Manufacturer source check

The applicable fire rating depends on the tested construction and installation detail. Confirm the required system with the project fire certifier or engineer before application; a headline rating or joint width alone does not establish compliance. AC, PA and XT are not automatically interchangeable because they share an up-to-240-minute headline. See the manufacturer product information and matching certification.

What is Selleys Fireblock XT, and where should you use it?

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 through construction joints and service penetrations, without solvents or isocyanates. It is fire rated to 240 minutes under AS1530.4-2014 and supports an acoustic rating of up to Rw 60.

Before anything else: prior to application of fire rated products, consult with the project fire certifier or engineer to ensure compliance with certification requirements. The rating belongs to a tested system and joint detail, not to the sealant on its own.

Key characteristics

• Fire rated to 240 minutes (4 hours) under AS1530.4-2014
• Acoustic rating up to Rw 60
• ±20% joint movement capability
• UV tracer included for verification and traceability
• Low toxicity and low VOC; solvent free and isocyanate free
• Free of isocyanates and carcinogens
• High performance SMP (silyl modified polymer) technology

The UV tracer earns its place on a fire-rated job. Once the joint is closed up and finished, there is normally no way to confirm which sealant went into it. A tracer means a certifier can check under UV light rather than take it on trust.

What the acoustic opinion says

Selleys publishes an acoustic opinion for Fireblock XT prepared by Acoustic Logic Pty Ltd, dated 27 March 2024, continuing an earlier opinion from 12 January 2015. It confirms that the change in the sealant's uncured theoretical specific gravity — from 1.499 to a theoretical 1.494 under the new formulation — makes no measurable difference in performance.

Installed in accordance with the manufacturer's recommendation, a single or double bead of Fireblock XT would typically be suitable for the following walls:

Bead	Sealant depth	Area of gap (% of wall area)	Suitable up to
Single	15mm	2%	Rw 45
Single	15mm	1%	Rw 47
Single	15mm	0.5%	Rw 50
Double	15mm	2%	Rw 55
Double	15mm	1%	Rw 57
Double	15mm	0.5%	Rw 60

Two things are worth taking from that table. A double bead is worth roughly 10 Rw over a single bead at the same gap area — so if your target is Rw 55 or above, the detail needs sealing on both sides, not a thicker bead on one. And performance falls as the gap grows: halving the gap area from 1% to 0.5% is worth about 3 Rw. The headline "up to Rw 60" is the best case in that table, not a general figure.

The opinion concludes that Fireblock XT is suitable for use in wall systems rated up to Rw 60, and adds that where a guarantee of acoustic performance is required, specific advice from an acoustic consultant should be sought as to the performance of the sealant in that application.

What ±20% movement capability means for your joint

±20% means the cured bead tolerates the joint widening or narrowing by a fifth of its width without failing. On a 20mm joint that is 4mm each way. It is a moderate figure — deliberately so, because a fire rated seal has to hold its geometry — and it sets the practical limit on which joints XT suits. Where a joint moves more than that, the Fireblock PA product in the same range carries a higher movement rating.

Where to use it

• Sealing construction joints
• Sealing service penetrations
• Sealing gaps to prevent both fire and sound transfer

See also Selleys Fireblock AC and the wider fire rated sealant range.

How to apply it

1. Prepare the joint — clean, dry and sound.
2. Insert a backing rod for joints exceeding the recommended depth or width. As with all sealants, joint dimensions must follow proper width and depth guidance to maintain the rating.
3. Cut the nozzle and extrude the sealant, ensuring full contact with the joint sides. Both faces must be wetted out — a bead touching only one side leaves a continuous path through the seal.
4. Tool off if required.
5. Allow to cure, then finish with paint if needed.

Limitations — where it should NOT be used

• Not suitable for PE, PP or PTFE plastics without surface testing. These are low-surface-energy plastics that most sealants will not grip.
• Joint dimensions must follow proper width and depth guidance to maintain the rating. A joint outside the tested geometry is not a lesser version of the tested detail — it is an untested one.

Hazard classification

Classified as hazardous under Safe Work Australia GHS 7, with the signal word Danger.

• Reproductive toxicity — Category 1B. H360: may damage fertility or the unborn child.
• Eye damage/irritation — Category 2A. H319: causes serious eye irritation.

H360 is the classification that governs how this product should be handled. Category 1B means the substance is presumed to be a human reproductive toxicant on the basis of animal evidence — a step below the Category 1A carried by FlameFlex, but far more serious than the "suspected" Category 2 (H361) seen on products like Tradie's Bog.

The practical consequence is unambiguous: anyone who is pregnant, may become pregnant, or is trying to conceive should not handle Fireblock XT. Where fire-rated sealant is being installed in volume — a floor of service penetrations, a run of head-of-wall joints — that belongs in the site risk assessment rather than being left to whoever picks up the cartridge.

What is in it

Chemical entity	CAS number	Proportion (w/w)
Boron zinc hydroxide oxide	138265-88-0	<1%
Calcium oxide	1305-78-8	<1%
Bis(2,2,6-tetramethyl-4-piperidinyl) decanedioate	52829-07-9	<1%
Silicic acid tetraethyl ester, reaction products with bis(acetyloxy)dioctylstannane	93925-43-0	<1%
N-[3-(trimethoxysilyl)propyl]ethylenediamine	1760-24-3	<1%
Non-hazardous or below reporting limits	—	Balance

Every declared hazardous ingredient is present at under 1% by weight. The organotin catalyst in that list — the dioctylstannane — is the component whose chemical class carries reproductive toxicity classifications; organotins are used as cure catalysts in silicone and hybrid sealants precisely because they work at very low concentrations. A trace ingredient is what drives the classification, which is why "less than 1%" should not be read as reassurance here.

What XT does not carry

Fireblock XT is free of solvents and isocyanates. That is a real advantage in a poorly ventilated service riser or ceiling space, where a solvent-borne or isocyanate product would bring flammability or respiratory sensitisation with it. It does not offset the reproductive hazard, and the two should be weighed separately rather than traded off.

AC, PA and XT have different formulations and safety classifications. Assess each matching SDS and certified installation detail. Do not substitute one for another solely on the basis of a shared headline fire rating.

Protection

• Gloves — nitrile rubber for intermittent contact.
• Eye protection — safety glasses or goggles; this is a Category 2A eye irritant and fire-stopping work is frequently overhead.
• Ventilation — work in a well-ventilated area.
• Do not eat, drink or smoke while handling it. Wash hands afterwards and wash contaminated clothing before reuse.

Cured sealant in a finished joint is not the exposure route of concern; handling the uncured material is.

First aid

Exposure	What to do
Eyes	Rinse cautiously with water for several minutes, removing contact lenses if present and easy to do, then keep rinsing. If irritation persists, get medical advice.
Skin	Wash with soap and water. Remove contaminated clothing and wash it before reuse. If irritation develops, get medical advice.
Inhalation	Move to fresh air. If irritation persists, seek medical advice.
Swallowed	Do not induce vomiting. Rinse the mouth and seek medical advice.

If poisoning occurs, contact a doctor or the Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand — with the container to hand. Selleys emergency line: 1800 220 770 in Australia, 0800 220 770 in New Zealand.

A note on finding XT's safety data

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 products carry H360, but at different sub-categories, so quoting one sheet for the other is still incorrect. The document that covers this product is SELLEYS_FIREBLOCK_XT-AUS_GHS.pdf — ask for it by name.

Common questions about Selleys Fireblock XT

How long is it fire rated for?

To 240 minutes — four hours — under AS1530.4-2014. Which rating applies to your detail depends on the tested system, so confirm with the project fire certifier or engineer.

What acoustic rating does it support?

Up to Rw 60. The independent acoustic opinion gives the breakdown by bead count and gap area in the table above.

How do I actually reach Rw 60?

A double bead at 15mm sealant depth where the area of gap is 0.5% of the wall area. A single bead at the same depth and gap area is rated up to Rw 50.

How much joint movement will it take?

±20%.

What is it made of?

A high performance SMP — silyl modified polymer — with UV tracer, free of solvents, isocyanates and carcinogens.

What is the UV tracer for?

Verification and traceability: it lets a certifier confirm under UV light that the correct product was installed.

Will it bond to plastic?

Not to PE, PP or PTFE without surface testing.

Can I paint over it?

Allow it to cure, then finish with paint if needed.

Do I need a backing rod?

Insert one for joints exceeding the recommended depth or width.

Does it contain isocyanates?

No. It is solvent and isocyanate free, and free of carcinogens.

Do I need sign-off before using it?

Selleys states that prior to application of fire rated products you should consult the project fire certifier or engineer. Where a guarantee of acoustic performance is required, seek specific advice from an acoustic consultant.

How does it compare with the other Fireblock products?

All three are rated to 240 minutes under AS1530.4-2014. XT is the SMP product with ±20% movement and up to Rw 60; PA is a polyurethane modified acrylic with ±40% movement and Rw 67; AC is a single-component acrylic with Rw greater than 60.