Which Selleys fire rated sealant do you need, and how do you choose?

The Selleys Fire Rated Silicone Range is a fire and acoustic rated sealant range for perimeter and penetration seals in fire rated construction. That phrasing sets the boundary of the category precisely. These are not general-purpose sealants that happen to resist fire — they are products specified into fire-rated building elements, where the seal is part of the rated construction rather than a finishing detail.

Two features run across all three products and are the reason to use this range rather than something else: a low toxicity formula, and UV Tracer technology. The tracer is the practically important one. It allows certifiers to verify that the specified fire rated sealants were used in the correct application — which means the seal can be checked after installation rather than taken on trust.

What this category covers

Three Fireblock products sit under this path, and they differ by chemistry. Because they are specified into rated construction, the choice between them is made against the construction specification and the approved system, not by preference or by what is on the shelf. What follows explains what distinguishes them so you can read a specification with confidence.

How to choose

Fireblock AC — acrylic, internal

Selleys Fireblock AC is a low toxic, single-component acrylic fire rated sealant featuring UV Tracer technology, developed for internal perimeter and penetration seals in fire-rated construction.

Single-component means no mixing on site — it comes ready to gun, which removes one of the more common sources of error in a rated installation. The internal qualifier is the constraint to note: this is the product for inside the building envelope.

Fireblock PA — polyurethane modified acrylic, fire and acoustic

Selleys Fireblock PA is a low toxic, low modulus, non-slumping polyurethane modified acrylic sealant featuring UV Tracer technology, developed for fire and acoustic rated construction.

Two properties distinguish it. Low modulus means it develops less stress as the joint moves, so it puts less load back into the substrate at the bond line — which matters where a rated joint has to accommodate structural movement without tearing away from its faces. Non-slumping means the bead holds its shape in a vertical or overhead joint instead of sagging before it cures, which is the normal orientation for a perimeter seal at a wall head.

Fireblock XT — silyl modified polymer, fire and sound

Selleys Fireblock XT is a low toxic, high performance SMP (silyl modified polymer) fire retardant sealant with UV Tracer technology, suitable for sealing gaps to prevent both fire and sound passing through.

It is the highest-specification product of the three by its own description, and it is the one named for addressing fire and sound together in the same seal.

The products in this range

• Selleys Fireblock AC — low toxic, single-component acrylic fire rated sealant with UV Tracer technology, for internal perimeter and penetration seals in fire-rated construction.
• Selleys Fireblock PA — low toxic, low modulus, non-slumping polyurethane modified acrylic sealant with UV Tracer technology, for fire and acoustic rated construction.
• Selleys Fireblock XT — low toxic, high performance SMP fire retardant sealant with UV Tracer technology, for sealing gaps against both fire and sound.

Related products elsewhere in the range

Selleys FlameFlex, in the construction silicone section, is described for sealing around pipes and other fire-rated applications to form a fire-resistant barrier.

Selleys Fireproof Cement, in specialist fillers and putty, is a different kind of product for a different job — it is described for high-risk areas such as fireplaces and chimneys, not for rated construction seals.

The headline numbers are identical — so they decide nothing

All three Fireblock products are fire rated to 240 minutes under AS1530.4-2014, all three support an acoustic rating up to Rw 60, and all three carry UV Tracer technology so an inspector can confirm the seal under UV light. The specification most people compare first is the same across the range.

That means the choice is made on the two things that do differ: how much the joint moves, and what the sealant is made of.

	Fireblock AC	Fireblock PA	Fireblock XT
Chemistry	Acrylic	Polyurethane-modified acrylic	SMP (silyl-modified polymer)
Fire rating	240 min, AS1530.4-2014	240 min, AS1530.4-2014	240 min, AS1530.4-2014
Acoustic	Up to Rw 60	Up to Rw 60	Up to Rw 60
Joint movement	See product page	±40%	See product page
Solvents / isocyanates	See product page	See product page	Free of both
Published GHS classification	Warning — H317	Warning — H317	Danger — H360, H319

Movement is the real differentiator

Fireblock PA publishes ±40% joint movement, which is high for a fire-rated sealant. Fire-rated products are usually formulated for integrity under heat rather than for flexibility, and movement capability is where they typically give ground. If the penetration you are sealing is also a genuine movement joint — a structural gap, a slab edge, a junction that will cycle seasonally — PA is the product in this range designed for both duties at once.

If the joint does not move, that capability buys you nothing, and the choice falls to chemistry instead.

What the range publishes about hazard, and what it does not

Fireblock AC and PA both carry a single classification: Warning, H317 — may cause an allergic skin reaction. In both cases it comes from isothiazolinone preservatives present at under 0.05% (BIT and MIT in AC; BIT, CMIT and MIT in PA). Under 0.05% reads as trivial and is not: skin sensitisation is generally permanent once acquired, is then triggered by far smaller exposures, and builds from repeated casual contact. On a floor of service penetrations or a long run of head-of-wall joints, that is exactly the exposure pattern. Wear gloves and tool the bead with a tool rather than a bare finger.

Fireblock XT is the most severely classified product in this range, and the one to read carefully. Its safety data sheet gives the signal word Danger and two classifications: reproductive toxicity Category 1B — H360, may damage fertility or the unborn child, and eye damage/irritation Category 2A (H319). Every declared hazardous ingredient sits below 1% by weight; the organotin catalyst in that list is the component whose chemical class carries reproductive toxicity classifications.

The practical consequence is unambiguous: anyone who is pregnant, may become pregnant, or is trying to conceive should not handle Fireblock XT. On a site installing fire-rated sealant in volume, that belongs in the risk assessment rather than being left to whoever picks up the cartridge. XT is also free of solvents and isocyanates, which is a genuine advantage in poorly ventilated spaces — but it does not offset the reproductive hazard.

If you need a fire-rated sealant in this range without a reproductive toxicity classification, Fireblock AC and Fireblock PA carry only Warning and H317, and both are rated to the same 240 minutes.

One caution when researching XT. Selleys sells a separate product called simply Selleys Fireblock (also branded Proseries Fireblock), with its own product code and its own safety data sheet. Both carry H360, but at different sub-categories, so quoting one for the other is still wrong. Fireblock XT has its own sheet — SELLEYS_FIREBLOCK_XT-AUS_GHS.pdf — and that is the document to ask for.

For completeness, FlameFlex carries the same H360 statement at the more severe Category 1A. It sits in the construction silicone category rather than here. Between the two, Fireblock AC and PA are the fire-rated options in this range that carry no reproductive toxicity classification at all.

Common questions

What are these sealants actually for?

Perimeter and penetration seals in fire rated construction. They are specified as part of the rated element rather than used as general-purpose sealants.

What is UV Tracer technology?

It allows certifiers to verify that the specified fire rated sealants were used in the correct application — an installed seal can be checked rather than assumed.

What is the difference between Fireblock AC, PA and XT?

AC is a single-component acrylic developed for internal seals. PA is a low modulus, non-slumping polyurethane modified acrylic developed for fire and acoustic rated construction. XT is a high performance SMP fire retardant sealant for sealing gaps against both fire and sound.

Which one should I use for an acoustic seal as well as a fire seal?

Fireblock PA is described for fire and acoustic rated construction, and Fireblock XT for preventing both fire and sound passing through a sealed gap.

Can I use Fireblock AC outside?

It is described as developed for internal perimeter and penetration seals. Follow that and the project specification rather than extending it to external work.

Can I substitute a general sealant in a fire-rated joint?

No. A fire rated seal forms part of the rated construction and has to be the specified product, installed in the approved system. Substitution is how a rated element stops being rated.

Are these low toxicity?

All three are described as low toxic formulations.

Is Fireproof Cement a fire rated sealant?

No. It is a specialist filler described for high-risk areas such as fireplaces and chimneys, and it sits in a different part of the catalogue. Do not use it in place of a rated sealant.