablative sealant. A single 50 mm batt is typically rated up to 2 hours, depending on the services and the opening, and it must be installed exactly as tested.
On this page
Open a riser cupboard in most commercial buildings and blocks of flats and you will see where the cables, trays, pipes and ducts pass through the wall or floor. If the opening is sealed properly, the chances are it is a fire batt: a grey or white board with sealant round every cable. If it is not, you may see open holes, loose mineral wool or expanding foam, and that is one of the most common defects on any compartmentation survey.
What a fire batt is
A fire batt is a board of high-density stone wool with an ablative coating on both faces. The common 50 mm batt has a dense stone wool core, around 160 kg/m³, and comes in 1200 mm by 600 mm sheets.
The stone wool resists the heat. The ablative coating chars and slowly wears away in a fire, absorbing heat and helping to keep the far side cooler. Together with the sealant round its edges and the services, it restores the fire resistance of the wall or floor.
Where batts are used
| Use | What it looks like |
|---|---|
| Multi-service openings | One large hole with cables, trays, small pipes and ducts passing through |
| Blank seals | Openings left after services were removed |
| Head-of-wall joints | The gap between the top of a wall and the slab above |
| Large framed voids | Openings in drylining or partition walls with a frame round them |
Batts are not load-bearing. They should not be used where someone could stand or where anything rests on them, unless the system is designed for it.
How a batt is fitted
The details vary by product and test, but the broad method in published data sheets is the same.
- Cut to size. The batt is cut to fit tightly in the opening, with cut-outs for each service.
- Coat the edges. Every cut edge, including round the services, is coated with the maker’s sealant through the full thickness of the batt.
- Fit and seal. The batt is pushed into place and a bead of sealant, often around 15 mm, is run around the perimeter and every service, on both sides.
- Treat the services. Plastic pipes passing through usually need a collar or wrap as well. Cables may need a coating for a set distance either side. Metal pipes may need insulation. Each must be part of the tested system.
- Label and record. A good installer photographs and labels each seal with a reference, the product and the rating.
Two batts, one on each face of a wall or at the top and bottom of a floor, are common in thicker or higher-rated constructions and for better sound insulation. Check the specific system for when two layers are needed.
Ratings
A single 50 mm batt is typically rated up to 2 hours for integrity and insulation, depending on the size of the opening and what passes through it. For longer periods, such as 4 hours, makers typically specify a thicker batt, around 60 mm. Products are tested to BS EN 1366-3 for penetration seals and BS EN 1366-4 for linear joints, and classified to BS EN 13501-2.
The rating that matters is the one for your exact arrangement: wall or floor type, batt layers, opening size, and every type of service passing through.
What goes wrong
- Services added later. A new cable punched through a batt and never sealed is the most common failure.
- No sealant on cut edges. Without it, the cut stone wool is a weak point.
- Wrong services. Batts tested with cables are not automatically tested with plastic pipes, which need fire collars or wraps.
- Mixed products. Sealant from one maker with a batt from another, outside any test.
- Foam instead. Expanding foam stuffed round the edges or used to fill gaps in the batt.
Repairing a batt seal
Small changes, such as adding a cable, can often be made by cutting a slot, passing the cable through and resealing with the same system. Badly damaged or mixed seals are usually cut out and replaced. Either way, ask the contractor to record what was done and update the label.
Who should install fire batts
Because the seal is only as good as its installation, many clients ask for a fire stopping contractor with third-party certification under a recognised installer scheme. Check the certificate number on the scheme’s own register, and ask for photographs of each seal.
Questions people ask
Is a fire batt the same as loft insulation?
No. Loft insulation is light and has no fire-stopping coating or test evidence for sealing openings. A fire batt is dense, coated and part of a tested system.
How long do fire batts last?
Undisturbed, for the life of the building. The risk is later work cutting through them, which is why a permit system for any work in risers helps.
Can a batt be used in a floor?
Yes, many are tested in floors, but they are not load-bearing. Openings that people could step on need a protected cover or a different system.
This bulletin is general guidance, not a design or specification.