A sealant is an elastomeric material applied to joints in building assemblies to create a flexible weathertight barrier that prevents air, water, dust, and thermal transfer while accommodating the compression and extension cycles that occur as the building moves.

Sealants are not gap fillers or cosmetic trim finishes. They are engineered products rated by ASTM standards for specific movement capacity, substrate adhesion, and environmental resistance. On a commercial construction project, the sealant specification determines whether a joint lasts twenty years or fails in three.

Professional contractors understand that sealants perform a structural function in the building envelope. A window perimeter seal is part of the weatherproofing system. An expansion joint sealant is part of the thermal management system. A curtainwall weather-seal is part of the air barrier system. When the sealant fails, the system fails. Water gets in, energy efficiency drops, and the callback starts.

This post explains what sealants are, how they differ from caulk and adhesives, what the ASTM ratings mean, and how to match sealant type to application on commercial and residential projects.

If you are specifying or installing sealant on a project with performance requirements and inspection schedules, understanding what a sealant actually does is the starting point for making the right product choice.

Quick Answer A sealant is a flexible elastomeric material that fills and seals joints between building components while allowing for thermal expansion, structural movement, and settlement. Sealants are rated by ASTM C-920 classification based on their movement capacity: Class 7.5 (±7.5% movement), Class 12.5 (±12.5%), Class 25 (±25%), or Class 35 (±35%). Professional-grade sealants bond to substrates without primer, cure to form a weathertight barrier, and maintain flexibility through decades of compression and extension cycles.

What a Sealant Does: Weatherproofing and Joint Movement

Sealants serve two primary functions in construction: they create weathertight barriers and they accommodate joint movement. Most people understand the first function. A sealant prevents water, air, and dust from passing through gaps between materials. What many contractors miss is the second function. Buildings move. Concrete expands and contracts with temperature. Wood swells and shrinks with moisture. Metal cladding grows and shrinks measurably between summer heat and winter cold. The joints between these materials need to absorb that movement without cracking, tearing, or losing adhesion.

This is where elastomeric performance matters. An elastomeric sealant is a material that stretches and compresses repeatedly without permanent deformation. When the joint opens, the sealant extends. When the joint closes, the sealant compresses. The sealant returns to its original shape after each cycle. A properly selected sealant maintains adhesion at the bond line through thousands of these cycles over a service life measured in decades.

The ASTM C-920 standard classifies sealants by their movement capacity. A Class 25 sealant can handle ±25% joint movement. For a 1/2-inch joint, that is 1/8 inch of compression and 1/8 inch of extension. A Class 35 sealant can handle ±35% movement.

The higher the class number, the greater the flexibility and the more demanding the application the sealant can handle. Exterior joints on commercial buildings typically require Class 25 or Class 35 sealants. Interior joints with minimal movement can use Class 12.5. Standard acrylic caulk at Class 7.5 is not appropriate for exterior weatherproofing work.

Sealant vs Caulk vs Adhesive: The Functional Difference

The terms sealant and caulk are used interchangeably in casual conversation, but they describe different products with different performance standards. Understanding the distinction matters when you are reading a spec or ordering material for a commercial project.

Sealant: Elastomeric Joint Sealing

A sealant is an elastomeric product engineered to handle joint movement of ±12.5% or higher as defined by ASTM C-920. Sealants maintain flexibility after cure, bond to a wide range of substrates, and provide long-term weatherproofing performance in exterior applications. Professional-grade sealants like silicone, polyurethane, and acrylic urethane fall into this category.

Caulk: Low-Movement Gap Filling

Caulk typically refers to latex or acrylic products rated at Class 7.5 to Class 12.5 joint movement. These products are paintable, easy to apply, and clean up with water, which makes them popular for interior trim work and finish applications. However, standard caulk does not have the elasticity or durability for exterior weatherproofing on commercial buildings. Caulk is appropriate for filling gaps around baseboards, crown molding, and painted interior trim. It is not appropriate for window perimeters, expansion joints, or exterior siding.

Adhesive: Bonding Not Sealing

An adhesive creates a structural bond between two materials. Adhesives are designed to resist shear and tensile loads. They cure to a rigid or semi-rigid state and are not intended to accommodate ongoing joint movement. Some products, like construction adhesive or structural glazing silicone, function as both adhesive and sealant because they create a bond while maintaining elasticity. But most adhesives are not sealants, and using an adhesive where a sealant is required will result in joint failure when the building moves.

Types of Sealants for Construction Applications

Sealants are classified by their base chemistry. Each type has specific performance characteristics that make it suitable for certain applications and unsuitable for others. Matching sealant chemistry to application is how professional contractors avoid callbacks.

Acrylic Urethane Elastomeric Sealants

Acrylic urethane sealants combine the elasticity of polyurethane with the paintability and water cleanup of acrylic. Tower AU-1 Commercial Construction Sealant is an acrylic urethane product rated at ASTM C-920 Class 35 with 800 percent elongation. These sealants bond to concrete, masonry, wood, aluminum, vinyl, and fiber cement without primer on most substrates. They are paintable within 24 hours, VOC compliant for all 50 states, and appropriate for window perimeters, expansion joints, EIFS, precast panels, and building envelope applications where the sealant will be painted.

Silicone Sealants

Silicone sealants are 100 percent silicone polymers that cure through moisture reaction. They remain permanently flexible, resist UV degradation, and bond to glass, metal, and ceramic surfaces. Silicone is not paintable, which makes it unsuitable for painted facades. It is the correct product for structural glazing, curtainwall weatherseals, glass-to-metal joints, wet areas like showers and tubs, and rooftop penetrations where UV exposure is constant. Tower Silicone carries a 60-year guarantee and is rated at ASTM C-920 Class 25 minimum.

Polyurethane Sealants

Polyurethane sealants cure to a tough, abrasion-resistant elastomer with excellent adhesion to concrete, masonry, and wood. Standard polyurethane is not paintable and requires mineral spirits for cleanup. It off-gasses during cure with fumes that can be a concern on occupied buildings. Polyurethane is appropriate for parking deck joints, plaza deck expansion joints, and high-traffic horizontal applications where abrasion resistance matters. For painted building envelope work, acrylic urethane provides similar performance with better application characteristics.

Butyl and Polysulfide Sealants

Butyl sealants are rubber-based products that never fully cure. They remain tacky and flexible throughout their service life. Butyl is used in specialized applications like metal roofing laps, HVAC ductwork, and glazing bedding where permanent flexibility is required. Polysulfide sealants are two-part products that cure to a very flexible state and can be applied underwater. They are expensive and rarely specified outside of industrial and marine applications. Neither product is common on standard commercial construction projects.

How ASTM C-920 Classification Works

The ASTM C-920 standard is the specification that defines sealant performance for building construction. Every professional-grade sealant carries an ASTM C-920 classification that tells you how much joint movement it can handle and which substrates it adheres to. Understanding how to read that classification is how you confirm that a product meets the performance requirements on your spec.

The classification format is: ASTM C-920, Type (S or M), Grade (P, NS, or P/NS), Class (7.5, 12.5, 25, 35, 50, or 100), Use (T, M, G, A, or O). Type S is single-component. Type M is multi-component. Grade P means the sealant self-levels and is appropriate for horizontal joints. Grade NS means non-sag and is appropriate for vertical joints. Class indicates movement capacity. Use indicates which substrates the sealant bonds to without primer.

For most building envelope applications, you are looking for Type S, Grade NS, Class 25 or Class 35, Use M (masonry), G (glass), or A (aluminum). Tower AU-1 is classified as ASTM C-920, Type S, Grade NS, Class 35, Use NT (non-traffic joints on the ground). This means single-component, non-sag, ±35% movement, tested for non-traffic bearing horizontal applications. That classification tells you the product is appropriate for expansion joints, window perimeters, precast panels, and EIFS where high movement capacity and multi-substrate adhesion are required.

Where Sealants Are Used in Commercial Construction

Sealants appear in virtually every exterior joint on a commercial building. The sealant specification varies by location based on movement requirements, substrate types, and weathering exposure.

• Window and door perimeters. The joint between the window frame and the surrounding wall assembly requires a paintable Class 25 or Class 35 elastomeric sealant like Tower AU-1.
• Precast concrete panel joints. Control joints and expansion joints in precast panel facades require Class 35 sealant to handle the thermal movement that large panels experience.
• EIFS control joints. Exterior Insulation and Finish Systems use elastomeric sealants at control joints to accommodate thermal expansion without cracking the finish coat.
• Curtainwall perimeter seals. Glass and aluminum curtainwall systems use silicone weatherseals at the perimeter to create the air and water barrier. These are non-structural weatherproofing joints.
• Structural glazing. Glass bonded directly to the structural frame with no mechanical retention uses silicone sealant as a structural adhesive. This is a load-bearing application that requires engineering review and ASTM C1184 testing.
• Masonry control joints and relief joints. Brick and CMU facades require elastomeric sealants at control joints to accommodate thermal and moisture-related movement without cracking the masonry.
• Rooftop penetrations and flashing terminations. HVAC curbs, vent pipes, and metal flashing terminations require UV-stable silicone sealant to maintain weathertightness through constant sun exposure and require a high surface temperature range to deal with rooftop heat.

Common Mistakes When Specifying or Applying Sealants

• Using Class 7.5 or Class 12.5 caulk on exterior building envelope joints. Standard acrylic caulk does not have the elasticity to handle exterior joint movement and will fail within a few years.
• Specifying silicone on painted facades. Paint will not bond to silicone. If the joint will be painted, specify acrylic urethane.
• Ignoring joint sizing requirements. Sealant joints need proper width-to-depth ratios to perform correctly. Too narrow and the sealant tears. Too wide and the sealant does not maintain adhesion.
• Skipping backer rod. Three-sided adhesion is one of the most common causes of premature sealant failure. Backer rod controls joint depth and prevents the sealant from bonding to the back of the opening.
• Applying sealant over dirty or damp substrates. Surface prep is not optional. Sealant will not bond to dirt, oil, or moisture.
• Substituting products without checking the ASTM classification. A Class 25 sealant is not a substitute for a Class 35 sealant on a joint designed for higher movement.

Frequently Asked Questions: What Is a Sealant

What is the difference between a sealant and caulk?

Sealants are elastomeric products rated at ASTM C-920 Class 12.5 or higher for joint movement. Caulk typically refers to latex or acrylic products rated at Class 7.5 to Class 12.5. Sealants are engineered for exterior weatherproofing and high-movement applications. Caulk is appropriate for interior trim work and low-movement finish applications. The terms are often used interchangeably, but the performance standards are different.

What does ASTM C-920 Class 35 mean?

ASTM C-920 Class 35 means the sealant can handle ±35% joint movement without losing adhesion or tearing. For a 1/2-inch joint, that is approximately 3/16 inch of compression and 3/16 inch of extension. Class 35 sealants are the professional standard for most building envelope applications including windows, doors, precast panels, and EIFS.

Can I paint over sealant?

It depends on the sealant type. Acrylic urethane sealants like Tower AU-1 are paintable within 24 hours of application. Silicone sealants are not paintable. Paint will not bond to cured silicone. If the joint will be painted, specify a paintable acrylic urethane sealant.

How long does sealant last?

Properly installed elastomeric sealant has a service life of 15 to 20 years for acrylic urethane and polyurethane products. Silicone sealants can last 50 to 60 years or more. Service life depends on proper surface prep, correct joint sizing, and environmental exposure. Joints exposed to extreme temperature cycling, UV radiation, or water immersion will degrade faster than protected joints.

What is the best sealant for windows?

For painted window perimeters, specify a Class 25 or Class 35 acrylic urethane sealant like Tower AU-1. It bonds to aluminum, vinyl, wood, masonry, and fiber cement without primer, accepts paint within 24 hours, and handles the thermal movement at window perimeters. For glass-to-metal curtainwall weatherseals, specify silicone.

Do I need primer for sealant?

Most professional-grade elastomeric sealants bond to common substrates without primer. Tower AU-1 bonds to concrete, masonry, wood, aluminum, vinyl, and fiber cement without primer on most surfaces. Smooth or non-porous substrates like glazed tile, glass, or powder-coated metal may require primer. Check the product data sheet for the specific substrate compatibility list.

What is elastomeric sealant?

Elastomeric sealant is a material that stretches and compresses repeatedly without permanent deformation. It returns to its original shape after each extension and compression cycle. Elastomeric performance is what allows a sealant to accommodate joint movement over decades of service. Standard acrylic caulk is not elastomeric and will crack or tear under repeated movement.

Can sealant be used as an adhesive?

Some sealants function as both sealant and adhesive. Structural glazing silicone is designed to bond glass to metal while maintaining flexibility. Most sealants are not designed to carry structural loads and should not be used where an adhesive is required. Check the product data sheet to confirm whether a sealant is rated for adhesive applications.

Conclusion: Understanding Sealants Is How You Prevent Callbacks

A sealant is not a gap filler or a cosmetic trim product. It is an elastomeric material engineered to create weathertight barriers while accommodating the compression and extension cycles that occur as buildings move through decades of thermal cycling, settlement, and wind load. The difference between a joint that lasts twenty years and a joint that fails in three comes down to matching sealant chemistry, movement class, and application technique to the specific demands of the joint.

Tower Sealants manufactures professional-grade elastomeric sealants for commercial and residential construction. Tower AU-1 Commercial Construction Sealant is a Class 35 acrylic urethane product that bonds to virtually every substrate without primer, cleans up with water, meets all-50-states VOC compliance, and accepts paint within 24 hours. Tower Silicone is a 100 percent silicone sealant with a 60-year guarantee for structural glazing, curtainwall weatherseals, and rooftop applications.

If you are specifying sealant for a commercial project or ordering material for a building envelope application, Tower Sealants has the product and the technical support to help you match sealant to application. Visit towersealants.com or call 866-897-7568 to talk through your project and confirm the right product for your spec.