Silicone Sealants for Construction: Neutral Cure, Structural Glazing and Weatherproofing

A silicone sealant is a polydimethylsiloxane joint sealant that cures with air moisture into an elastic rubber, rated under ASTM C920-18(2024) and EN 15651 as a weatherseal and under ASTM C1184-23 and EN 15434-1:2023 for structural glazing. Which grade fits a joint depends on its cure chemistry, its movement class and the faces it must bond to.

Two cure families exist: acetoxy grades release acetic acid, neutral grades release an oxime or an alcohol, and only neutral grades suit metals, concrete and mortar. In UK usage "silicone" stands for any gun-applied sealant, whatever the chemistry. Class 100/50 of ASTM C920, +100 % and -50 % movement, is the top US class, and silicone facade grades such as DOWSIL 790 declare it. Silicone sealants are one family in this reference of construction chemicals.

This page follows a specification, from identity and the EN and ASTM classes to joint size, yield, cure by temperature, inspection, safety, dated costs and sourcing.

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.

The summary table below lists the attributes a specifier checks first on any silicone sealant.

What Is a Silicone Sealant?#

A silicone sealant is an elastomeric joint sealant whose binder is silanol-terminated polydimethylsiloxane, crosslinked by a silane that sets the cure by-product and the substrates it suits. The silicon-oxygen backbone is inorganic, which separates silicone from the organic polymers of the other sealant chemistries. Data sheets call it RTV silicone, neutral cure silicone, weatherproofing silicone or structural silicone sealant.

Silicone is one sealant chemistry beside polyurethane, silyl-terminated hybrid, polysulfide, acrylic and butyl sealants, so "silicone" and "sealant" are not synonyms. Each chemistry carries its own cure, substrate range and paint behavior, compared across the family of construction sealants.

Neutral cure vs acetoxy silicone: what does each crosslinker release?#

Acetoxy silicones release acetic acid as they cure, while neutral-cure silicones release an oxime or an alcohol, so only neutral grades suit metals, concrete and mortar. Acetic acid corrodes lead, zinc, copper, brass and ferrous metals and reacts with alkaline mortar and concrete, which rules acetoxy grades out on those substrates.

Six crosslinker types exist: acetoxy, oxime, alkoxy, benzamide, amine and lactate. Oxime grades release butanone oxime (MEKO), a cure by-product whose harmonised classification includes H350, covered under safety below. DOWSIL 999A is a named acetoxy grade with ±25 % movement and Shore A 25, half the ±50 % of the neutral DOWSIL 795 in the performance table.

One-part, two-part, self-leveling and precured silicone forms#

Silicone sealants come in four forms: one-part moisture-cure, two-part, self-leveling and precured extrusions. The four forms share the silicone polymer and differ in packaging, cure start and joint orientation, while the silane crosslinkers behind all of them are explained under silanes, siloxanes and silicones.

The four forms are listed below.

• One-part (1K) moisture-cure grades in cartridges, sausages and pails, cured by air moisture.
• Two-part (2K) grades for structural glazing and insulating glass, cured after mixing.
• Self-leveling traffic grades, poured into horizontal joints.
• Precured silicone extrusions, such as DOWSIL 123 Silicone Seal, supplied as cured profiles.

How Does a Silicone Sealant Cure and Bond?#

A one-part silicone cures from the exposed surface, where air moisture reaches the crosslinker, forming a skin first and then an elastic rubber. The reaction is a room-temperature vulcanizing (RTV) condensation cure, and its by-product leaves the joint as acetic acid, an oxime or an alcohol. Two-part grades start to cure when mixed.

Sika's facade compendium states that silicone keeps a nearly constant modulus from -30 to +80 °C (-22 to 176 °F), that partial crystallization starts below -50 °C (-58 °F) and that the rubber stays elastic to +150 °C (302 °F) in dry air. That temperature range comes from the inorganic silicon-oxygen backbone, which also gives silicone its UV stability. The bond forms on two joint faces only: a backer rod or bond-breaker tape prevents three-sided adhesion, because a sealant bonded on three sides cannot stretch freely as the joint moves.

Where Are Silicone Sealants Used in Buildings?#

Silicone sealants are used where joints move, see sunlight and meet non-porous faces: curtain walls and glazing, metal and coated aluminum panels, window perimeters, primed precast and EIFS joints, commercial sanitary joints and fire-rated joints. Natural stone takes only non-staining grades, and acetoxy grades stay off metals and concrete.

The six building uses are listed below.

• Curtain walls, glazing and anodized or coated aluminum: weatherseals and structural glazing (sections below).
• Metal panels and window perimeters: neutral weatherseals bonded to metal, glass and frames.
• Precast and stone facades: primed precast joints and non-staining grades on natural stone, part of masonry waterproofing.
• Insulated facades: primed joints in EIFS and ETICS.
• Pavement and slab joints: cold-applied silicone to ASTM D5893/D5893M-16(2021), Types NS and SL, covered under expansion joints in concrete.
• Fire-rated joints: silicone firestop sealants tested as listed systems.

Structural silicone glazing: bite, glueline and design stress#

In structural silicone glazing the sealant itself carries the glass or panel, so the bite and glueline are calculated, not chosen. Sika's design guidance sets both the bite and the glueline thickness at 6 mm (0.24 in) or more and sizes the bite as h = a x w / (2 x σdyn), the approach of ETAG 002. The dynamic design stress σdyn sits in the denominator, so a sealant with a higher declared design stress needs a narrower bite for the same panel and wind load.

Sikasil SG-550 declares 0.20 N/mm² (29 psi) dynamic design stress in tension, a manufacturer claim and the highest ETA design stress claim in the source guide. Structural glazing is work for a factory or a glazing contractor that keeps a quality-control log, with the tests listed under inspection below, while facade weatherseals are placed by a sealant contractor.

Sanitary and tiled wet-room joints (EN 15651-3)#

EN 15651-3 classes sanitary sealants S1 to S3 and XS1 to XS3 by microbiological growth, with 1 the top rating, and requires at least 25 % elongation after water immersion. XS classes declare higher proven properties than S classes. This page covers commercial wet-room joints, not household bathroom caulk: silicone in wet rooms picks up dirt and mold on its surface, so shower and washroom joints are specified by S or XS class together with the tile installation systems they seal.

Which Properties Define Silicone Sealant Performance?#

Six properties decide a silicone joint: movement capability, modulus, hardness, temperature range, UV and weathering resistance, and water-vapor permeability. Movement, modulus and hardness come from the product data sheet; the temperature, vapor and expansion values below come from one maker's compendium.

The table below compares the six properties with their test basis and named source.

Standard requirement or product value: how to read silicone data sheets#

A class threshold is what every product of the class must pass; a data-sheet value is what one product reached in its own test. ASTM C920-18(2024) sets Shore A 25 to 50 for Use T and 15 to 50 for Use NT, a weight loss of 7 % or less and no cracking or chalking. DOWSIL 790 declares C920 Class 100/50 with Shore A 15 and DOWSIL 795 ±50 % with Shore A 35, both tested by ASTM C719. A Shore A of 15 sits on the Use NT floor and below the Use T range, so the declared Use list decides where a grade may go, not the class alone.

Which Standards and Classes Apply to Silicone Sealants? EN and ASTM Side by Side#

Silicone weatherseals are classed under ASTM C920-18(2024) in the US and EN 15651 in Europe, structural silicones under ASTM C1184-23 and EN 15434-1:2023, and no table maps one system onto the other.

The table below sets the US, EU and international documents side by side with their classes, requirements and status.

No 1:1 mapping exists between the ASTM classes and the EN and ISO classes, so a project names the system its jurisdiction uses. The full designation rules of the US system, from Type to Use, are set out under ASTM C920 elastomeric joint sealants.

Weatherseal classes: C920 movement class and Use, EN 15651 type and class#

ASTM C920 names a sealant by Type, Grade, Class and Use, and silicone weatherseals reach the top class, 100/50: +100 % extension and -50 % compression in the ASTM C719-22 cycle. The C719 (Hockman) cycle combines cyclic movement, water immersion and temperature conditioning. Type S is single-component and M multicomponent, Grade P pours and Grade NS is non-sag, and Uses T, NT, I, M, G, A and O stand for traffic, non-traffic, immersion, mortar, glass, aluminum and other substrates. C920 covers buildings, plazas and decks and excludes highway and airfield pavements and bridges.

EN 15651-1 classes facade sealants as type F, EXT-INT or INT, from 25LM down to 7.5P, and silicone facade grades declare classes such as F 25LM. The 25 classes require elastic recovery of at least 70 %, flow of 3 mm (0.12 in) or less and loss of volume of 10 % or less; the suffix CC marks cold-climate testing at -30 °C (-22 °F), and type G under EN 15651-2 covers glazing joints. The harmonised references remain the 2012 editions under CPR 305/2011, assessed under AVCP system 3, while CEN published the 2017 editions on 2017-02-14, so a declaration of performance cites EN 15651-1:2012 or EN 15651-2:2012.

Structural silicone standards: C1184, C1401, EN 15434-1 and the SSG kit route#

ASTM C1184-23 specifies structural silicone sealants as Types S and M for Uses G and O, and ASTM C1401-23 guides their design and installation on walls within 15° of vertical. C1401-23 applies to structural silicone only, and ASTM C1193-25, the general guide for joint sealants, excludes structural glazing and points to C1401.

EN 15434-1:2023, published 2023-04-11, replaced EN 15434:2006+A1:2010 for bonded glazing sealants exposed to direct light. It requires shrinkage under 10 %, elastic recovery of at least 95 % and an aged breaking stress of at least 0.75 times the reference value with at least 90 % cohesive failure. A complete structural sealant glazing kit takes the European Technical Assessment route under EAD 090010-00-0404, the successor to ETAG 002 part 1. Regulation (EU) 2024/3110 applies from 2026-01-08, and standards cited under 305/2011 stay valid until a product family moves under a 2024/3110 implementing act.

Is ASTM C920 Class 100/50 the same as EN 15651 25LM?#

No: C920 Class 100/50 is a US movement class tested by ASTM C719-22, while EN 15651 and ISO 11600 classes stop at 25, so 25LM marks a low-modulus sealant at ±25 %, not the same claim. SikaHyflex-250 Facade carries both labels, C920 Class 100/50 and ISO 25LM, and Sikaflex-1A pairs Class 35 with 25HM, so each system is read on its own terms.

How do you check a silicone sealant claim on a DoP or data sheet?#

Five checks confirm what a silicone sealant claims, and they are listed below in reading order.

1. Read the full ASTM C920 designation: Type, Grade, Class and every Use, not the class alone.
2. Read the EU declaration of performance: it cites EN 15651-1:2012 or EN 15651-2:2012 with type and class under AVCP system 3, as the EU Construction Products Regulation requires for CE marking.
3. Confirm a structural claim: conformity with ASTM C1184-23 or a European Technical Assessment under EAD 090010-00-0404.
4. Look for an SWR Institute validation certificate, which reports the ASTM C719 cyclic movement result.
5. Read the substrate adhesion and staining notes: the primer for each substrate and a non-staining grade for natural stone.

The same checks answer whether a named grade is structural.

How Do You Choose a Silicone Sealant?#

Choose silicone where the joint faces sunlight, takes large movement and bonds to glass, metal or coated aluminum; choose another chemistry where the joint is painted, porous and unprimed, or under traffic or immersion. Silicone is rated limited for traffic and immersion in the chemistry comparison, and standard silicone cannot take paint.

The table below matches joints and exposures with the silicone grade and the class to ask for.

Sika's substrate guidance puts silicone on non-porous glass and metal, polyurethane on porous concrete and masonry, and silane-modified polymers on a wide range of both. Its facade compendium also limits coating over any sealant to joints with about 5 % movement, which keeps painted joints away from silicone. The two chemistries are set against each other criterion by criterion in polyurethane vs silicone sealants.

When is silicone the wrong sealant?#

Four situations call for another sealant, and they are listed below.

• Joints that will be painted: hybrid sealants (MS polymer) and polyurethanes accept paint.
• Porous concrete and masonry without primer: polyurethane sealants bond to porous faces.
• Traffic and immersion joints: grades of another chemistry that declare ASTM C920 Use T or Use I.
• Acetoxy grades on metals, mortar or concrete: acetic acid corrodes lead, zinc, copper and brass and reacts with alkaline substrates.

How Is a Silicone Sealant Applied? Preparation, Joint Size, Yield and Cure#

A silicone joint is only as durable as its bond, so the specification fixes four things: clean and primed faces, joint geometry, quantity per meter, and temperature and moisture at installation. The four sections below follow that order.

Cleaning, priming and backer rod for silicone joints#

Silicone joint faces are cleaned with solvent, primed per substrate, backed with rod and tooled dry. Dow's Contractor's Handbook cleans with isopropyl alcohol or MEK in cold weather and xylene or toluene in warm weather and primes per its substrate table, for example with DOWSIL 1200 OS; for DOWSIL 790 it bars alcohol cleaners, specifies open-cell polyurethane backer rod and allows dry tooling only.

Backer rod sets the sealant depth and stops three-sided adhesion. ASTM C1330-23 classes cylindrical sealant backing as Type C closed cell, Type O open cell and Type B bi-cellular, so the rod type on the drawing must match the one the sealant data sheet names.

Silicone joint width, depth and structural bite#

Silicone weatherseal joints are at least 6.4 mm (1/4 in) wide with a width-to-depth ratio of at least 2:1 under Dow's rule for DOWSIL 756, 790, 791 and 795. That rule suggests a depth of 9.5 mm (3/8 in), never more than 12.7 mm (1/2 in), and allows widths up to 101.6 mm (4 in). Joint width itself follows the expected movement: at least 4 times the anticipated movement and at least 6 mm (1/4 in) for a ±25 % sealant, a manufacturer rule set out under sealant joint design.

The table below lists the joint and bite rules with their source.

Structural joints are not sized by the weatherseal rule: their bite and glueline come from the design formula in the structural glazing section. Manufacturer guidance seals every moving joint at the mid-range of its movement, because a joint sealed at its lowest temperature sees only compression and one sealed at its highest sees only extension.

Silicone sealant yield per meter of joint#

A silicone joint takes width x depth in milliliters per meter, so a 12 x 6 mm (0.47 x 0.24 in) joint needs 72 mL per meter in theory. In US units one US gallon (231 in³, 3.785 L) fills 308 ft (93.9 m) of 1/4 x 1/4 in joint, theoretical and without waste.

The table below converts three joint sizes into length per cartridge and per sausage, theoretical and without waste.

Worked example: a perimeter of 120 m (393.7 ft) with a 12 x 6 mm joint needs 120 x 72 = 8,640 mL in theory, and with the full 25 % waste allowance 10,800 mL, which is 36 cartridges of 300 mL or 18 sausages of 600 mL. Counts for other joint sizes and lengths come from the sealant joint calculator.

Silicone application temperature and cure times#

Silicone application windows and cure times are product values, and the table below lists each with its product, temperature and humidity. The widest window in the table belongs to the listed neutral weatherseal grades, at substrate temperatures from -28.9 to 48.9 °C (-20 to 120 °F), and the narrowest to structural glazing at +5 to +40 °C (41 to 104 °F).

Listed weatherseal grades go onto faces free of moisture, condensation, dew and frost, with a frost check below 4.4 °C (40 °F). Structural cure times are quoted at 23 °C (73.4 °F) and 50 % relative humidity.

How Are Silicone Joints Inspected, and Why Do They Fail?#

Installed silicone joints are checked by field adhesion pulls under ASTM C1521-19(2025) and the hand-pull of ASTM C1193-25 Appendix X1, and a failure is read as adhesive, cohesive, substrate, staining or coating cracking. ASTM C1521-19(2025) covers destructive and non-destructive field adhesion evaluation, and mock-ups on the project substrates come before the full run.

Dow's handbook pulls DOWSIL 790 to 76.2 mm (3 in), an elongation of 300 %, without bond loss, and pulls DOWSIL 795, 995 and 121 to break. Structural glazing adds factory and site tests, from butterfly and snake tests to Shore A, peel and H-specimen tests, with the H-specimen target given by product in the table.

The table below lists the failure classes, the silicone-specific failure modes and the checks that find them.

What Health, Safety and VOC Rules Apply to Silicone Sealants?#

The hazards of a silicone sealant come from what it releases while curing and what it may contain: MEKO from oxime grades, acetic acid from acetoxy grades, and the cyclic siloxanes D4, D5 and D6. The polydimethylsiloxane (PDMS) polymer itself has no Annex VI entry, so the classification of a sealant is product-specific and its line is given per constituent in the table.

The table below collects the classifications, restrictions and VOC rules with their instrument and date.

MEKO from oxime grades and acetic acid from acetoxy grades#

MEKO (butanone oxime, CAS 96-29-7, EC 202-496-6) has a harmonised CLP Annex VI classification: H301, H312, H315, H317, H318, H336, H350, H370, H373. Oxime-cure silicones release MEKO while they cure, so indoor joints are sealed with ventilation during cure. Under CLP, H350 marks a carcinogen and H317 skin sensitization.

Acetoxy grades release acetic acid; for its classification, see the product SDS. Alkoxy grades release an alcohol instead, and the SDS of the specific product, not a class-wide hazard set, decides labeling and protective measures.

D4, D5 and D6 siloxanes under REACH#

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. The entry lists derogations in the regulation, and the polymer itself, polydimethylsiloxane, is not on the Candidate List. Three linear and branched siloxanes followed in 2025: octamethyltrisiloxane (L3) on 2025-01-21, and decamethyltetrasiloxane (L4) and M3T on 2025-06-25, all as vPvB.

VOC limits for silicone sealants#

SCAQMD Rule 1168, as amended 2022-11-04, limits architectural joint sealants, silicones included, to 50 g/L (0.42 lb/gal) less water and exempt compounds in the All Other Architectural Sealants category. Architectural sealant primers carry their own limits: 250 g/L (2.09 lb/gal) on non-porous and 775 g/L (6.47 lb/gal) on porous substrates. Product made before a limit takes effect may be sold for up to 3 years and used for up to 4 years after the effective date under paragraph (d)(3). DOWSIL 983, a two-part structural silicone, declares less than 18 g/L (0.15 lb/gal) mixed. A preliminary 2026 draft of proposed amended Rule 1168 would restrict volatile methyl siloxanes, but it is not adopted and sets no limit in force. The federal EPA AIM rule, 40 CFR 59 subpart D, has no joint-sealant category and so no VOC content limit for sealants. Limits for other categories and jurisdictions are compiled under VOC limits for construction chemicals.

How Much Does Silicone Sealant Work Cost?#

A window perimeter reseal, installed, costs USD 32.81 to 52.49 per m (USD 10 to 16 per linear ft) in British Columbia, Canada, on a contractor page dated 2025-07, with prices published in US$. A US contractor page puts commercial sealants and caulking, installed, at USD 16.40 to 59.06 per m (USD 5 to 18 per linear ft) in 2026, without the sealant type stated. Neither range isolates silicone, so silicone jobs are quoted from joint length and size, access, removal of old sealant and testing. Joint work is priced per linear meter or foot, never per square meter.

The table below lists the three dated installed ranges with region, date and source type.

Installed prices from contractor pages, each with its region and date; the commercial and Texas ranges do not state the sealant type.

What Are the Alternatives to Silicone Sealants?#

Five product classes replace silicone where paint, porous substrates, traffic, immersion or slab joints rule it out. Polyurethane is the closest substitute on porous facades, and hybrids sit between the two chemistries: paintable, free of isocyanate and available in Class 50 grades. Hybrids resist UV and heat less than silicone and discolor on bitumen.

The table below lists each alternative, where it wins over silicone and the standard it declares.

Silicone Sealant Grades, Packaging and Sourcing#

Silicone sealants are sold in 10.1 US fl oz (about 300 mL) cartridges, 600 mL sausages and pails for one-part grades and as two-part kits for structural and insulating-glass work. Buyers compare grades on the ASTM C920 or EN 15651 class, C1184 status for structural work, cure type, color range and substrate adhesion data. Colors such as black and "100 % silicone" labels say nothing about class or Use, so they never replace the declaration. Comparing suppliers on their declarations is set out under construction chemicals manufacturers and suppliers.

Five items belong in every silicone sealant request, and they are listed below.

1. Standard and class: ASTM C920 Type, Grade, Class and Use, or EN 15651 type and class.
2. Structural status: ASTM C1184-23 or an ETA under EAD 090010-00-0404.
3. Cure type: neutral (oxime or alkoxy) or acetoxy.
4. Package and color: cartridge, sausage, pail or two-part kit.
5. Documents: DoP, data sheet and SDS, plus adhesion and staining data for the project substrates.

Where Does a Silicone Sealant Fit in a Facade or Glazing System?#

A silicone weatherseal is one line of defense in a facade: it closes joints between glass, metal and panels, while the barrier and glazing system behind it handle water that passes the seal. Seal, joint design and primers are specified together, since one face losing adhesion lets water past.

Pavement and slab joints sit outside the facade and use cold-applied silicone to ASTM D5893/D5893M-16(2021), which FHWA HIF-18-019 (2019) sizes at a 2:1 width-to-depth ratio. Hot-applied pavement crack sealants are asphalt products, not covered here.

Silicone sealant FAQs#

Five questions about silicone sealants recur; short answers follow.

Can silicone sealant be painted?#

No: standard silicone cannot be painted, and Sika limits coating over any sealant to joints with about 5 % movement.

Paint crawls on a silicone surface, so joints planned for paint take a hybrid or polyurethane sealant.

Is silicone sealant a roof coating?#

No: a silicone sealant fills joints, while silicone roof coatings are fluid-applied films with their own standard, ASTM D6694/D6694M-25.

Silicone roof films are covered under liquid roof coatings.

Is silicone sealant waterproof?#

Silicone weatherseals keep rain out of building joints, but immersion calls for a sealant declaring ASTM C920 Use I, and silicone is rated limited for immersion.

A silicone joint is watertight against rain but not vapor-tight: the performance table gives about 20 g/(m²·d) at 2 mm (0.08 in).

Will new silicone bond to cured silicone?#

Yes, silicone renews silicone: Sika's renewal rule replaces silicone with silicone and lets up to 0.5 mm (0.02 in) of sound old sealant remain.

Polyurethane cured next to curing silicone loses adhesion, so the two chemistries are not mixed in a renewal.

Is silicone the right sealant for concrete joints?#

Not as a rule: silicone suits non-porous glass and metal, porous concrete and masonry joints take polyurethane under the substrate rule above, and acetoxy grades react with concrete.

Slab and pavement joints, including cold-applied silicone to ASTM D5893, are covered under expansion joints in concrete.