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60-Second Decoder Step 1: Movement Step 2: Chemistry Step 3: Class Match Step 4: Use Match Step 5: Joint Design Application Recipes Failure Mistakes FAQ

How to Choose a Joint Sealant: Types, Grades & Movement

Translate your joint into a Type, Grade, Class, and Use spec in five steps.

Last updated: August 19, 2026

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If you are caulking siding gaps, baseboards, trim, or general weatherproofing around the home or shop, the Sealant and Caulking Selection Guide is the better starting point.
If you are sealing a moving expansion joint in a building, parking deck, curtain wall, plaza, or pool, stay here.
If you are specifying a bridge deck expansion joint system (strip seal, modular, finger plate, asphaltic plug, compression seal), see the Bridge Expansion Joint Selection Guide.

The 60-Second Decoder: Type, Grade, Class, Use

ASTM C920 is the performance standard for elastomeric joint sealants used in building construction. Every C920 sealant carries a four-part designation that tells you what it is and what it can do. Read the four parts in order and you have a complete spec.

Part	Options	Plain English
Type	S or M	Single-component (ready to use) or Multi-component (mix before use)
Grade	P or NS	Pourable (self-leveling, horizontal only) or Non-Sag (gun grade for any orientation)
Class	12.5, 25, 35, 50, 100/50	Movement capability as percent of joint width
Use	T, NT, I, M, A, O, G	Application environment (Traffic, Non-Traffic, Immersed, Metal, Adhesion, Other, Glazing)

Worked example

Type S, Grade NS, Class 50, Use NT

Translation: a single-component, gun-grade sealant rated for plus or minus 50 percent movement, intended for non-trafficked vertical or horizontal joints. Typical for curtain walls and high-movement building envelope joints.

For the full classification breakdown with every Type, Grade, Class, and Use code defined, jump to the ASTM C920 classification table.

Step 1: Calculate Joint Movement

Movement is the single most important number in sealant selection. A joint that moves more than the sealant can stretch will tear. The basic formula is:

Movement = Coefficient of Thermal Expansion × Joint Length × Temperature Range

The result is the absolute movement at the joint. Divide by joint width to get the percent movement the sealant must accommodate. ASTM C1193 (Standard Guide for Use of Joint Sealants) recommends including a safety factor of 1.25 to 1.5 on the calculated value.

Worked example: 30 ft aluminum panel run

Aluminum coefficient of thermal expansion is 0.0000128 in/in per degree F. Surface temperature swing on a south-facing wall is 130 degrees F (from a 0 degree winter morning to 130 degree summer surface temperature). Joint width is 1/2 inch.

Movement = 0.0000128 × 360 in × 130 = 0.60 in
Movement per joint at 0.5 in width = 0.60 / 0.5 = 120 percent
Apply 1.25 safety factor: 150 percent required
Result: a Class 50 sealant alone is not enough. Either widen the joint to 1.25 in (which brings required movement under 50 percent) or specify Class 100/50 with engineered joint design.

Common coefficients of thermal expansion

Material	CTE (in/in/°F)	Movement per 100 ft at 100°F swing
Aluminum	0.0000128	1.54 in
Steel	0.0000065	0.78 in
Concrete	0.0000055	0.66 in
Brick masonry	0.0000036	0.43 in
Glass	0.0000045	0.54 in
Granite	0.0000044	0.53 in

Watch for moisture movement and creep. Concrete continues to shrink for years after placement, and brick masonry can expand over time from moisture absorption. Add 1/8 in per 10 ft of run to the thermal calculation for masonry on a new building, and consult ACI 224 for concrete.

Step 2: Choose Chemistry

Once you know the movement, the chemistry decision narrows fast. Each chemistry family has a movement ceiling, a UV story, a paintability story, and a substrate compatibility story. Match the chemistry to the application constraints, not the other way around.

Chemistry	Movement Class Ceiling	UV Resistance	Paintable	Best For
Silicone (neutral cure)	50 to 100/50	Excellent (20+ years)	No	Curtain wall, glazing, high-movement metal joints
Polyurethane	25 to 50	Good (10 to 20 years)	Yes	Concrete and masonry expansion joints, traffic decks
Polysulfide	25 to 50	Good	Yes (limited)	Fuel-resistant joints, immersed conditions
Hybrid (MS Polymer, SPUR)	25 to 50	Excellent	Yes	Mixed substrates, low-VOC requirements, fast cure
Acrylic latex	12.5 (mostly)	Fair (interior)	Yes	Interior trim, low-movement static joints

Silicone vs polyurethane: the most common decision

Silicone wins on UV exposure, high movement, and dissimilar substrate compatibility (glass, metal, glazed surfaces).
Polyurethane wins when paint must adhere over the sealant, when trafficked horizontal joints need abrasion resistance, and when concrete or masonry is the substrate.
Silicone will not bond to itself across cures unless freshly tacky. Polyurethane will not paint cleanly with most silicones.
Neutral-cure silicones are required against alkaline substrates (fresh concrete, mortar, stone) because acid-cure silicones release acetic acid that attacks alkaline materials and corrodes metals.

Substrate compatibility test. Even within a chemistry, not every formulation bonds to every substrate. Request the manufacturer's tested-substrate list and run an ASTM C794 adhesion-in-peel test or an ASTM C1521 field-applied adhesion test before committing on an unusual substrate.

Step 3: Match Class to Movement Range

Class is the percent movement the sealant tolerates without failure. Take the percent movement from Step 1, add a safety margin, and pick the next Class up. Picking down from your calculated movement guarantees a tear.

Class	Movement (Tension / Compression)	Typical Use	Required Joint Width vs Movement
12.5	±12.5%	Interior trim, static joints	Joint width ≥ 8 × movement
25	±25%	Window perimeters, light exterior joints	Joint width ≥ 4 × movement
35	±35%	Intermediate movement envelope joints	Joint width ≥ 2.86 × movement
50	±50%	Curtain wall, glazing, metal panel joints	Joint width ≥ 2 × movement
100/50	+100% / -50%	Asymmetric large-movement joints	Joint width ≥ 1 × tensile movement

A Class 100/50 sealant accommodates 100 percent extension but only 50 percent compression. It is the right choice for joints that open more than they close or that close to a hard stop, such as precast panel joints with shim contact.

Class is not a stand-in for joint width. Even a Class 100/50 sealant fails if the joint is too narrow. Per ASTM C1193, joint width should be sized so that calculated movement (with safety factor) does not exceed the sealant's rated class.

Step 4: Match Use Code to Environment

The Use code captures what the sealant has to live with day to day: vehicle traffic, full water immersion, glass-to-metal bond stresses, or none of the above. Multiple Use codes can apply to a single sealant; specify all that match the application.

Code	Meaning	Specify When
T	Traffic	Parking decks, plazas, sidewalks, any horizontal joint with vehicle or pedestrian load
NT	Non-Traffic	Wall joints, window perimeters, vertical or horizontal joints without traffic
I	Immersed	Pool joints, fountain basins, reservoirs, planters, any joint that stays wet
M	Metal	Metal panel systems, metal curtain wall, metal-to-metal joints
A	Adhesion	Difficult or unusual substrates requiring extra adhesion qualification
O	Other	Anything outside the standard categories, qualified by manufacturer test data
G	Glazing	Glass-to-frame sealing in window and curtain wall systems

Example combined Use spec: a plaza deck over occupied space might be specified as Use T (vehicle traffic) and Use I (water immersion at planters and fountains). The sealant must qualify for both, and it has to be compatible with the deck waterproofing membrane requirements for the assembly beneath it.

Step 5: Joint Design (Width, Depth, Backer Rod, Primer)

A correctly chosen sealant still fails in a poorly designed joint. ASTM C1193 sets the geometry rules. The two that matter most are the width-to-depth ratio and three-sided adhesion prevention.

Width-to-depth ratio

For joints up to 1/2 in wide: depth equals width (1:1 ratio).
For joints 1/2 in to 1 in wide: depth is 1/2 the width but at least 1/4 in (2:1 ratio).
For joints over 1 in wide: depth is 1/2 in regardless of width.
A sealant bead deeper than wide cannot stretch and tears prematurely.

Backer rod

Backer rod sets the depth, prevents three-sided adhesion, and gives the sealant the hourglass profile it needs to stretch. Use closed-cell rod for most applications, open-cell for very deep joints with no water exposure, and bi-cellular for joints where the sealant could trap moisture. Rod diameter is 25 percent larger than joint width.

Three-sided adhesion

A sealant bonded to the bottom of the joint (in addition to the two sides) cannot stretch. The sealant tears as the joint opens. Backer rod or a bond breaker tape on the joint floor is mandatory. This is the single most common design mistake on field-installed expansion joints.

Primer

Porous substrates (concrete, CMU, mortar, stone) almost always require primer.
Dense, nonporous substrates (glass, anodized aluminum, glazed tile) often do not.
Field-applied adhesion testing per ASTM C1521 is the only reliable way to confirm primer is or is not needed on an unfamiliar substrate.
Skipping primer where required is the second most common cause of sealant failure after width-to-depth violations.

Surface preparation matters as much as primer. Substrates must be clean, dry, and free of dust, oils, form release, curing compounds, and old sealant residue. SSPC and NACE surface preparation standards apply to metal substrates. For concrete, mechanical roughening or sandblast often outperforms chemical cleaning.

Common Application Recipes

Use these as starting points. Verify against project conditions, substrate test data, and any project-specific spec language.

Application	C920 Spec	Why
Window or door perimeter (residential or light commercial)	Type S, Grade NS, Class 25, Use NT	Moderate movement, vertical orientation, paintable polyurethane or hybrid
Curtain wall vertical and horizontal joints	Type S, Grade NS, Class 50, Use NT and M	High movement, mixed metal and glass substrates, neutral-cure silicone
Glazing (glass to frame structural)	Type S, Grade NS, Class 50, Use G (and ASTM C1184 for structural)	UV exposure, glass adhesion, structural glazing requires C1184 in addition to C920
Parking deck horizontal expansion joint	Type S, Grade P, Class 25, Use T	Self-leveling for horizontal pour, traffic-rated polyurethane
Plaza deck over occupied space	Type M, Grade P, Class 50, Use T and I	Multi-component for fast cure under traffic schedule, immersed-rated for ponded water
Pool or fountain coping joint	Type S, Grade NS, Class 25, Use I and NT	Continuous water immersion, chlorine resistance, neutral-cure silicone
Concrete-to-concrete vertical expansion joint	Type S, Grade NS, Class 25 to 50, Use NT	Polyurethane bonds well to concrete and accepts paint or coatings
Metal panel joint (rainscreen, ACM)	Type S, Grade NS, Class 50, Use NT and M	High movement on aluminum, UV exposure, neutral-cure silicone preserves coatings

Mistakes That Cause Sealant Failure

Most failed joints fail for one of seven reasons. Recognizing the failure mode in the field tells you what to fix on the replacement, not just what sealant to reorder.

Three-sided adhesion. Sealant bonded to the joint floor as well as the two sides cannot stretch and tears in the middle. Mandatory fix: backer rod or bond breaker tape on every replacement.
Wrong width-to-depth ratio. A bead deeper than wide cannot stretch. Per ASTM C1193, the bead should be no more than half as deep as it is wide for joints over 1/2 in.
Class undersized for movement. Class 25 sealant in a joint that moves 40 percent will tear within one or two thermal cycles. Always size up after Step 1 calculation, including a safety factor.
No primer where required. Porous substrates (concrete, CMU, mortar, stone) usually need primer. Field adhesion check per ASTM C1521 is the only way to verify.
Wrong chemistry for substrate. Acid-cure silicone on alkaline substrates (fresh concrete, masonry, stone) attacks the substrate and corrodes adjacent metals. Specify neutral-cure silicone or polyurethane.
Sealant applied over old sealant. Different chemistries are incompatible and even matched chemistries from different manufacturers may not bond. Remove all old sealant to clean substrate before reapplying.
Surface contamination. Form release, curing compounds, oils, dust, and moisture all destroy adhesion. Clean substrates per the manufacturer's data sheet and the SSPC or NACE standard for the substrate type.

Diagnose before you replace. Cohesive failure (sealant tears down the middle, both edges stuck) means movement exceeded the rated class. Adhesive failure (sealant pulls cleanly off one substrate) means primer or surface prep failed. The two failure modes need different fixes.

Need help sourcing C920 sealant for a project?

We work with US-made building product distributors across the country. Email partnerships@usmadesupply.com with the spec line (Type, Grade, Class, Use), substrate, joint dimensions, and quantity. We will confirm availability and pricing.

Frequently Asked Questions

What's the difference between Class 25 and Class 50, and when is the upgrade worth it?

Class 25 tolerates plus or minus 25 percent movement; Class 50 tolerates plus or minus 50 percent. The upgrade is worth it whenever calculated movement (with a 1.25 to 1.5 safety factor per ASTM C1193) exceeds 25 percent of joint width. That happens on long runs of aluminum, on south-facing walls in high thermal swing climates, and on most curtain wall and metal panel systems. Class 50 sealants cost more per cartridge but eliminate replacement labor over the building life.

Should I use silicone or polyurethane for an exterior expansion joint?

Use silicone (neutral-cure) when you need maximum UV resistance, the highest movement class, or you are bonding to glass or anodized metal. Use polyurethane when paint must adhere over the sealant, when the substrate is concrete or masonry, or when the joint sees vehicle or foot traffic. Polyurethane is also typically lower cost per linear foot. Both meet ASTM C920; the chemistry difference drives substrate compatibility, not standard compliance.

Can I caulk over old sealant or do I have to remove it?

Remove it. Different chemistries are incompatible, and even the same chemistry from a different manufacturer may not bond. New sealant applied over weathered or contaminated old sealant fails by adhesive separation within months. Mechanical removal followed by solvent wipe is required. Verify adhesion of the new sealant per ASTM C1521 on the cleaned substrate before completing the run.

Why does my sealant keep failing in the same joint?

Repeated failure in the same joint almost always means the joint design is wrong, not the sealant. Check three things in order: (1) is the joint wide enough that calculated movement falls within the sealant's class, (2) is the bead the right depth (no deeper than half the width for joints over 1/2 in), (3) is there bond breaker or backer rod preventing three-sided adhesion. If all three are correct, then test the sealant chemistry against the substrate per ASTM C1521.

What's a typical Type/Grade/Class/Use spec for a window perimeter?

For a residential or light commercial window perimeter, Type S, Grade NS, Class 25, Use NT covers most cases. Single-component for ease of field use, non-sag for the vertical orientation, Class 25 because window perimeter movement rarely exceeds 20 percent, Use NT because the joint is not in a traffic path. Step up to Class 50 in tall metal-frame windows on south-facing walls in high-swing climates, and add Use M when bonding directly to anodized aluminum frames.

Related Resources

Standards reference: ASTM C920 standard page (full classification table, testing requirements, common spec language).

Movement-class deep dive: ASTM C920 Class 25 vs Class 50 for the full movement-class ladder and when it is safe to substitute up.

Caulking-focused companion: Sealant and Caulking Selection Guide for siding gaps, interior trim, kitchen and bath, and product comparisons by chemistry.

Cladding joints on homes and light commercial: best caulk for exterior siding covers gap sizing, paintability, and chemistry picks for vinyl, fiber cement, wood, and aluminum siding.

Roofing applications (metal flashing seams, vent stacks, fire-rated penetrations): Commercial Roof Flashing & Penetration Sealants for the specific details where 100% silicone and intumescent firestop are the right chemistry, and where they aren’t.

Bridge-deck systems (different problem, different products): Bridge Expansion Joint Selection Guide for strip seal, modular, finger plate, asphaltic plug, and compression-seal systems.

Parking garages (mixed traffic, chloride exposure): parking structure maintenance under ACI 362.2R for corrosion repair, deck joint sealing, and traffic-coating decisions.