Understanding Sealant Movement Capability: Class 25 vs Class 50
Sealant movement capability is the single most overlooked specification in curtain wall design — and the most expensive to get wrong. A sealant that cannot accommodate joint movement will fail by adhesion loss or cohesive tear, usually within the first seasonal cycle. This guide explains what Class 25 and Class 50 really mean, and how to specify the right grade for your project.
What Is Movement Capability?
Movement capability is the percentage of the joint width a cured sealant can stretch or compress without failure. A Class 25 sealant accommodates ±25% of the joint width; a Class 50 sealant handles ±50%. The number is tested under controlled conditions, not claimed by the manufacturer — it is verified against standards such as GB 16776 for structural grades and ISO 11600 for building sealants.
Class 25 vs Class 50: What the Numbers Mean
Class 25 covers most standard applications: insulated glass units, curtain wall weatherseals and window perimeter joints where thermal movement is moderate. Class 50 is reserved for joints with high movement demand — large aluminium composite panels, precast concrete panels and structural glazing in climates with wide temperature swings.
The practical difference: a 10 mm joint sealed with a Class 25 product tolerates roughly 2.5 mm of movement; the same joint with Class 50 tolerates 5 mm. Choose based on the substrate, panel size and local climate — not on price.
How to Choose the Right Class
Three questions decide the class: What is the maximum panel dimension? What is the expected thermal range on site? Is the joint structural or weathersealing? For structural glazing, always verify the grade against GB 16776 or the local equivalent — a Class 50 rating on paper means nothing without a batch certificate (COA) and matching test data.
Ask Your Supplier for the Test Data
Every SIKEN structural and weatherproof grade ships with movement class verified by third-party testing, plus a COA on every batch. If your supplier cannot show you the movement test report, treat the class number as marketing copy.
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