+---
+title: Exterior Weather Sealants
+category: Building Envelope
+toc_depth: 3
+description: >
+ When to use: Selecting, specifying, and installing cold-applied elastomeric
+ sealants that weatherproof perimeter, transition, and penetration joints in
+ exterior building assemblies, on new construction and on restoration/recaulking
+ of commercial and institutional buildings. Covers window and curtain wall
+ perimeter joints, door frame perimeters, above-grade control and expansion
+ joints in concrete, masonry, metal panel, EIFS, and wood-framed walls,
+ through-wall penetrations, thresholds, and horizontal ledge joints, using
+ single- and multicomponent silicone, polyurethane, and polyurethane-hybrid
+ systems with backer rods, bond-breaker tape, and primers.
+ Not intended for: Interior, acoustical, and fire-rated through-penetration
+ seals (coordinate ratings with sync/fire-rated-wall-and-floor-assemblies);
+ structural glazing and structural silicone curtain wall perimeter joints
+ (sync/glazed-curtain-walls); storefront/entrance perimeter sealant supplied
+ with the glazing package (sync/aluminum-entrances-and-storefronts); building
+ expansion joint cover assemblies (sync/expansion-joints); the broader
+ non-weatherproofing sealant vocabulary (sync/joint-sealants); horizontal
+ traffic-bearing pavement joint sealants; and roof-membrane base-of-wall
+ flashings (sync/fluid-applied-roofing).
+---
+
+# Scope {toc}
+
+## This standard governs cold-applied elastomeric sealants used to weatherproof the exterior building envelope at perimeter, transition, and penetration joints. {note}
+
+## It applies to both new construction and the restoration or recaulking of existing commercial and institutional buildings. {note}
+
+## The following exterior joint conditions are within scope: window and curtain wall perimeter joints (frame-to-wall), door frame perimeters, above-grade control and expansion joints in concrete, masonry, metal panel, EIFS, and wood-framed walls, through-wall penetrations such as pipes, conduit, and louvers, thresholds, and horizontal ledge joints. {note}
+
+## Single-component and multicomponent silicone, polyurethane, and polyurethane-hybrid sealant systems, together with backer rods, bond-breaker tape, and primers, are within scope. {note}
+
+## The following work types are excluded from this standard: {note}
+
+- Interior joint sealants, acoustical sealants, and fire-rated through-penetration seals — coordinate assembly ratings with [[sync/fire-rated-wall-and-floor-assemblies]].
+- Structural glazing sealants and structural silicone curtain wall perimeter joints — see [[sync/glazed-curtain-walls]].
+- Aluminum storefront and entrance frame perimeter sealants furnished as part of the glazing system package — see [[sync/aluminum-entrances-and-storefronts]].
+- Building expansion joint cover assemblies (cover plates, precompressed foam, bellows) — see [[sync/expansion-joints]].
+- The broader non-weatherproofing sealant vocabulary (acoustical, setting beds, paint caulk) — see [[sync/joint-sealants]]; this standard is the narrower exterior-weatherproofing subset.
+- Horizontal traffic-bearing pavement joint sealants (parking deck, highway), which fall outside the ASTM C920 scope addressed here.
+- Roofing-membrane flashings and fluid-applied roof coatings at base-of-wall — see [[sync/fluid-applied-roofing]].
+
+# Referenced Standards {toc}
+
+## Materials and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited.
+
+## Where referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
+
+| Standard | Title |
+|----------|-------|
+| ASTM C920-18(2024) | Standard Specification for Elastomeric Joint Sealants |
+| ASTM C1193-25 | Standard Guide for Use of Joint Sealants |
+| ASTM C1248-12(2018) | Standard Test Method for Staining of Porous Substrate by Joint Sealants |
+| ASTM C1481-12(2024) | Standard Guide for Use of Joint Sealants with Exterior Insulation and Finish Systems (EIFS) |
+| ASTM C794-18 | Standard Test Method for Adhesion-in-Peel of Elastomeric Joint Sealants |
+| ASTM C1401-14(2019) | Standard Guide for Structural Sealant Glazing |
+| ASTM C679-20 | Standard Test Method for Tack-Free Time of Elastomeric Sealants |
+| ASTM C1516-06(2019) | Standard Guide for Selection of Sealants for Use in Air Barrier Systems |
+| AAMA 800-15 | Voluntary Specifications and Test Methods for Sealants (includes AAMA 808.3) |
+
+## ASTM C920 is the controlling product specification and classifies every elastomeric sealant by Type, Grade, Class, and Use; specifying that classification, not a bare reference to C920, is the foundation of a usable sealant specification. {note}
+
+## ASTM C1193 is the companion installation guide governing joint design, substrate preparation, priming, backer rod selection, tooling, and field quality control; it is cited throughout this standard for the "how" behind the requirements. {note}
+
+# Submittals {toc}
+
+## The Contractor shall submit the following Action Submittals for review before ordering or installing any sealant:
+
+- Product data sheets for each sealant, primer, backer rod, and bond-breaker tape, including the manufacturer's ASTM C920 Type, Grade, Class, and Use designations.
+- Manufacturer's color chart, and a custom-color request where a factory-matched or custom-tinted color is required.
+- Joint design schedule correlating each joint type to its sealant product, movement class, joint width, and backer rod.
+- Substrate compatibility and adhesion test data, or the manufacturer's written confirmation of primer requirements, for each project-specific substrate.
+- Manufacturer's approved-sealant list confirmation where AAMA 800 compatibility with the fenestration system is required.
+
+```datasheet
+label: Action Submittals
+type: checkbox
+options:
+ - Product data (sealant, primer, backer rod, tape)
+ - Color chart and custom-color request
+ - Joint design schedule
+ - Substrate compatibility / adhesion test data
+ - Fenestration manufacturer approved-sealant confirmation
+```
+
+## The Contractor shall submit the following Informational Submittals:
+
+- Field adhesion (peel) test reports for each substrate at project startup and at the intervals scheduled below.
+- Sample warranty for the installed sealant system.
+- Applicator qualification statement documenting comparable completed projects.
+
+```datasheet
+label: Informational Submittals
+type: checkbox
+options:
+ - Field adhesion (peel) test reports
+ - Sample warranty
+ - Applicator qualification statement
+```
+
+## The Contractor shall submit the following Closeout Submittals before final acceptance:
+
+- Executed sealant manufacturer's warranty and the installer's workmanship warranty.
+- As-installed joint locations and product log keyed to the joint design schedule.
+- Maintenance instructions for inspection and recaulking intervals.
+
+```datasheet
+label: Closeout Submittals
+type: checkbox
+options:
+ - Executed manufacturer and installer warranties
+ - As-installed joint and product log
+ - Maintenance / recaulking instructions
+```
+
+# Quality Assurance {toc}
+
+## Installer qualifications
+
+### The sealant installer shall have completed at least three exterior weatherproofing-sealant projects of comparable scope within the preceding five years.
+
+### The installer shall employ workers trained in the specific sealant systems used on the project, including manufacturer field instruction where required to validate the warranty.
+
+## Mock-ups and field adhesion testing
+
+### Field adhesion verification is the single most reliable defense against the adhesion failures that account for roughly 40% to 50% of all sealant warranty claims; it confirms in place what a data sheet only promises in the laboratory. {note}
+
+### The Contractor shall perform a field-applied mock-up of each representative joint condition and obtain acceptance before production sealant work begins.
+
+### The Contractor shall perform a field adhesion (peel) test in accordance with ASTM C1193 at the start of work, at each new substrate first encountered, and at the scheduled production interval below.
+
+### A cohesive failure mode, in which the sealant tears within itself rather than releasing from the substrate, is the passing result; an adhesive failure, in which the sealant releases cleanly from the substrate, is a failing result requiring corrective action. {note}
+
+### Sealant that fails the field adhesion test shall be removed and the substrate re-prepared, re-primed, and re-sealed before work continues at that condition.
+
+```datasheet
+label: Field adhesion test interval (linear ft of joint per test)
+type: range
+unit: ft
+min: 100
+max: 1000
+step: 50
+default: 500
+```
+
+# Environmental and Service Conditions {toc}
+
+## Application temperature
+
+### Sealant shall be applied only when the substrate surface temperature is within the manufacturer's published application range; substrate surface temperature, not air temperature, governs whether application may proceed. {note}
+
+### Single-component polyurethanes shall not be applied below 40°F (4°C) substrate surface temperature unless the manufacturer's data sheet permits a lower limit.
+
+### Sealant shall not be applied when the substrate surface temperature exceeds 100°F (38°C), above which cure rates and tooling behavior of single-component silicone are impaired.
+
+```datasheet
+label: Minimum substrate surface temperature at application
+type: range
+unit: °F
+min: 35
+max: 50
+step: 5
+default: 40
+```
+
+## Service temperature range
+
+### Selected sealants must remain elastic across the full service temperature swing the joint will see; silicone retains elasticity over a far wider range than polyurethane, which is one reason it is preferred at sun-loaded aluminum frames. {note}
+
+### Silicone sealants shall maintain published movement capability over a service temperature range of at least -65°F to +400°F (-54°C to +204°C).
+
+### Polyurethane sealants shall maintain published movement capability over a service temperature range of at least -40°F to +180°F (-40°C to +82°C).
+
+## Cure and weather exposure
+
+### Tack-free time per ASTM C679 is the point at which the sealant surface will shed water and resist dirt pickup, but it is not full cure; deep joints continue curing for days and must be protected accordingly. {note}
+
+### Freshly placed sealant shall be protected from rain and standing water until it reaches tack-free condition per ASTM C679.
+
+### Joints shall not be subjected to building movement, traffic, or hydrostatic exposure until the sealant has reached full cure per the manufacturer's data sheet.
+
+```datasheet
+label: Time to tack-free at 75°F (24°C), 50% RH
+type: range
+unit: min
+min: 30
+max: 240
+step: 15
+default: 60
+```
+
+```datasheet
+label: Full cure time
+type: range
+unit: days
+min: 7
+max: 21
+step: 1
+default: 14
+```
+
+# Sealant Selection and Classification {toc}
+
+## The ASTM C920 classification system
+
+### Specifying a sealant by ASTM C920 alone, without Type, Grade, Class, and Use, leaves the entire product selection to the contractor and generates an RFI on every product submittal; the classification fields below are mandatory, not optional refinements. {note}
+
+### Type designates component count: Type S is single-component (cures by reaction with atmospheric moisture); Type M is multicomponent (cures by mixing two parts and offers a longer working pot life for large restoration and deck joints). {note}
+
+### Grade designates flow: Grade NS (non-sag) holds its shape on vertical and overhead joints; Grade P (pourable, self-leveling) flows into horizontal deck and ledge joints. {note}
+
+### Class designates movement capability, expressed as the joint movement the sealant accommodates: Class 100/50 (+100%/-50%), Class 50, Class 35, Class 25, and Class 12.5, in descending order of movement. {note}
+
+### Use designates qualified substrate and exposure: T (traffic-bearing horizontal), NT (non-traffic horizontal), I (immersion), M (mortar/masonry), G (glass), A (aluminum), and O (other); a single joint may require several Use designations. {note}
+
+### The Contractor shall provide for each joint a sealant whose ASTM C920 Type, Grade, Class, and Use designations match the scheduled values for that joint.
+
+```datasheet
+label: Component type (ASTM C920 Type)
+type: radio
+options:
+ - Type S - single-component
+ - Type M - multicomponent
+default: Type S - single-component
+```
+
+```datasheet
+label: Flow grade (ASTM C920 Grade)
+type: radio
+options:
+ - Grade NS - non-sag (vertical / overhead)
+ - Grade P - pourable / self-leveling (horizontal)
+default: Grade NS - non-sag (vertical / overhead)
+```
+
+```datasheet
+label: Movement class (ASTM C920 Class)
+type: select
+options:
+ - Class 100/50
+ - Class 50
+ - Class 35
+ - Class 25
+ - Class 12.5
+default: Class 25
+```
+
+```datasheet
+label: Use designations (ASTM C920 Use)
+type: checkbox
+options:
+ - NT - non-traffic horizontal
+ - T - traffic-bearing horizontal
+ - M - mortar / masonry
+ - A - aluminum
+ - G - glass
+ - O - other substrate
+ - I - immersion
+```
+
+## Base polymer (chemistry) selection
+
+### Chemistry choice is driven by three things: the calculated joint movement, whether the surface must be painted, and whether the joint abuts porous stone. Silicone wins on movement and weathering but cannot be painted and may stain stone; polyurethane is paintable and stone-friendly but accommodates less movement. {note}
+
+### For masonry and concrete perimeter and control joints, the 80%-case default is single-component polyurethane, Type S, Grade NS, Class 25, Use NT/M/A/O. {note}
+
+### For aluminum frame and metal panel perimeter joints, the 80%-case default is single-component silicone, Type S, Grade NS, Class 50. {note}
+
+### Where a high-movement curtain wall perimeter joint requires Class 100/50 performance, a two-part structural silicone may be required; that condition crosses into [[sync/glazed-curtain-walls]] and ASTM C1401 territory and shall be coordinated with the glazing scope. {note}
+
+### The Contractor shall select the base polymer to satisfy simultaneously the movement, paintability, and staining requirements scheduled for the joint.
+
+```datasheet
+label: Base polymer (chemistry)
+type: radio
+options:
+ - Single-component polyurethane (NS)
+ - Single-component silicone (NS)
+ - Polyurethane-hybrid / modified silicone (paintable)
+ - Self-leveling polyurethane (Grade P, horizontal)
+ - Multicomponent polyurethane (NS, restoration/deck)
+default: Single-component polyurethane (NS)
+```
+
+## Paintability
+
+### Silicones cannot be painted; paint will not adhere to a cured silicone surface and will fish-eye over it. Where a sealed joint falls within a painted field or will receive paint overspray, a polyurethane or a paintable polyurethane-hybrid must be specified and coordinated with the painting scope. {note}
+
+### Where the sealed joint occurs in a surface scheduled to be painted, the Contractor shall provide a paintable polyurethane or polyurethane-hybrid sealant, not silicone.
+
+```datasheet
+label: Paintability requirement
+type: radio
+options:
+ - Paintable required (polyurethane / hybrid)
+ - Not paintable acceptable (silicone permitted)
+default: Not paintable acceptable (silicone permitted)
+```
+
+## Non-staining performance at porous stone
+
+### Silicone in contact with limestone, marble, sandstone, or granite can migrate oils into the stone and leave a permanent stain that cannot be removed; the only defense is to verify non-staining performance by test before the sealant ever touches the stone. {note}
+
+### Where sealant contacts porous stone, the Contractor shall provide a sealant verified non-staining to that substrate in accordance with ASTM C1248.
+
+```datasheet
+label: Non-staining verification (ASTM C1248) required
+type: radio
+options:
+ - Required - sealant abuts porous stone
+ - Not required - no porous stone contact
+default: Not required - no porous stone contact
+```
+
+# Joint Design and Geometry {toc}
+
+## Width and depth
+
+### A weatherproofing sealant works by stretching and compressing as the joint moves; that movement capacity comes from the sealant's shape, so the joint geometry is as much a part of the specification as the product. An over-deep, undersized joint cannot move and tears in an hourglass pattern; a correctly proportioned joint flexes freely. {note}
+
+### The joint width shall be a minimum of 3/8 in. (10 mm); perimeter joints at aluminum frames are typically 1/2 in. to 3/4 in. (12 mm to 19 mm).
+
+### Joints wider than 1-1/2 in. (38 mm) shall not be sealed with a field-applied bead alone; use a precompressed foam sealant or expansion joint cover assembly per [[sync/expansion-joints]].
+
+### The sealant depth at the bead center shall be one-half the joint width, with a minimum of 1/4 in. (6 mm) and a maximum of 1/2 in. (13 mm) for joint widths up to 1 in. (25 mm), per ASTM C1193.
+
+### The sealant depth shall not exceed the joint width; a 1:1 width-to-depth ratio is the deepest permitted.
+
+```datasheet
+label: Design joint width
+type: range
+unit: in.
+min: 0.375
+max: 1.5
+step: 0.0625
+default: 0.5
+```
+
+```datasheet
+label: Sealant depth at bead center
+type: range
+unit: in.
+min: 0.25
+max: 0.5
+step: 0.0625
+default: 0.25
+```
+
+## Movement accommodation
+
+### The selected movement class shall equal or exceed the joint movement calculated from thermal expansion and structural drift for the assembly and climate.
+
+### Joint locations, spacing, and extents shall be as shown; they cannot be reduced to a product field. [[drawing: exterior elevation joint plan]]
+
+```datasheet
+label: Default movement class by substrate
+type: radio
+options:
+ - Class 25 polyurethane - masonry / concrete
+ - Class 50 silicone - aluminum frame / metal panel
+ - Class 100/50 silicone - high-movement curtain wall
+default: Class 25 polyurethane - masonry / concrete
+```
+
+## Backer rod and bond breaker
+
+### A sealant that bonds to three sides of a joint, including the back, cannot move; the back-of-joint bond restrains it and it tears at its face. The backer rod (or bond-breaker tape in very shallow joints) exists to break that third-side bond and to tool the bead to its proper hourglass profile. {note}
+
+### Closed-cell polyethylene backer rod is the default for exterior weatherproofing joints; open-cell polyurethane backer rod is used as an off-gassing bond breaker in deep joints, and bond-breaker tape is used only where the joint is too shallow for any rod. {note}
+
+### The Contractor shall install a backer rod or bond-breaker tape in every joint to prevent three-sided adhesion.
+
+### The backer rod diameter shall be 25% to 33% larger than the joint width so the rod compresses into the joint and cannot be displaced behind the sealant.
+
+### The backer rod shall not be punctured, stretched, or twisted during installation, as any of these changes its depth and disturbs the bead profile.
+
+```datasheet
+label: Backer rod type
+type: radio
+options:
+ - Closed-cell polyethylene
+ - Open-cell polyurethane (bond breaker, deep joints)
+ - Bond-breaker tape (shallow joints only)
+default: Closed-cell polyethylene
+```
+
+```datasheet
+label: Backer rod oversize relative to joint width
+type: range
+unit: '%'
+min: 25
+max: 33
+step: 1
+default: 30
+```
+
+# Substrate Preparation and Priming {toc}
+
+## Cleaning
+
+### The joint substrate shall be clean, dry, sound, and free of dust, oils, form-release agents, old sealant residue, coatings, and frost before any primer or sealant is applied.
+
+### Existing failed sealant in restoration work shall be completely removed and the substrate re-prepared; new sealant applied over old sealant bonds cohesively to the old material and fails at that interface. {note}
+
+### In restoration work the Contractor shall remove all existing sealant to sound substrate and re-prepare the joint before applying new sealant.
+
+## Priming
+
+### Failure to prime is the most common single cause of adhesion failure, particularly on concrete, CMU, EIFS, and anodized aluminum; primer is cheap and adhesion failure is the most expensive defect in the assembly. {note}
+
+### The Contractor shall determine the primer requirement for each substrate from the manufacturer's adhesion testing on the project-specific substrates, not from a generic assumption.
+
+### Where primer is required, the Contractor shall apply it per the manufacturer's instructions and allow it to dry to tack-free, typically 30 min to 60 min, before placing sealant.
+
+### Sealant shall be placed within the manufacturer's published open time after the primer dries; a primed surface left exposed too long shall be re-primed.
+
+```datasheet
+label: Primer requirement (per manufacturer adhesion testing)
+type: radio
+options:
+ - Primer required
+ - Primer not required (verified by test)
+default: Primer required
+```
+
+## EIFS substrates
+
+### EIFS demands special attention: an incompatible sealant chemically attacks the EPS insulation board, and generic specifications routinely omit the compatibility check that ASTM C1481 requires. {note}
+
+### Where the joint abuts an EIFS surface, the Contractor shall provide a sealant and joint design conforming to ASTM C1481 and approved in writing by the EIFS manufacturer.
+
+# Installation {toc}
+
+## Application
+
+### Sealant shall be gun-applied in a continuous operation that fully wets both joint faces and leaves no voids, skips, or air pockets behind the bead.
+
+### The sealant shall contact solid substrate over a minimum bond width of 3/8 in. (10 mm) on each side of the joint, per ASTM C1193.
+
+### The bead shall be tooled immediately after application, before skinning, to force the sealant against both joint faces and to form a slightly concave, hourglass profile.
+
+### Masking tape, where used to produce clean bead edges, shall be removed immediately after tooling and before the sealant skins.
+
+```datasheet
+label: Minimum substrate bond width each side
+type: range
+unit: in.
+min: 0.375
+max: 0.75
+step: 0.0625
+default: 0.375
+```
+
+## Penetrations and transitions
+
+### Through-wall penetrations, thresholds, and dissimilar-material transitions are where the weather barrier is most often broken; each requires a fully tooled, continuous bead bridging the joint with backer rod behind it, exactly as a field joint does. {note}
+
+### The Contractor shall seal each through-wall penetration of pipe, conduit, or louver with a continuous tooled bead.
+
+### The backer rod used at through-wall penetrations shall be sized to the annular gap between the penetrating element and the sleeve or rough opening.
+
+### Sealant at horizontal ledge and threshold joints shall be sloped or detailed to shed water and shall not be left in a configuration that ponds water against the bead.
+
+## Coordination of perimeter sealant responsibility
+
+### At curtain wall and storefront perimeters, both the glazing installer and the general contractor's sealant crew can claim the perimeter joint, and a joint claimed by no one is the joint that leaks; the specification must name the responsible party unambiguously. {note}
+
+### The Contractor shall designate responsibility for the curtain wall and storefront perimeter weatherseal, typically to the glazing installer, and shall reflect that designation in the joint schedule; coordinate with [[sync/aluminum-entrances-and-storefronts]] and [[sync/glazed-curtain-walls]].
+
+### Perimeter sealant compatibility with architectural aluminum fenestration shall conform to AAMA 800 and to the fenestration manufacturer's approved-sealant list.
+
+# Field Quality Control {toc}
+
+## Inspection
+
+### The Contractor shall inspect every completed joint for full, continuous, void-free fill, correct bead profile, and clean edges.
+
+### Beads exhibiting gaps, blisters, three-sided adhesion, incorrect profile, or adhesive failure shall be cut out and replaced.
+
+## Adhesion verification during production
+
+### Field peel testing must continue into production, not stop after the startup test, because substrate conditions, weather, and workmanship all change as the job proceeds and adhesion can fail on a later wall that passed at startup. {note}
+
+### The Contractor shall perform production-stage field adhesion tests at the scheduled interval and shall record the failure mode (cohesive or adhesive) for each.
+
+# Delivery, Storage, and Handling {toc}
+
+## Delivery and storage
+
+### Sealants, primers, and backer rods shall be delivered in unopened original containers bearing the manufacturer's name, product designation, ASTM C920 classification, batch number, and shelf-life expiration date.
+
+### Materials shall be stored in a dry, shaded location within the temperature range published by the manufacturer and protected from freezing where the data sheet so requires.
+
+### Sealant whose shelf-life expiration date has passed shall not be used and shall be removed from the site.
+
+## Handling
+
+### Multicomponent sealant shall be mixed strictly per the manufacturer's ratio and mixing-time instructions; under-mixed or off-ratio material will not cure properly.
+
+# Warranty {toc}
+
+## Warranty coverage
+
+### The sealant manufacturer shall warrant the sealant material against adhesive and cohesive failure, and against weathering and color change beyond published limits, for the scheduled term.
+
+### The installer shall warrant the workmanship of the sealant installation against leakage and adhesive or cohesive failure for the scheduled term.
+
+```datasheet
+label: Manufacturer material warranty term
+type: range
+unit: years
+min: 5
+max: 20
+step: 5
+default: 10
+```
+
+```datasheet
+label: Installer workmanship warranty term
+type: range
+unit: years
+min: 2
+max: 5
+step: 1
+default: 3
+```
+
+# Maintenance and Recaulking {toc}
+
+## Maintenance program
+
+### Exterior sealant is a wearing element of the envelope, not a permanent one; even a 20-year sealant should be inspected periodically and recaulked at end of service life, and the owner should be told so at closeout. {note}
+
+### The Contractor shall provide maintenance instructions identifying recommended inspection intervals and the indicators of sealant end of life (loss of elasticity, surface crazing, adhesive release, and joint leakage).
+
+### Recaulking of failed joints shall follow the same removal, preparation, priming, backer rod, and tooling requirements as new work; sealant shall not be reapplied over existing failed sealant.