Construction Sealants: Types, Movement Classes, Joint Design and Selection

Construction sealants are elastic or plastic joint materials, from polyurethane and silicone to acrylic, butyl and semi-rigid fillers, that bond to the two faces of a building joint and take its movement. Which of the 9 classes fits a joint depends on how far the joint moves, what it is bonded to and what it is exposed to, so how is that decided?

The 9 classes fall into 3 groups: 4 elastomeric sealants, 2 low-movement sealants and 3 joint fillers, seals and backing products. US trade speech calls low-movement products "caulk", and UK and Indian usage calls a sealant a "mastic"; a specification names the standard and class instead of either word. Sealants and joint products are one of the 15 families of construction chemicals.

ASTM C920-18(2024) rates elastomeric sealants from Class 12.5 to Class 100/50, while EN 15651 and ISO 11600 classes stop at 25. A manufacturer rule sizes a joint at least 4 times as wide as its expected movement for a ±25 % sealant, and 1 L fills 1,000 / (W × D) meters of joint before waste. In the South Coast air district, architectural sealants carry a VOC limit of 50 g/L (0.42 lb/gal) under SCAQMD Rule 1168, and the EU requires documented training for diisocyanate sealants since 2023-08-24. Published installed costs, such as USD 16.40 to 59.06 per m (USD 5 to 18 per linear ft) for commercial sealant and caulking work (US contractor page, 2026), are dated in the complete list below.

Standards, codes and regulations are checked against their issuing bodies, product values against the named data sheet and prices against the dated source named in the sentence; see our methodology and fact-checking.

What Is a Construction Sealant?#

A construction sealant is a paste or liquid placed in a building joint that cures or dries to an elastic or plastic seal, bonds to the two joint faces and accommodates the joint's cyclic movement.

Construction sealants come in 5 forms: one-component (1K) cartridges, sausages and pails; two-component (2K) kits of base and activator; non-sag gun grades (ASTM C920 Grade NS); pourable self-leveling grades (Grade P) for horizontal joints; and preformed butyl tapes. Facade joints take non-sag grades; floor and deck joints take pourable grades.

Sealant, caulk and mastic: what each term means#

In the trade, "caulk" (US) names low-movement products and "mastic" (UK, India) is a regional word for sealant, while a specification names the standard and class instead. A specifier writes "ASTM C920, Type S, Grade NS, Class 25, Use NT" or "EN 15651-1 F EXT-INT 25LM", never "caulk".

In UK usage "silicone" stands for any gun-applied sealant, whatever the chemistry. Low-movement acrylic and butyl products, the closest match to "caulk", carry their own ASTM documents, C834-26 and C1311-22. Retail bathtub and window caulk is not covered here.

How Do Construction Sealants Work?#

Construction sealants work in two steps: the material cures by a chemical reaction or dries by losing water or solvent, and the cured bead bonds only to the two joint faces so it can stretch and compress as the joint opens and closes.

Cure chemistries: moisture cure, two-component reaction and drying#

Polyurethane, silicone and hybrid sealants cure with moisture from the air, two-part polysulfides and other 2K grades cure after mixing, and acrylic and butyl sealants dry by losing water or solvent.

A 1K polyurethane releases carbon dioxide as its isocyanate end groups react with moisture, so it bubbles on wet or outgassing substrates. Silicones release acetic acid (acetoxy cure) or an oxime or alcohol (neutral cure), and hybrids cure through silyl end groups into siloxane crosslinks without isocyanate. Polysulfides cure by oxidation of their thiol end groups, with manganese dioxide as the common curing agent. The isocyanate prepolymer chemistry behind urethane sealants is set out under polyurethane resins.

Two-sided adhesion: why the joint base stays unbonded#

A sealant must bond to the two joint faces and not to the joint base, because a bead held on three sides cannot stretch and tears. A bead fixed to the base has no free length to elongate.

Two products keep the base unbonded. A cylindrical backer rod, classified by ASTM C1330-23 as Type C closed cell, Type O open cell or Type B bi-cellular, sets the depth and gives the bead its hourglass profile. A bond-breaker tape does the same job where the joint is too shallow for a rod. Inspectors record three-sided adhesion beside adhesive, cohesive and substrate failure.

9 Types of Construction Sealants and Joint Products in 3 Groups#

The 9 types of construction sealants and joint products fall into 3 groups: 4 elastomeric sealants rated by ASTM C920 class, 2 low-movement sealants, and 3 joint fillers, seals and backing products that work with or instead of a sealant. The group table below compares how each group works and which document governs it.

1. Elastomeric sealants (4)#

Elastomeric sealants are reactive sealants that cure to a rubber bonded to both joint faces; this group has 4 classes: polyurethane, silicone, silyl-terminated hybrids and polysulfide. Firestop sealants for fire-rated joints are tested as systems to UL 2079 and ASTM E1966-24 rather than as sealants, and they are covered under firestop sealants and penetration seals.

Polyurethane (1K and 2K)#

Polyurethane sealants are isocyanate-terminated prepolymers that cure with air moisture (1K) or with a polyol component after mixing (2K). They are rated under ASTM C920-18(2024), bond well to porous concrete and masonry, and come in Use T and Use I grades for traffic and immersion. Light colors yellow under UV.

Silicone (acetoxy, oxime and alkoxy cure)#

Silicone sealants are polydimethylsiloxane (PDMS) polymers that cure at room temperature (RTV) with air moisture through acetoxy, oxime or alkoxy crosslinkers. Weather seals are rated under ASTM C920-18(2024), and structural glazing silicones under ASTM C1184-23, the C1401-23 guide and EN 15434-1:2023. Standard silicone cannot be painted.

Silyl-terminated hybrids (MS, SMP, STPE, STPU)#

Silyl-terminated hybrids are polyether or polyurethane polymers with silyl end groups that cure with air moisture, free of isocyanate and without a silicone backbone. They are rated under ASTM C920-18(2024) and accept paint. They discolor on bitumen, are incompatible with EPDM and resist UV and heat less than silicone.

Polysulfide (two-part and one-part)#

Polysulfide sealants are thiol-terminated polysulfide polymers cured by manganese dioxide or another oxidant, in two-part and one-part grades. They are rated under ASTM C920-18(2024), and their very low gas and moisture permeability suits immersion, fuel and tank joints. A primer goes on wherever the maker requires one.

2. Low-movement sealants (2)#

Low-movement sealants are drying sealants that lose water or solvent, stay plastic and take small joint movement; this group has 2 classes: acrylic latex and butyl. ASTM C834-26 and C1311-22 govern them instead of C920.

Acrylic latex#

Acrylic latex sealants are acrylic emulsions that dry by water evaporation to a plastic or plasto-elastic seal that shrinks as it dries. ASTM C834-26 covers one-component latex sealants in Types OP and C and Grades -18 °C (0 °F), 0 °C (32 °F) and NF, and the plastic classes 12.5P and 7.5P fit this chemistry. They accept paint.

Butyl and solvent-release#

Butyl sealants are butyl rubber in solvent that skin and set as the solvent evaporates, with high shrinkage, while non-skinning grades stay tacky. ASTM C1311-22 covers solvent-release sealants and places them at about 7.5 % movement in its scope. Butyl tapes and sealants bed laps and concealed joints with very low vapor transmission.

3. Joint fillers, seals and backing (3)#

Joint fillers, seals and backing are products that fill, shape or back a joint with or instead of a sealant; this group has 3 classes: expansion and isolation fillers, backer rod and bond breakers, and semi-rigid floor fillers. Joints that must hold water under pressure take waterstops cast into the concrete, compared under waterstops for concrete joints.

Expansion and isolation joint fillers#

Expansion and isolation joint fillers are preformed compressible boards placed full depth so a slab can move away from walls, columns and adjacent slabs. ASTM D1751-23 covers bituminous fiber fillers, D1752-18(2023) Types I to IV of sponge rubber, cork, self-expanding cork and recycled PVC, and D994/D994M-11(2022) bituminous fillers. An isolation joint that will be sealed takes no asphalt-impregnated filler (ACI PRC-302.1-15).

Backer rod and bond breakers#

Backer rod is a cylindrical foam backing that sets the sealant depth and keeps the bead off the joint base, and bond-breaker tape replaces it in shallow joints. ASTM C1330-23 covers Type C closed-cell, Type O open-cell and Type B bi-cellular rod. A rod never goes under a floor joint that carries hard wheels.

Semi-rigid floor joint fillers#

Semi-rigid floor joint fillers are 100 % solids two-component epoxy or polyurea poured full depth into saw-cut joints, with no backer rod, to support the joint edges under hard wheels. ACI PRC-302.1-15 recommends a hardness of at least Shore A 80 (ASTM D2240): a guide recommendation, not an ASTM requirement.

How Do Sealant Chemistries Compare? Movement, Substrates, Paint and UV#

Sealant chemistries differ in four things a joint decides: how much movement they take (elastic C920 grades vs plastic latex and butyl), which substrates they bond to, whether they can be painted, and how they age in sunlight. The verdict table below compares the 6 sealant chemistries on standards and these four attributes, without single-product values.

Polyurethane and silicone split on substrate, paint and sunlight, and hybrids sit between the two with an isocyanate-free cure. The pair is compared head to head under polyurethane vs silicone sealants.

Elastic vs plastic sealants#

Elastic sealants recover their shape after stretching, while plastic sealants stay deformed, and EN 15651 draws the line at 40 % elastic recovery. Classes 25LM and 25HM need at least 70 % recovery, 20LM and 20HM at least 60 %, and 12.5E at least 40 %; 12.5P and 7.5P fall below 40 % and are plastic.

Why does recovery matter? A joint that cycles daily pulls a plastic bead out of shape each time, so it thins and splits. The 4 elastomeric chemistries in ASTM C920-18(2024) are elastic; acrylic latex under ASTM C834-26 and butyl under ASTM C1311-22 are plastic and belong in joints that barely move.

Porous vs non-porous substrates, paint and UV#

Polyurethanes suit porous concrete and masonry, silicones suit glass and metal, and silyl-terminated hybrids bond to a wide range of both. This rule is manufacturer guidance, so adhesion is still proven on site.

Paint and sunlight narrow the choice further. Polyurethane and hybrid sealants can be painted, standard silicone cannot, and acetoxy silicone attacks reactive metals and alkaline concrete. Under UV, light-colored polyurethanes yellow, hybrids age faster than silicone, and silicone's inorganic silicon-oxygen backbone keeps its properties. The siloxane chemistry behind that stability is covered under silanes, siloxanes and silicones.

Where Are Construction Sealants Used? Joints by Structure and Exposure#

Construction sealants are used in 6 joint settings (facades and masonry, glazing, floors and parking decks, tanks, roofs, and pavements and bridges), and the movement, traffic and water at each one should set the class and Use code. The table below matches each setting with its exposure, the classes that suit it and the rule that should govern the choice.

Hot-applied asphaltic pavement crack sealants are covered on asphaltadditives.net.

Facades, precast panels and masonry#

Facade and precast panel joints take the largest thermal movement on a building, so they get elastomeric sealants classed for movement, with a primer chosen by adhesion test where the substrate needs one. They carry an ASTM C920 class or an EN 15651-1 type F class.

Windows, curtain walls and glazing#

Glazing joints use Use G sealants, and structural glazing, where the sealant carries the glass, uses structural silicone under ASTM C1184-23 and C1401-23 or EN 15434-1:2023. ASTM C1401-23 covers walls within 15° of vertical and structural silicone only; EN 15434-1:2023 replaced EN 15434:2006+A1:2010.

Floors, slabs and parking decks#

Floor joints under hard wheels take semi-rigid fillers, while expansion, isolation and deck joints take elastomeric Use T sealants over a backer rod. ASTM C920-18(2024) sets Shore A 25 to 50 for Use T sealants, which separates them from the harder fillers at Shore A 80 or more.

Timing matters on floors: ACI PRC-302.1-15 puts most slab shrinkage within the first year and especially the first 60 to 90 days, so filling waits as long as possible. Parking decks add chlorides and wear, and the membranes and wear coats around their joints are compared under traffic coatings for parking decks.

Tanks and immersion joints#

Joints under water take Use I sealants, polysulfide or polyurethane among them, with the primer the maker requires for immersion.

Potable water changes the VOC rule. SCAQMD Rule 1168 gives potable-water sealants their own category at 100 g/L (0.83 lb/gal), half the 250 g/L (2.09 lb/gal) of the all-other-sealants category.

Roofs and metal roof laps#

Metal roof laps are sealed with butyl tapes and sealants that stay tacky and keep out water, while roof sealants have their own VOC categories. SCAQMD Rule 1168 sets 300 g/L (2.50 lb/gal) for all other roof sealants and 250 g/L (2.09 lb/gal) for single-ply roof membrane sealants since 2023-01-01; whole-roof restoration is covered under liquid roof coatings.

Which Standards Classify Construction Sealants?#

Construction sealants are classified by ASTM C920-18(2024) in the US and by EN 15651-1 to -4 and ISO 11600 in Europe and internationally, with ASTM C834-26 and C1311-22 for latex and solvent-release sealants and ASTM C1193-25 as the use guide. A data sheet can carry both designations.

ASTM C920 type, grade, class and use#

ASTM C920-18(2024) classes an elastomeric sealant by Type (S single, M multicomponent), Grade (P pourable, NS non-sag), Class (100/50, 50, 35, 25 or 12.5 % movement) and Use (T, NT, I, M, G, A, O). Class 100/50 means +100 % extension and -50 % compression.

The standard also sets hardness and aging limits: Shore A 25 to 50 for Use T and 15 to 50 for Use NT, and a weight loss of at most 7 %. The class test is ASTM C719-22, which cycles the bead with water immersion and temperature conditioning. C920 covers buildings, plazas and decks and excludes highway and airfield pavements and bridges; each designation is decoded under ASTM C920.

ASTM C1193-25 is the guide for using cold liquid-applied sealants in buildings, plazas, decks and pavements other than highways, airfields and bridges. Its wording that sealants "should meet" C834, C920 or C1311 is advisory.

EN 15651 parts and ISO 11600 classes#

EN 15651 has 5 parts by use (F facade, G glazing, S sanitary, PW pedestrian walkways, and conformity) and grades sealants 25LM, 25HM, 20LM, 20HM, 12.5E, 12.5P or 7.5P, the class names of ISO 11600:2002 + Amd 1:2011. ISO 11600 was last confirmed in 2025.

The classes carry three limits besides movement. Elastic recovery runs from at least 70 % (class 25) to below 40 % (12.5P and 7.5P); loss of volume is at most 10 % for classes 25 and 20 (at most 25 % for water-based dispersions) and at most 25 % for 12.5 and 7.5; resistance to flow is at most 3 mm (0.12 in).

LM and HM split by modulus: EN 15651-4 rates a sealant HM when its secant modulus exceeds 0.4 N/mm² (58 psi) at +23 °C (73 °F) or 0.6 N/mm² (87 psi) at -20 °C (-4 °F). The suffix CC marks cold-climate tests at -30 °C (-22 °F), and EN 15651-3 grades sanitary sealants S1 to S3 and XS1 to XS3 for microbiological growth, 1 being best. The class tests are set out under EN 15651 and ISO 11600.

Why ASTM and ISO movement classes do not map one to one#

ASTM C920 and ISO 11600 test movement differently, so a C920 class cannot be converted to an ISO class: ISO and EN classes stop at 25, while C920 runs to 100/50. ISO also adds elastic recovery and loss of volume limits that C920 does not set.

Each rating is read against its own standard. A C920 Class 50 or 100/50 rating has no ISO class above 25 to land in, and an ISO 25LM rating says nothing about a C920 class until the product passes the C719-22 cycle.

CE marking of sealants under the CPR#

Joint sealants are CE marked to the 2012 editions of EN 15651-1 to -4, which remain the harmonised references, although the 2017 editions are current at CEN. The 2012 editions stand on the Commission's summary list of 2026-02-09; the 2017 editions, published 2017-02-14, are not cited.

Conformity follows AVCP system 3, initial type testing by a notified laboratory. Regulation (EU) 2024/3110 entered into force on 2025-01-07 and applies from 2026-01-08, and standards cited under Regulation (EU) No 305/2011 stay valid until a product family moves under the new regulation. Dates and transition rules are set out under EU Construction Products Regulation.

Field adhesion and laboratory tests#

Four ASTM documents check sealant performance on site and in the lab, listed below.

• ASTM C1521-19(2025): destructive and non-destructive field adhesion evaluation of installed sealant joints.
• ASTM C1193-25 Appendix X1: the hand-pull tab test for field adhesion.
• ASTM C719-22: laboratory cyclic movement with water immersion and temperature conditioning, the C920 class test.
• ASTM D2240: the Shore A durometer method behind the C920 hardness limits and the floor-filler recommendation.

How Are Sealant Joints Sized? Width, Depth and Yield#

A sealant joint is sized from its movement: a manufacturer rule makes the joint at least 4 times the expected movement and at least 6 mm (1/4 in) wide for a ±25 % sealant, and the depth follows the width.

Joint width from movement and depth by width#

ASTM C1472-16(2022) is the guide for calculating joint movement and tolerances, and the sealant's movement class then sets the minimum width. As a worked example, take an expected movement of 3 mm (0.12 in) and a ±25 % sealant: the input movement × 4 gives a minimum width of 12 mm (0.47 in), above the 6 mm (1/4 in) floor.

Depth follows the width. Industry association guidance from IVD gives the metric depths below, measured from the bead face to the backer rod:

• width 10 to 15 mm: depth 8 ± 2 mm
• width 15 to 20 mm: depth 10 ± 2 mm
• width 20 to 25 mm: depth 12 ± 2 mm
• width 25 to 30 mm: depth 15 ± 3 mm
• width 30 to 35 mm: depth 15 ± 3 mm

Theoretical yield per meter and per gallon#

A joint needs width × depth in mL per meter, so 1 L fills 1,000 / (W × D) meters with W and D in mm, before waste. A 10 × 10 mm joint needs 100 mL per m, so a 600 mL sausage fills 6 m in theory.

The US check uses 1 US gal = 231 in³ (3.785 L): a gallon fills 308 ft of a 1/4 × 1/4 in joint and 154 ft (47 m) of a 1/4 × 1/2 in joint. Both figures are theoretical. Cartridges and sausages per joint length are counted in the sealant joint calculator.

VOC Limits and Safety Rules for Construction Sealants#

Joint sealants have VOC content limits in the South Coast air district, 50 g/L (0.42 lb/gal) for architectural sealants under SCAQMD Rule 1168 as amended 2022-11-04, but no category in the federal EPA AIM rule. The safety rules come from EU chemicals law.

Sealant VOC rules in the US and EU#

SCAQMD Rule 1168 sets separate limits for architectural, clear paintable water-resistant, roof, roadway and potable-water sealants and for sealant primers. The table below lists each category in g/L (lb/gal), less water and exempt compounds, beside the federal EPA AIM rule.

Product made before a limit took effect may be sold for 3 years and used for 4 years after it (Rule 1168 (d)(3)).

The clear, paintable and immediately water-resistant category dropped to 250 g/L (2.09 lb/gal) on 2026-01-01, and a 2026 draft amendment proposing a later date has not been adopted. The EPA AIM rule in 40 CFR 59 subpart D has no joint-sealant category in Table 1, so no federal VOC content limit applies to sealants. Every category across product families is listed under VOC limits for construction chemicals.

Diisocyanate, MEKO and siloxane rules#

Polyurethane sealants with at least 0.1 % diisocyanates by weight need documented training before professional use in the EU since 2023-08-24 (REACH Annex XVII entry 74, Regulation (EU) 2020/1149). The label statement has been required since 2022-02-24, and the training is renewed at least every 5 years.

The isocyanate monomers carry harmonised CLP entries: 4,4'-MDI (index 615-005-00-9), HDI (615-011-00-1), TDI (615-006-00-4), IPDI (615-008-00-5) and H12MDI (615-009-00-0) are each harmonised with H334 and H317, while pMDI and HDI oligomers have no Annex VI entry; see the product SDS. Oxime-cure silicones release butanone oxime (MEKO, CAS 96-29-7) as they cure, and acetoxy grades release acetic acid.

Silicone sealants carry their own line: no Annex VI entry for PDMS (notified not classified); per constituent: D4 (index 014-018-00-1): H361f, H410; MEKO (index 616-014-00-0): H301, H312, H315, H317, H318, H336, H350, H370, H373; see the product SDS. D4, D5 and D6 have been on the REACH Candidate List since 2018-06-27, and Annex XVII entry 70, replaced by Regulation (EU) 2024/1328, bars them at 0.1 % by weight or more each after 2026-06-06, with derogations listed in the regulation. Training levels and exposure limits for diisocyanates are set out under diisocyanates rules.

Complete List of Construction Sealants and Joint Products: All 9 Classes#

The complete list below gives all 9 classes of construction sealants and joint products with their chemistry, US and European standards, joints served, applicator need and a dated installed cost where one is published.

Published sealant costs are by job type, not by chemistry: commercial sealant and caulking work runs USD 16.40 to 59.06 per m (USD 5 to 18 per linear ft), installed, on an Associated Concrete Coatings contractor page (US, 2026). Joint sealant replacement in concrete runs USD 26.25 to 65.62 per m (USD 8 to 20 per linear ft), installed, on a Texas Structural Concrete page (US, Texas, 2026-03), and joint costs are quoted per linear meter or foot, never per m².

Which Construction Sealant Is Best for a Joint?#

No sealant is best for every joint: the right one has a movement class at least equal to the joint's movement, a Use code for its exposure and proven adhesion to both faces. A specification names it by standard and class, such as ASTM C920 Type S, Grade NS, Class 25, Use NT, not by brand. Adhesion is proven with an ASTM C1521-19(2025) field test, because a class rating says nothing about the primer a substrate needs.

Is silicone the same as a sealant?#

No: silicone is one sealant chemistry, the polydimethylsiloxane class, alongside polyurethane, hybrid, polysulfide, acrylic and butyl sealants. Each chemistry carries its own cure, substrate range and paint behavior.

Is a concrete sealer a joint sealant?#

No: a concrete sealer treats a surface against water and stains, while a joint sealant fills and seals a moving joint. Penetrating and film-forming surface treatments are compared under concrete sealers.

What is the difference between a joint filler and a joint sealant?#

A joint filler fills the joint (a preformed board cast in an expansion joint, or a semi-rigid resin poured full depth in a floor joint), while a sealant seals only the top of the joint over a backer rod. Boards follow ASTM D1751-23, D1752-18(2023) or D994/D994M-11(2022); floor fillers follow the ACI PRC-302.1-15 Shore A 80 recommendation.

When should silicone sealant not be used?#

Standard silicone is not used where the joint will be painted, on natural stone unless the grade is non-staining, or, for acetoxy grades, on concrete and reactive metals. Acetoxy cure releases acetic acid, which corrodes metals and reacts with alkaline concrete.

Does a joint sealant repair cracks in concrete?#

Not as a rule: a joint sealant is designed for a formed joint with two clean faces, while random cracks are routed and filled or injected. Static cracks and their fillers are covered under concrete crack fillers and crack sealants.

Do construction sealants need a specialist applicator?#

It depends on the joint: facade, structural glazing and traffic joints are installed by sealant or glazing contractors, and EU users of diisocyanate sealants need documented training. Low-movement acrylic and butyl joints do not need a specialist trade.