Intumescent sealants swell when heated and are used to close the small gaps that fire and smoke would otherwise travel through: around door frames, at service penetrations, at the perimeter of a fire resisting wall. They work only in the configuration they were tested in, which is why the tested detail rather than the tube is what you are actually buying.
What they are for
Sealing linear gaps and small penetrations in fire resisting construction. The perimeter joint between a door frame and the wall. The gap where a wall meets a soffit. A small hole where a cable passes through. In each case the sealant restores the fire resistance of the element it is repairing, provided it is used in a detail matching a tested one.
The tested detail is the product
A sealant is tested in a specific construction, at a specific gap width, to a specific depth, with or without backing material. Applied to a wider gap, a thinner bead, or a different substrate it has not been tested and nobody can say what it does. The manufacturer's field of application document is the thing to ask for, and a contractor who cannot produce one for the detail in front of them is guessing.
Intumescent fire sealant, filler and foam are not interchangeable
Ordinary expanding foam is not fire resisting and has no place in fire separating construction, though it is found there constantly because it is quick. Fire rated foams exist and are tested for specific gap sizes. Fire rated mastics handle small movement joints. Fire rated board and batt handle larger openings. Using the quick product because the correct one needs more work is the commonest defect in passive fire protection.
Acoustic and fire performance together
Many products are sold as acoustic and intumescent, because the same joints frequently need both. The two performances are tested separately and a product meeting one does not automatically meet the other. Read which standards the product was tested to and for which construction, rather than assuming the combined description covers your case.
Intumescent filler, and where it differs from a sealant
Both expand under heat and they are sold for different geometries. An intumescent sealant is a gun applied mastic for a joint with movement in it: the perimeter gap around a frame, a linear joint, a small service penetration, tooled to a depth the certification states. An intumescent filler, sold as a foam or an acrylic filler, is for filling a void rather than sealing a joint, typically a larger irregular opening where a mastic bead would slump, and it is finished over rather than left as the fire seal in some systems. The rule that matters for both is the same and is where most site work fails: the product is certified as part of a tested SYSTEM, which specifies the substrate, the gap width, the depth and whether backing is required. A sealant rated for a ten millimetre gap has no rating in a thirty millimetre one, and a filler used to pack out a hole it was never tested in is decoration.
Intumescent expanding foam, and where it is not the answer
Fire rated expanding foam exists and it has a narrow proper use: filling an irregular void around a service penetration where the foam is part of a tested system for that substrate, that gap size and that service. Outside that, it is the material most often used wrongly on site. It is not a joint sealant, because it has no movement capability and cracks away from the frame. It is not a fire stop around a metal pipe on its own, since a copper or steel pipe conducts heat and usually needs a collar or a wrap as well. It is not a filler for a gap wider than the system was tested to. And ordinary building foam is not fire rated at all, whatever colour the can is, so the certification and the batch marking are what matter rather than the appearance of the bead. Where a hole is bigger than a fist, the answer is normally a board and mastic system or a proprietary pillow rather than foam.
Intumescent acoustic sealant, and where both ratings are needed
Acoustic and fire performance are different tests and a sealant can be certified for one, the other or both. The places where both matter are the same places: the perimeter joint of a fire rated partition in a flat or an office, the head of a wall to the soffit, and a service penetration through a wall separating dwellings. If the wall has to hold sound as well as fire, and a separating wall between homes does, then a fire only sealant leaves an acoustic flanking path and an acoustic only sealant leaves a fire path, and either failure is invisible once the wall is skimmed. Two practical points. The certification is for the SYSTEM, so the joint width, the depth of the bead and any backing rod are part of what was tested and cannot be varied on site. And the acoustic performance is easily lost by the next trade: a socket box cut into the wall afterwards undoes what the sealant achieved, which is why the penetration detail matters more than the mastic.
Questions people ask about intumescent sealant
Can I use ordinary expanding foam around a fire door frame?
No. Standard expanding foam is not fire resisting. Fire rated foams and sealants exist and are tested for specific gap sizes and constructions, and the tested detail is what makes them work.
What is a field of application document?
The manufacturer's statement of the constructions, gap sizes and depths a tested product may be used in. It is what tells you whether your detail is covered by the test evidence.
Is intumescent sealant the same as fire mastic?
They overlap. Some products intumesce, others are simply fire resisting. What matters is the test evidence for the joint you are sealing rather than the marketing word on the tube.
Is intumescent foam the same as intumescent sealant?
Different products for different gaps. A gun applied sealant or mastic is for a small linear joint and a pipe or cable penetration, and it is tested as part of a system with a stated joint width and depth. An expanding foam, even a fire rated one, is for filling a larger void and is not a substitute for the tested detail around a fire door frame or a service penetration, which is the commonest misuse on site. The rule is the same for both: the product has to be the one named in the tested detail for that construction, and the evidence is the manufacturer's data rather than the colour of the tube.