Sealant failures are almost never a product quality problem. They are a specification problem. The wrong chemistry in the right joint, or the right chemistry in a poorly prepared joint, produces the same result: adhesion loss, cohesive tearing or hardening that cracks under thermal cycling. Getting it right starts before the cartridge is opened.
Movement Accommodation Factor
Every joint sealant has a movement accommodation factor (MAF), expressed as a percentage of the joint width. A sealant rated ±25% MAF installed in a 20 mm joint can tolerate 5 mm of movement in either direction, for a total movement range of 10 mm. If the joint moves more than that, the sealant will fail, regardless of how well it was applied.
AS 4296 covers the testing of joint sealants for building construction, and MAF ratings in manufacturer datasheets should be read against that standard or an equivalent. Common ratings are ±12.5%, ±20%, ±25% and ±35%. Higher MAF products are not automatically better; they are softer and may not suit trafficked joints or joints subject to compression loading.
Calculating Joint Width from Expected Movement
The formula is straightforward. Measure or calculate the total expected movement, divide by the MAF expressed as a decimal, and that gives you the minimum joint width.
For example: a facade panel with a thermal movement of 6 mm total, sealed with a ±25% MAF sealant:
- Minimum joint width = 6 mm ÷ 0.25 = 24 mm
In practice, designers add a construction tolerance allowance on top of that. A joint designed for 24 mm minimum is typically specified at 25 to 30 mm to account for panel placement variation. If the joint ends up narrower than designed because of installation error, the sealant is already over-stressed from day one.
Thermal movement itself is calculated from the coefficient of thermal expansion of the substrate, the panel length and the temperature differential. For aluminium composite panels in Queensland, a 3 m panel can move 3 to 4 mm over a 60°C temperature swing. Concrete panels move less per metre but are often much larger, so total movement can be comparable.
The 2:1 Width-to-Depth Rule
Joint geometry controls how the sealant deforms under movement. A sealant that is too deep relative to its width will try to resist movement rather than accommodate it, because the stress concentrates at the bond faces instead of distributing through the body of the sealant.
The standard ratio is 2:1, width to depth. A 20 mm wide joint should have a sealant depth of 10 mm. For joints wider than 20 mm, a maximum depth of 12 to 15 mm is generally recommended; going deeper adds cost and increases the risk of tooling defects without improving performance.
For very narrow joints under 10 mm, a 1:1 ratio is sometimes acceptable, but check the manufacturer's datasheet. The geometry recommendation exists because sealants are designed to deform in an hourglass shape under tension. If the depth is too great, the sealant cannot form that shape and cohesive failure becomes likely.
Backer Rod and Bond Breaker Tape
Three-sided adhesion is one of the most common causes of sealant failure and one of the most preventable. When a sealant bonds to the back of the joint as well as both faces, it cannot deform freely. Movement that should stretch the sealant in an hourglass profile instead tears it at the bond line or causes cohesive splitting through the centre.
Backer rod, typically closed-cell polyethylene foam, serves two purposes. It controls sealant depth to the 2:1 ratio, and it provides a surface the sealant will not bond to. The rod diameter should be 25% larger than the joint width so it sits under compression and does not fall out during application.
Where a joint has a solid backing that cannot accept a backer rod, bond breaker tape is applied to the back surface before sealing. The tape prevents adhesion to the substrate behind the sealant without affecting the depth control. Do not use masking tape as a substitute; it can leave residue that contaminates the bond face.
Neither backer rod nor bond breaker tape is optional. They are part of the sealant system.
Chemistry Comparison
Polyurethane
Polyurethane sealants are the most widely used general-purpose option in Australian construction. They offer MAF ratings typically in the ±20% to ±25% range, good adhesion to concrete, masonry, fibre cement and timber, and they accept paint once cured. Cure time is moisture-dependent; in humid conditions they cure faster, in dry inland climates more slowly.
Polyurethanes are trafficable once fully cured and are available in formulations rated for pedestrian and light vehicular traffic in expansion joints. UV stability is moderate. Exposed polyurethane will chalk and degrade on the surface over years, which affects appearance but not necessarily adhesion or function in the short term. For permanently exposed facade joints, a UV-stable topcoat or a UV-resistant formulation is worth specifying.
Polyurethane is not suitable for joints in contact with bituminous materials or some plasticised substrates, as plasticiser migration can soften the cured sealant.
Polysulphide
Polysulphide sealants have been used in construction since the 1950s and remain the standard for glazing rebates, insulating glass unit edge seals and fuel-resistant applications. They offer good chemical resistance, particularly to fuels, oils and water immersion, and they perform well in permanently wet environments such as water-retaining structures.
MAF ratings are generally ±12.5% to ±20%, lower than polyurethane or silicone. Polysulphide is paintable and bonds well to glass, aluminium and concrete with appropriate primer. It is slower to cure than polyurethane and has a characteristic odour during application that requires adequate ventilation.
Polysulphide is not the first choice for high-movement facade joints. Its strength is chemical resistance and compatibility with insulating glass, not movement capacity. Specifying it for a wide facade panel joint to save cost over silicone is a common mismatch.
Silicone
Silicone sealants offer the highest MAF ratings available in standard construction products, typically ±25% to ±50% depending on the formulation. They are UV stable without any topcoat, remain flexible at low temperatures and do not harden with age in the way polyurethane can. For exposed facade joints, curtain wall perimeter seals and structural glazing, silicone is the default chemistry.
The drawbacks are well known. Silicone cannot be painted. Once cured, it repels paint and any coating applied over it will peel. It also cannot be oversealed with most other sealant chemistries without surface preparation or a compatible primer. If a silicone joint fails and needs remediation, the old silicone must be removed completely before resealing.
Silicone also has poor adhesion to porous substrates without primer, and some formulations contain acetic acid cure systems (the vinegar smell) that can corrode certain metals. For aluminium and steel substrates, a neutral-cure silicone is the correct choice.
Hybrid Chemistries
Silyl-terminated polyether (STPE) and silyl-terminated polyurethane (STPU) hybrids have grown in specification over the past decade. They combine silicone-like UV stability and flexibility with polyurethane-like paintability and adhesion to porous substrates. MAF ratings of ±25% are common, and some products reach ±35%.
Hybrids are not a universal solution. They cost more than standard polyurethane, and compatibility with existing sealants in remedial work needs to be verified. Where the specification requires a paintable sealant on an exposed facade joint with high movement, a hybrid is often the most practical answer.
Primer Requirements on Porous Substrates
Concrete, masonry, fibre cement and some stone substrates are porous. Without primer, sealant adhesion depends on mechanical keying into the surface, which is unreliable. Primer penetrates the substrate, consolidates the surface and provides a chemical bond layer for the sealant.
Most sealant manufacturers publish a primer compatibility chart. The primer is chemistry-specific; a polyurethane primer will not prepare a substrate correctly for silicone, and vice versa. Using the wrong primer is worse than using none, because it can create a weak interface layer.
Primer application requires a clean, dry surface. Apply with a brush or lint-free cloth, allow the specified flash-off time (typically 20 to 60 minutes depending on temperature and humidity), and seal within the open time window, usually 4 to 8 hours. Sealing over primer that has been left too long, or that has been contaminated by dust after application, negates the benefit.
Joint Face Preparation: Where Most Failures Begin
Dust, laitance, form release oil, moisture and previous sealant residue are the four most common contaminants found on joint faces at the time of sealing. Any one of them is enough to cause adhesion failure.
Joint faces should be mechanically abraded where possible, blown clean with dry compressed air and wiped with a clean solvent-dampened cloth before primer or sealant application. The solvent must be compatible with the substrate; isopropyl alcohol is safe for most surfaces and leaves no residue.
Moisture is particularly problematic with polyurethane sealants. Polyurethane reacts with surface moisture during cure, and a damp joint face can cause bubbling, pinholing or foam formation in the sealant body. Concrete that looks dry on the surface may still carry moisture from recent rain or curing water. Allow adequate drying time, and if in doubt, check with a surface moisture meter.
Sealing in direct sun on a hot day is also problematic. Substrate temperatures above 40°C accelerate cure, reduce open time for tooling and can cause outgassing from some substrates that creates voids in the sealant. Early morning application on shaded faces is preferable in summer conditions.
Fosroc Sealant Range
Fosroc manufactures a range of joint sealants covering polyurethane, polysulphide and hybrid chemistries, with products suited to facade joints, concrete construction joints, water-retaining structures and trafficked expansion joints. CSA supplies the Fosroc range and can provide datasheet-level guidance on product selection for specific joint types. The Fosroc technical data sheets specify MAF ratings, primer requirements, substrate compatibility and application temperature ranges, which are the documents to work from when specifying.
Selecting by Joint, Not by Price
The decision tree is straightforward. Start with the joint: what substrates, what movement, what exposure, what finish requirement? Movement capacity and substrate compatibility narrow the chemistry. UV exposure and paintability often make the final call. Price is a factor in volume, but specifying a ±12.5% polysulphide in a joint that moves ±20% is not a saving; it is a callback.
For product selection, datasheets and project-specific advice, visit constructionsupplies.group/au or contact the CSA trade desk with your joint specification details.