How to Read Sealant Movement Capability Grades (25LM, 25HM and ISO 11600 Classes Explained)

Quick answer: A sealant's movement capability is how much joint movement the cured sealant can repeatedly handle, expressed as a percentage of the joint width. In the grade codes you see on datasheets — like 25HM or 20LM — the number is the movement class (±25%, ±20%, ±12.5%) and the letters are the modulus (LM = low modulus, softer; HM = high modulus, firmer). This convention is shared by the international standard ISO 11600 and China's national standard GB/T 14683, so a grade like 25HM means the same thing in both. Below we decode a real third-party-tested grade as a worked example.

What does "movement capability" actually mean?

Building joints are never static. Thermal expansion, structural settlement and wind load make joints widen and narrow continuously over their life. Movement capability is the cured sealant's ability to stretch and compress through those cycles without splitting or losing adhesion.

It is expressed as a percentage of the original joint width. A ±25% sealant in a 10 mm joint can accommodate the joint opening to 12.5 mm and closing to 7.5 mm, repeatedly, without failing. The higher the class, the more movement the joint can take — which is why high-rise façades and long curtain-wall spans demand higher classes than a small internal joint.

What do the numbers 25, 20 and 12.5 mean?

They are the movement classes, defined by the percentage of movement the sealant can repeatedly accommodate:

• Class 25 → ±25% movement capability
• Class 20 → ±20% movement capability
• Class 12.5 → ±12.5% movement capability (further split into 12.5E elastic and 12.5P plastic in ISO 11600)
• Class 7.5 → ±7.5% (lower-movement applications)

For demanding exterior work — curtain walls, expansion joints, large panels — class 25 is the common specification. Lower classes suit joints that see little movement.

What is the difference between LM (low modulus) and HM (high modulus)?

Modulus describes how much stress the sealant transfers to the substrate as it moves:

• LM (low modulus) — softer and more flexible. It deforms easily, placing less stress on the substrate. This is preferred for weatherproofing joints, weak or brittle substrates (natural stone, render), and situations where protecting the bond line matters more than firmness.
• HM (high modulus) — firmer and stronger. It resists deformation and transfers more load, suited to applications wanting a more rigid joint, such as certain glazing work on sound substrates.

Neither is "better" — they are matched to the job. A low-modulus sealant on a fragile substrate and a high-modulus sealant in a firm glazing joint are each the correct choice for their context.

How do ISO 11600 and China's GB/T 14683 compare?

For an overseas buyer, this is the practical question — and the answer is reassuring: the two standards use the same movement-class and modulus language.

• ISO 11600 classifies construction sealants by type (F for façade joints, G for glazing) and by class (25LM, 25HM, 20LM, 20HM, 12.5E, etc.).
• GB/T 14683-2017 ("Silicone and modified silicone sealants for building") classifies silicone (SR) and modified-silicone (MS) sealants using the same movement-class + modulus suffixes — 25LM, 25HM, 20LM, 20HM.

So a product tested to GB/T 14683 grade 25HM carries the same core meaning a buyer expects from ISO 11600 class 25HM: ±25% movement capability, high modulus. A Chinese national-standard test report from an accredited laboratory is therefore directly readable by an international specifier.

Worked example: decoding a real grade, SR-Ⅰ-Gn-25HM

Here is how one of our weatherproofing silicones is classified under GB/T 14683-2017, character by character:

In plain terms: a neutral-cure silicone weatherproofing sealant with ±25% movement capability and high modulus — verified by independent third-party testing.

What the third-party test report shows

The grade above was tested by an accredited third-party laboratory (China Testing & Certification Group / CTC) bearing CMA, CNAS and ilac-MRA accreditation, against GB/T 14683-2017:

All tested items met the standard's requirements for the SR-Ⅰ-Gn-25HM classification. (The full report carries a unique report number and a scannable online verification code; these are provided to buyers on request so the report can be independently verified at source.)

A note on reading results: the tested values sit within the standard's limits — for example tack-free time and sag are well inside the thresholds, and extrudability above the minimum indicates good gunnability during application. The ±25% movement class itself is established by the standard's movement test that underpins the 25HM grading.

Why does accredited third-party testing matter to an overseas buyer?

Because it converts a supplier's claim into an independently verifiable fact:

• CMA — the testing body is officially qualified to issue valid test reports.
• CNAS — the laboratory is accredited to recognised competence standards.
• ilac-MRA — that accreditation is internationally mutually recognised, so the report is accepted across participating economies, not only domestically.

For a buyer importing from China, a report carrying all three marks — and naming the exact grade you are purchasing — is far stronger evidence than a glossy datasheet alone. (See our sourcing guide on how to verify a sealant manufacturer.)

How do I choose the right movement class for my joint?

A practical sequence:

1. Estimate the joint's movement — driven by joint width, substrate thermal expansion, and exposure. High-rise façades and long spans move more.
2. Match the movement class — exterior curtain-wall and expansion joints typically call for class 25; smaller, sheltered joints can use lower classes.
3. Choose modulus by substrate — low modulus to protect weak or brittle substrates (stone, render); high modulus where a firmer joint on sound substrates is wanted.
4. Confirm with a third-party report for the exact grade, and run your own adhesion test on your actual substrates.

FAQ

Q: What does ±25% movement capability mean in practice? A: The cured sealant can repeatedly accommodate the joint widening and narrowing by 25% of its original width without failing. In a 20 mm joint, that is movement between roughly 15 mm and 25 mm, cycle after cycle.

Q: Is a higher movement class always better? A: Not necessarily — it should match the joint. Over-specifying adds cost without benefit on a low-movement joint, while under-specifying risks failure on a high-movement façade. Match the class to the joint's actual movement.

Q: What's the difference between low modulus and high modulus sealant? A: Low modulus is softer and puts less stress on the substrate as it moves, which protects weak or brittle materials like stone. High modulus is firmer and transfers more load, suited to firmer joints on sound substrates. Choose by substrate and application, not by assuming one is superior.

Q: My supplier's report is to GB/T 14683, not ISO 11600 — is that a problem? A: Not for reading the grade. Both standards use the same movement-class and modulus convention, so a GB/T 14683 "25HM" conveys the same ±25%, high-modulus meaning as ISO 11600 "25HM". An accredited report (CMA/CNAS/ilac-MRA) is internationally recognised. Confirm the specific certification your project or market formally requires.

Q: How do I verify a test report is genuine? A: Accredited Chinese reports carry a report number and often a scannable verification code that checks the result against the issuing laboratory's records. A credible supplier will provide the full, verifiable report for the exact grade you are buying.

Q: Does the grade tell me everything about the sealant? A: No. The grade tells you movement class, modulus and type, but not cure chemistry details, colour, substrate-specific suitability or certification for a particular market. Read the grade alongside the TDS, SDS and substrate compatibility information.

This guide explains standard grade conventions for general reference. Movement requirements vary by project; confirm the appropriate class and the certification your market requires, and conduct an adhesion test on your actual substrates before full application. For project-specific recommendations, contact our technical support team.