ASTM C920 Class 25 vs 50: Which Sealant Class You Need

Overview

You have a spec callout that reads something like "ASTM C920, Type S, Grade NS, Class 25, Use NT," or you have a joint that moves and you need to know which sealant will hold up. The Class number is the part that decides whether the sealant survives or tears. Class is the movement rating: it tells you how far the joint can open and close, as a percentage of its width, before the sealant fails.

ASTM C920 rates movement in five classes: Class 12.5 (±12.5%), Class 25 (±25%), Class 35 (±35%), Class 50 (±50%), and Class 100/50 (+100% extension, −50% compression). The two you will see most often on real building jobs are Class 25 and Class 50, which is why most of the confusion lands there. This guide walks the full ladder, settles the Class 25 vs Class 50 question, and shows you how to match a class to your joint and joint type.

Class is one of four parts in the C920 designation (Type, Grade, Class, Use). For the full classification system, the Type/Grade/Use definitions, and the testing requirements behind the rating, see the ASTM C920 standard reference.

Quick Reference: The C920 Movement-Class Ladder

Higher class numbers accommodate more movement. The percentage is measured against the joint width, so a Class 25 sealant in a 1/2-inch joint can handle about ±1/8 inch of movement (±25% of a 1/2-inch joint), a 1/4-inch total range.

Class	Movement capability	Typical Joints
12.5	±12.5%	Interior trim, molding, low-movement joints
25	±25%	Most expansion joints, window and door perimeters, precast panel joints
35	±35%	Intermediate-movement joints, special applications
50	±50%	High-movement joints, curtain wall, glazing, wide metal-panel joints
100/50	+100% / −50% (asymmetric)	Very high, asymmetric movement: joints that open far more than they close

The class is a percentage, not a fixed dimension. The same Class 25 sealant handles more total movement in a wide joint than in a narrow one, because 25% of a wider joint is a larger number. Always design the joint width and the class together.

Class 25 vs Class 50, Head to Head

Class 25 and Class 50 cover the bulk of commercial building-envelope work, and picking between them is where most spec questions come up. The short version: Class 25 handles ±25% movement and Class 50 handles ±50%, so a Class 50 sealant accommodates twice the joint movement.

	Class 25	Class 50
Movement	±25% of joint width	±50% of joint width
Common chemistry	Polyurethane, silicone	Usually silicone; Class 50 polyurethanes also exist
Typical use	Expansion joints, window and door perimeters, precast and panel joints	Curtain wall, glazing, wide metal-panel joints, high-movement facades
Cost	Lower; widest product selection	Higher; usually a silicone formulation

Can you use a Class 50 sealant where Class 25 is specified? Movement-wise, yes: a higher class meets a lower class's movement requirement, so Class 50 covers a Class 25 spec. But the substitution still has to match the rest of the designation, the Use code (T, NT, I, G, A, and so on), the Grade (NS for vertical and overhead joints, P for self-leveling horizontal joints), the chemistry, paintability, and the color, and on a spec'd job it should get the architect's approval. A Class 50 silicone is non-paintable, for example, so it is the wrong substitute on a joint that has to be painted. Class 50 products also cost more, so substituting up the ladder is fine for performance but is not free.

The reverse invites failure. Once the joint exceeds the sealant's rated movement the sealant is prone to cohesive tearing or adhesive pull-away. When the calculated movement is near a class boundary, specify up, not down.

Which Class Do I Need?

The class you need comes from the joint, not from habit. Movement depends on the substrate's thermal expansion, the temperature swing the joint sees, and the joint width. A wide joint on a metal panel in a climate with large temperature swings moves far more than a narrow joint on masonry in a mild climate, and the two need different classes even though both are "expansion joints."

The basic logic:

Estimate total joint movement from substrate, temperature range, and joint width.
Divide expected movement by joint width to get a movement percentage.
Apply a safety factor by sizing up: pick a class whose rated movement is about 1.25 to 1.5 times the calculated joint movement, so the sealant is not run at its limit (ASTM C1193 covers the joint-design method).
Pick the lowest class that still covers the result, then round up if you land near a class boundary.

The full calculation, the thermal-coefficient values, the C1193 joint-design method, and the joint width-to-depth design rules live in the Joint Sealant Selection Guide. It walks the C920 Type/Grade/Class/Use designation step by step, including the movement worksheet, so use it when you need to put a number on the joint rather than estimate.

By Joint Type

A quick read on the class most commonly specified for each joint type, with US-made sealants that meet the C920 movement rating. Confirm the calculated movement for your own joint before ordering: these are the typical starting points, not a substitute for the calculation above.

Expansion joints

Most building expansion joints are designed for Class 25 (±25%) movement, handled well by polyurethane or silicone. Wide joints, large temperature swings, or metal-to-metal joints push the requirement toward Class 50 silicone. Grade NS for vertical joints; Grade P (self-leveling) for horizontal joints.

Precast and panel joints

Precast concrete and architectural-panel joints are typically Class 25, sometimes Class 50 on tall or wide panels with large thermal movement. Match the Use code to the substrate (Use M for masonry and concrete, Use A for aluminum framing) and use a non-sag (Grade NS) sealant for vertical joints.

Control joints

Control joints in concrete and masonry generally see modest movement and are usually specified Class 25. Interior, low-movement trim and control joints can drop to Class 12.5. For self-leveling control joints in horizontal slabs, use a Grade P pourable sealant; for vertical control joints, use Grade NS.

Window, curtain wall, and glazing perimeters

Curtain wall and glazing perimeters are high-movement joints, usually specified Class 50 silicone (Use G for glass, Use A for aluminum framing). Standard window and door perimeters in lower-movement walls are commonly Class 25. Silicone is the typical chemistry here because it holds up under UV and large cyclic movement. Note that load-bearing structural sealant glazing, where the sealant holds the glass in place, is a separate scope governed by ASTM C1184 and C1401, not C920 weatherseal.

Compliant Products

US-made elastomeric joint sealants tested to ASTM C920, grouped by the property that drives your class decision: chemistry and food-grade certification. We stock these from American manufacturers, so they carry the same Made-in-USA sourcing as the rest of the catalog.

Specifying C920 sealant by the case or pallet?

Tell us the spec (Type, Grade, Class, Use) and order size and we'll quote case and pallet pricing on US-made C920 sealants. Typical turnaround: one business day.

or call 714-248-6555 · email partners@usmadesupply.com

Frequently Asked Questions

What is ASTM C920 Class 25?

Class 25 is the ASTM C920 movement rating for a sealant that accommodates ±25% of the joint width in extension and compression. It is the most common class for building expansion joints, window and door perimeters, and precast panel joints, and it is available in both polyurethane and silicone chemistries.

What is ASTM C920 Class 50?

Class 50 is the ASTM C920 movement rating for a sealant that accommodates ±50% of the joint width. It is specified for high-movement joints such as curtain wall and glazing perimeters and wide metal-panel joints, and it is usually a silicone formulation because silicone holds up under large cyclic movement and UV exposure, though Class 50 polyurethanes also exist. Load-bearing structural sealant glazing is a separate scope (ASTM C1184 / C1401), not C920 weatherseal.

What is the difference between Class 25 and Class 50 sealant?

The difference is movement capability. Class 25 handles ±25% of the joint width; Class 50 handles ±50%, or twice the movement. Class 25 covers most expansion and perimeter joints and comes in polyurethane or silicone. Class 50 is for high-movement facades and glazing and is usually silicone, though Class 50 polyurethanes also exist. Class 50 products cost more.

Can I use a Class 50 sealant where Class 25 is specified?

Movement-wise, yes: a higher class meets a lower class's movement requirement, so Class 50 covers a Class 25 spec. But the substitution still has to match the rest of the designation (Use, Grade, chemistry, paintability, color) and, on a spec'd job, get the architect's approval. A Class 50 silicone is non-paintable, so it is the wrong substitute on a joint that must be painted. Class 50 products also cost more than Class 25.

How do I know what movement class I need?

Calculate the joint movement from the substrate's thermal expansion, the temperature range the joint sees, and the joint width, then express that movement as a percentage of the joint width. Apply a safety factor by sizing up: pick a class whose rated movement is about 1.25 to 1.5 times the calculated joint movement, so the sealant is not run at its limit (ASTM C1193 covers the joint-design method). Pick the lowest class that still covers the result, and round up if you land near a class boundary. The Joint Sealant Selection Guide has the full worksheet with thermal-coefficient values.

What does Type S vs Type M and Grade NS vs Grade P mean?

Type describes components: Type S is single-component (ready to use from the container) and Type M is multi-component (mixed before use). Grade describes consistency: Grade NS is non-sag, the gun-grade sealant that stays put on vertical and overhead joints, and Grade P is pourable, a self-leveling sealant for horizontal joints such as floor and plaza-deck joints. Movement class is independent of Type and Grade, so confirm all four parts of the designation.

What is Class 100/50?

Class 100/50 is the asymmetric ASTM C920 movement rating: +100% extension and −50% compression. It is for very high-movement joints that open far more than they close, such as high-rise facade movement joints and curtainwall stack joints where thermal and structural movement is not symmetric. C920-18(2024) does not include a standalone "Class 100"; the correct designation is Class 100/50.