$$ \textRequired Class \ge \frac\textTotal Anticipated Movement\textInstalled Joint Width \times 100 $$
The "Class" designation in ASTM C920 refers to the expressed as a number.
Marcus called the structural engineer, Dr. Patricia Okonkwo. “Pat, why Class 50 on the whole building? The north face sees half the movement.”
“What’s the spec?” Marcus asked, pulling his collar against a sudden gust. astm c920 class 25 vs class 50
This represents a "high-movement" tier. It can handle a ±50% change in joint width. That same 1-inch joint can now safely range from 1.5 inches to 0.5 inches. Head-to-Head Comparison Expansion/Compression ±25% of joint width ±50% of joint width Typical Chemistries Polyurethanes, standard Silicones Premium Silicones, MS Polymers Best For Residential windows, perimeters, concrete control joints Metal curtain walls, glazing, extreme temperature zones Failure Risk High failure risk in high-movement commercial joints Excessive for simple interior or low-movement trim Why It Matters: Stress vs. Success
But for the tower that had to survive Seattle’s wind and sun? Only Class 50 would do.
ASTM C920 is the industry standard for elastomeric joint sealants used in building construction. The classification numbers (25 and 50) represent the sealant's ability to accommodate movement. The primary distinction is that Selecting the incorrect class leads to adhesive or cohesive failure, resulting in air/water infiltration and subsequent substrate damage. “Pat, why Class 50 on the whole building
To decide between the two, you must calculate the of your joint.
A sealant qualifying under this specification shall be classified as to type, grade, class and use as follows: type S - a single-c... ASTM International Choosing A Construction Sealant Technology - OSI Tough 2. Joint Dimensions * Class 12.5: 12.5% Expansion or Contraction. * Class 25: 25% Expansion or Contraction. * Class 35: 35% Expans... OSI Tough ADHESIVES AND SEALANTS - Architectural Metal fabrication POLYURETHANE SEALANT Chem-Calk 2020 is a one-component, moisture curable advanced hybrid seal- ant that offers superior adhesion t... Division 7, Inc. Guide Specification-Joint Sealants(Long Form)-Section 07 92 00 Color: White. Specifier: DOWSILTM 999A Silicone Building & Glazing Sealant is a one-part, weather-resistant silicone sealant formu... BuildSite A New Era of Exterior Sealants Nov 10, 2017 —
Curtain wall systems, metal panel siding, high-rise glazing, and joints between dissimilar materials (e.g., aluminum to glass). It can handle a ±50% change in joint width
Class 50 sealants are high-performance materials designed for dynamic, high-movement joints. They possess superior "elastic memory," meaning they return to their original shape after being stretched or compressed without permanent deformation.
Marcus Chen, a senior project manager for a high-rise in downtown Seattle, stood on the windswept 30th-floor balcony. 400 feet below, traffic crawled along Elliott Avenue. Above him, the new aluminum curtain wall gleamed—thousands of panels designed to withstand the Pacific Northwest’s mood swings: freezing rain, summer heat, and the perpetual damp.