Choosing silicone sealant: why the right type matters more than the brand

Silicone sealant is easy to overlook. It is cheap relative to everything else in a project, it is applied in minutes, and most people only think about it when it fails — as mould in a shower recess, a cracked bead letting water behind cladding, or a fire inspector flagging an uncertified penetration seal. The sealant itself is rarely "wrong" in the sense of being faulty. It is usually the wrong product for the job it was asked to do. Silicone is a family of products with genuinely different chemistry, movement capability and certification, and the differences matter.

It is not one product — it is a chemistry choice

The first split is the cure type, and it decides what the sealant can safely touch and where it can be used.

• Acetoxy (acetic) cure sets quickly and stays clear, which is why it is the default in pre-finished bathroom and kitchen cartridges. It releases acetic acid as it cures — the vinegar smell — which is mildly corrosive and will attack natural stone, concrete, and copper or brass fittings over time. It is not suitable for structural glazing, submerged joints, or external movement joints exposed to sustained UV and weathering.
• Neutral cure (alkoxy or oxime) has a wider adhesion range, does not corrode metals, masonry or stone, and holds up better outdoors. It costs more and takes longer to skin over, which is the only reason acetoxy still dominates the cheap end of the market.

Picking the wrong one is the single most common sealant mistake on a renovation: acetoxy bathroom sealant used on a stone benchtop or a window reveal will eventually discolour the stone or corrode the frame fixings it sits against (neutral vs acetic silicone comparison, AusCaulking).

Matching the sealant to the application

Wet areas (bathrooms, kitchens, laundries). Needs a sanitary-grade, mould-resistant neutral or acetoxy sealant rated for continuous moisture. This sits alongside — not instead of — the waterproofing membrane under the tiles; see our piece on wet area waterproofing for where the membrane's job ends and the sealant's begins.

External movement joints and cladding. Exterior joints move with thermal expansion, building settlement and wind load, so the sealant needs a published movement capability (how far it can stretch and compress without tearing), strong UV and weathering resistance, and good adhesion to the specific substrates either side of the joint — masonry, render, metal cladding or glazing. A neutral-cure weatherproofing grade is the standard choice; acetoxy is not suitable outdoors long-term.

Structural glazing and curtain walls. Where silicone is doing structural work — holding glass to its frame rather than just sealing the gap — it is governed by AS 1288:2021 Glass in buildings — Selection and installation. Appendix F sets out the design method for structural silicone joints, sizing the bead against wind load and the sealant's own tested tensile strength (Viridian's breakdown of AS 1288 for glaziers). This is not a product a glazier should be substituting on site — it has to be the specific system the structural calculation was based on, installed to the manufacturer's cure schedule.

Fire-rated wall and floor penetrations. Where a pipe, cable or duct passes through a fire-rated wall or floor, the sealant is part of a fire-stopping system and has to be tested and certified as a complete assembly to AS 1530.4, installed in accordance with AS 4072.1. The fire resistance level (FRL) of the penetration seal has to match or exceed the FRL of the wall or floor it passes through — a 90-minute-rated wall with a 60-minute seal fails compliance at every one of those penetrations, however good the rest of the installation is. The National Construction Code's Specification for penetrations sets out the installation requirements this sits under (Section C — Fire Resistance, NCC 2022).

Natural stone benchtops and cladding. Always neutral cure. Acetic acid will etch and stain polished stone over time, which is why acetoxy sealant around a stone benchtop is a common and entirely avoidable defect.

Why this matters beyond the joint itself

A sealant failure is rarely just a sealant problem:

• A failed bathroom seal lets water track into the substrate behind the tiles — the same pathway covered in our damp and mould article.
• A failed external joint is one of the common entry points covered in our building envelope leak points piece.
• An uncertified fire-stopping product is a compliance failure that can surface years later at a fire safety audit or a strata insurance review, well after the installer has moved on.

Questions worth asking before sealant goes in

1. Is this joint doing a structural job (glazing), a weatherproofing job (external movement joint), a fire job (penetration) or a wet-area job — and is the product rated for that specific use?
2. Is it acetoxy or neutral cure, and is that compatible with every substrate either side of the joint?
3. For structural glazing or fire-rated penetrations, is there a test certificate or compliance declaration for the exact system installed, not just the brand name?
4. What is the sealant's published movement capability, and does it suit the joint width and expected movement?
5. What is the expected service life, and when should it be inspected or replaced?

How we approach it

We specify sealant by application, not by habit — checking the manufacturer's data sheet against the joint it is going into, and keeping the compliance documentation for anything structural or fire-rated on file. See our Quality & Building Compliance policy for how this fits into our broader inspection process.

If you are scoping a renovation, a new build or ongoing building maintenance and want to talk through the details that are easy to get wrong, get in touch.

Sources

• AS 1288:2021 — Glass in buildings: Selection and installation, Standards Australia
• Understanding AS 1288: A practical breakdown for glaziers, Viridian Glass
• Neutral cure vs acetic silicone: key differences and best uses, AusCaulking
• NCC 2022, Section C — Fire Resistance, Australian Building Codes Board

This article is general information, not advice for your project. Sealant selection depends on the specific substrates, joint design, exposure and fire rating involved, so confirm the correct product and system with your supplier, glazier or fire engineer. Full website disclaimer