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 AC, and where should you use it?

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 up to 240 minutes in accordance with AS1530.4-2014. It also carries an acoustic rating higher than 60 dB (Rw > 60). The product was formerly known as Acryseal.

Before anything else: prior to application of fire rated products, consult with the project fire certifier or engineer to ensure compliance with certification requirements. That instruction is Selleys' own, and it comes first for good reason — a fire rating belongs to a tested system, not to a tube of sealant on its own.

Key characteristics

• Fire rated up to 240 minutes (4 hours) under AS1530.4-2014
• Acoustic rating higher than 60 dB (Rw > 60)
• UV tracer to verify the correct product was used
• Low toxicity, low VOC, solvent-free and isocyanate-free
• Paintable with both oil-based and water-based paints
• Single-component acrylic — no mixing

The UV tracer is the feature that is easiest to underrate. On a fire-rated job the seal disappears behind linings and finishes, and there is normally no way to confirm afterwards that the sealant in a given joint is the rated product. A tracer lets a certifier check under UV light rather than take it on trust.

What the acoustic opinion says

Selleys publishes an acoustic opinion for Fireblock AC prepared by Acoustic Logic Pty Ltd, dated 27 March 2024. Its conclusions are worth reading before you specify a depth:

• The cured density is (theoretically) sufficient for a 13mm depth of sealant to seal panels having a surface density of up to approximately 40 kg/m² without measurably reducing the acoustic performance of the panel.
• Given a typical plasterboard sheet is around 10 kg/m² — depending on whether it is standard or fire-rated, and on thickness — Fireblock AC would be suitable for use in systems lined with up to 3–4 sheets of plasterboard, and in systems exceeding Rw 60.
• Heavier panels can be treated to a higher rating by increasing the depth or sealing both sides.

That last point is the practical lever. If your wall is heavier than the sealant's 13mm depth will match, the fix is more sealant depth or a bead on each face — not a different assumption about the same detail.

The opinion adds an important qualifier: 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.

Where to use it

• Internal perimeter and penetration seals in fire-rated construction
• Non-trafficable control joints in concrete, masonry, brick and plasterboard constructions

For a polyurethane modified acrylic with higher movement capability, see Selleys Fireblock PA; for an SMP-based option, see Selleys Fireblock XT. All three are in the fire rated sealant range.

Joint dimensions

Geometry is part of the rating, not a detail left to the installer's judgement:

• Joint width should be between 10mm and 40mm for rated performance.
• Fit a backing rod for joints wider than 20mm.
• Maintain a minimum joint depth of 10mm, or half the width for wider joints.

The manufacturer states plainly that the joint must be within the specified width and depth ratios to perform correctly. A joint outside those dimensions is not a lesser version of the tested detail — it is an untested one.

How to apply it

1. Prepare the joint — ensure surfaces are clean and sound.
2. Fit a backing rod for joints wider than 20mm, and maintain the minimum joint depth of 10mm, or half the width for wider joints.
3. Cut the nozzle and extrude the sealant, ensuring all substrate sides are wetted with sealant. Wetting out both faces is what makes the seal continuous; a bead that only touches one side has a path straight through it.
4. Allow the sealant to form a thick skin before painting.
5. Use a flexible paint, and allow full cure before applying flat or ceiling paint to avoid cracking.

That painting sequence is the step most often rushed. Flat and ceiling paints are relatively brittle films; applied over a sealant that is still shrinking as it cures, they crack along the joint line and the finish has to be redone.

Limitations — where it should NOT be used

• Not for trafficable joints.
• The joint must be within the specified width and depth ratios to perform correctly. Outside 10–40mm width, the rated performance does not apply.
• It is specified for internal perimeter and penetration seals.

Hazard classification

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

• Sensitisation — skin — Category 1. H317: may cause an allergic skin reaction.

That is the only classification it carries. It is not classified as Dangerous Goods by the criteria of the Australian Code for the Transport of Dangerous Goods by Road & Rail or New Zealand NZS5433.

What is in it

Chemical entity	CAS number	Proportion (w/w)
1,2-Benzisothiazol-3(2H)-one (BIT)	2634-33-5	<0.05%
2-Methyl-2H-isothiazol-3-one (MIT)	2682-20-4	<0.05%
Non-hazardous ingredients	—	Balance

The whole hazard profile of this product rests on two preservatives present at less than one part in two thousand each. Everything else in the formulation is non-hazardous. BIT and MIT are isothiazolinones, the biocide class routinely used to stop a water-based product spoiling in the tube, and isothiazolinones as a chemical class are the component group most commonly behind skin-sensitisation classifications in water-based sealants and paints. The safety data sheet does not attribute H317 to a named ingredient, so read that as the obvious chemical explanation rather than a sourced one.

Sensitisation is worth understanding precisely because the percentages look trivial. It is not irritation, and it does not work on a dose-response you can feel. A sensitiser can teach the immune system to react; once that has happened it is generally permanent, and thereafter exposures far smaller than the one that caused it will set it off. It develops through repeated casual contact rather than a single incident — and fire sealing is repetitive work by nature. A fire stopper on a floor of service penetrations does not do one bead, they do three hundred, cartridge after cartridge, and water clean-up makes it easy to wipe a joint with a bare thumb because the product comes off under a tap.

Worth comparing across the range while you are choosing. Fireblock PA carries the identical classification — Warning, skin sensitisation Category 1, H317 — from the same isothiazolinone preservative group. So the choice between AC and PA is a decision about technology, movement capability and where the joint is, not a decision about exposure: the gloves are the same either way.

Protection

• Gloves — nitrile rubber is suitable for intermittent contact. This is the control that matters for an H317 product.
• Eye protection — safety glasses, and worth insisting on for overhead penetration seals.
• Ventilation — natural ventilation is adequate under normal use conditions.
• Avoid eye contact, skin contact and inhalation of dust.
• Safety shoes and overalls on site; wash hands before eating, drinking or smoking, and wash contaminated clothing before reuse.

First aid

Exposure	What to do
Eyes	Wash out immediately with water. Seek medical advice in all cases of eye contamination, as a precaution.
Skin	Effects may be delayed. Remove contaminated clothing and flush skin and hair with running water. If swelling, redness, blistering or irritation develops, seek medical assistance immediately.
Inhalation	Remove the person from exposure without becoming a casualty yourself. Loosen tight clothing, keep them comfortable, warm and at rest until recovered. Seek medical advice if effects persist.
Swallowed	Rinse the mouth with water. Do NOT induce vomiting. Give a glass of water. Never give anything by mouth to an unconscious person. 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.

Common questions about Selleys Fireblock AC

How long is it fire rated for?

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

What acoustic performance does it give?

Higher than 60 dB (Rw > 60). The independent acoustic opinion concludes it is suitable for use in wall systems exceeding Rw 60.

How deep does the sealant need to be for acoustic performance?

The acoustic opinion is based on a 13mm depth sealing panels of up to approximately 40 kg/m² surface density. Heavier panels need greater depth, or sealing on both sides.

Will it work in a wall lined with several sheets of plasterboard?

The opinion states it would be suitable for systems lined with up to 3–4 sheets of plasterboard, taking a typical sheet as around 10 kg/m².

What is the UV tracer for?

It lets you verify that the correct product was used in a joint after the fact, under UV light.

Can I paint over it?

Yes, with both oil-based and water-based paints. Let it form a thick skin first, use a flexible paint, and allow full cure before applying flat or ceiling paint so the finish does not crack.

What joint width does it need?

Between 10mm and 40mm for rated performance.

When do I need a backing rod?

For joints wider than 20mm. Maintain a minimum depth of 10mm, or half the width on wider joints.

Can I use it in a floor joint that gets walked on?

No. It is not for trafficable joints.

Does it contain solvents or isocyanates?

No. It is low toxicity and low VOC, and is solvent-free and isocyanate-free.

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 to ensure compliance with certification requirements. Where a guarantee of acoustic performance is required, seek specific advice from an acoustic consultant.

Is this the same product as Acryseal?

Fireblock AC was formerly known as Acryseal.