A guide to joint sealants in construction

General

From towering buildings and expansive bridges to everyday homes and industrial facilities, joints are inherent in construction.

In our guide, we’ll delve into the world of joint sealants, exploring what they are, their various types, how they function, and the critical considerations for their selection and application. Whether you are a seasoned contractor or an experienced builder, understanding the nuances of joint sealants in construction is fundamental in delivering durable, compliant, and high-performance structures.

What are joint sealants?

A joint sealant is a material used to fill gaps or joints between two or more building components. Its primary purpose is to create a durable, flexible, and impermeable barrier that accommodates movement while preventing liquids, gases, and solids from passing through.

Sealants are designed to maintain their integrity and adhesive bond even when the joint experiences dynamic stresses, such as those caused by thermal expansion and contraction, structural settlement, or wind loading.

The correct use of sealants contributes significantly to:

Weatherproofing: Preventing water ingress into the building envelope.
Airtightness: Reducing uncontrolled air leakage, which improves energy efficiency and indoor air quality.
Acoustic Isolation: Dampening sound transmission through joints.
Fire Resistance: Certain sealants contribute to fire compartmentation.
Hygiene: Preventing the accumulation of dirt, dust, and contaminants in gaps, especially in sanitary and cleanroom environments.
Aesthetics: Providing a clean, finished appearance to joints.

Types of joint sealants

You can get an array of joint sealants, each formulated with specific chemical compositions to suit different applications, environmental conditions, and movement requirements. Understanding these variations is key to selecting the right product for your project.

Elastomeric Sealants

These are the most common and versatile joint sealants, known for their excellent flexibility and ability to accommodate significant joint movement. They cure to form a rubber-like, elastic material.

Silicone sealants:

Properties: Highly durable, excellent UV resistance, wide temperature stability, very good movement capability, good chemical resistance. They adhere well to glass, ceramics, metals, and some plastics.
Common Sses: Glazing, façade joints, movement joints in concrete, sanitary applications (bathrooms, kitchens), cleanrooms, and curtain walling. Neutral-cure silicones are generally preferred for building and construction due to their non-corrosive nature and broader adhesion range.

Polyurethane sealants (PUs):

Properties: Excellent adhesion to a wide range of substrates (concrete, masonry, wood, metal), good abrasion and tear resistance, can be painted over, and good chemical resistance.
Common uses: Expansion joints in concrete flooring, civil engineering structures (bridges, roads), general construction joints, pre-cast concrete elements, industrial floors, roofing, and perimeter sealing around windows and doors.

Modified Silane Polymers (MS Polymer sealants / Hybrid sealants):

Properties: A newer generation of sealants combining the best attributes of silicones and polyurethanes. They are solvent-free, odourless, paintable, have excellent adhesion to various substrates, good UV resistance, and excellent elasticity. They are also more environmentally friendly.
Common uses: Highly versatile, suitable for internal and external applications, general construction, perimeter sealing, bonding, damp surfaces, and often used as an alternative to silicones or polyurethanes where painting or improved environmental credentials are required.

Acrylic sealants

Acrylic sealants are made from water-based acrylic resins, making them easy to handle and environmentally friendly. They gained popularity in the 20th century as a versatile, paintable option for interior finishing. Their formulation makes them best suited to areas with minimal movement.

Properties: Water-based, easy to apply and clean up, paintable. However, they tend to have lower movement capability.
Common uses: Filling static cracks, sealing gaps around window and door frames internally, skirting boards, and other low-movement interior joints. Not suitable for external or wet areas unless formulated explicitly for these uses..

Butyl sealants

These are based on synthetic rubber compounds that remain permanently tacky and flexible. First developed for industrial and roofing applications, they have proven reliable for weatherproofing overlaps and joints. Their non-curing nature ensures they stay pliable for years!

Properties: Non-curing or slow-curing, providing a permanently flexible, tacky seal. Good water resistance and adhesion to many surfaces. Limited movement capability.
Common uses: Sealing overlaps in metal cladding, jointing membranes, specific applications in roofing, and low-movement joints where a permanent, flexible, and waterproof seal is needed.

Bituminous sealants

These are derived from bitumen, a by-product of crude oil refining, which gives them their black tar-like look. They have been used for decades in civil engineering and roofing for robust waterproofing. Their composition makes them ideal for areas exposed to harsh weather.

Properties: Black, tar-like sealants with excellent waterproofing properties. Typically used in specific external applications where asphalt or bitumen materials are present.
Common uses: Sealing around asphalt, concrete, and metal in roofing, foundations, and civil engineering works where waterproofing is paramount.

Epoxy sealants

Epoxy sealants are formulated as two-part systems that cure to form a rigid, high-strength bond. They became widely used in industrial and structural applications where durability and chemical resistance are important. With low flexibility, they’re best suited for static joints.

Properties: Epoxy sealants are usually two-part, rigid, high-strength sealants with excellent chemical resistance and compressive strength. They offer very little flexibility and are primarily used for static joints.
Common Uses: Industrial flooring, chemical containment areas, structural repairs where a rigid, strong bond is required rather than movement accommodation.

How are joint sealants used in construction?

Joint sealants play a vital role in many different applications in construction settings. Their specific application depends on the types of joints and the surrounding environment. Before the development of modern sealant technologies, builders relied on traditional materials such as putty, tar, bitumen, and natural fibres like hemp and oakum to seal gaps and joints, which usually required frequent maintenance.

The widespread use of synthetic polymer-based sealants began in the mid-20th century, driven by advances in chemistry and the need for buildings to withstand greater movement and weather.

Today, modern sealants are engineered to meet specific demands, including movement, moisture, fire protection, and structural bonding. Their popularity has grown as construction techniques have evolved towards lighter, larger, and more flexible structures, which place greater demands on joint movement and long-term durability.

Some ways we use joints today include:

Movement joints (Expansion, Contraction, Isolation joints): These are arguably the most critical applications. Buildings are constantly subjected to movement due to thermal expansion and contraction, seismic activity, settlement, and wind loading. Sealants in these joints (e.g., between concrete slabs, façade panels) must have high elasticity and movement capability to absorb these stresses without tearing or detaching.
Construction joints: Formed where one concrete pour meets another, these joints require sealing to prevent water ingress. While they may not experience significant movement, they still benefit from a flexible, durable seal.
Perimeter sealing: Sealing around window and door frames, pre-cast concrete panels, and other penetrations in the building envelope is crucial for airtightness and weatherproofing. This prevents air and water from bypassing the primary façade materials.
Sanitary and wet areas: In bathrooms, kitchens, and other areas exposed to moisture, sealants prevent water penetration, inhibit mould growth, and provide a hygienic, easy-to-clean finish around sinks, showers, and tiling.
Flooring: Sealants are used in control joints within concrete floors to manage cracking, as well as perimeter joints where the floor meets walls, accommodating differential movement.
Roads and pavements: Flexible sealants are essential for filling joints in concrete pavements and asphalt surfaces, protecting them from water ingress, debris, and frost damage caused by freeze-thaw cycles.
Roofing and cladding: Sealants are used to waterproof laps, penetrations, and joints in various roofing and cladding systems.
Firestopping: Certain fire-rated sealants are used to maintain the fire resistance of walls and floors where services (pipes, cables) penetrate, preventing the spread of fire and smoke.
Glazing: Structural glazing silicones are specifically formulated to bond glass panes to frames or structural elements, bearing the weight of the glass and resisting wind loads while accommodating thermal movement.

How to select the right joint sealants for your project

It’s important to understand the right product for your project and applications. A misstep here can lead to premature failure, requiring repairs further down the line. Here's a structured approach to selection:

Understanding the joint types and expected movement

Is it an expansion joint, a control joint, a perimeter joint, or a static crack?
What is the anticipated maximum movement (expansion and contraction) the joint will experience?

Identify the substrates

What materials form the joint? (e.g., concrete, brick, stone, metal, glass, wood, PVC).
Does the sealant have good adhesion to all of these substrates?
Are the substrates porous or non-porous?

Assess environmental exposure

UV Radiation: Will the sealant be exposed to direct sunlight? If so, select a UV-stable sealant (e.g., silicone, high-quality MS polymer).
Temperature Extremes: What are the maximum and minimum temperatures the sealant will experience? Ensure the sealant's service temperature range is suitable.
Moisture/Water Immersion: Will the sealant be in contact with water (e.g., in wet rooms, external joints)? Choose a waterproof and moisture-resistant sealant.
Chemical Exposure: Are there any chemicals (cleaning agents, industrial spills) that the sealant might encounter? Select a sealant with appropriate chemical resistance.
Traffic/Abrasion: Will the joint be subject to foot or vehicular traffic? If so, a durable, abrasion-resistant sealant (e.g., some polyurethanes) is needed.

Consider aesthetics

Does the sealant need to match the surrounding materials in colour?
Does it need to be paintable?
What kind of finish is desired (matte, glossy)?

Review application conditions and method

What are the ambient temperature and humidity during application? These can affect curing time.
Is the joint horizontal (pourable sealant) or vertical (thixotropic/gun-grade sealant)?
Are there any specific ventilation requirements due to solvent content?

Consult the manufacturer’s data sheets

What are the ambient temperature and humidity during application? These can affect curing time.

How to apply joint sealants

The performance of any joint sealant, regardless of its quality, is heavily reliant on correct application. Even the best sealant will fail if applied improperly. While general steps are outlined below, it is absolutely crucial to always refer to the manufacturer's specific instructions, technical data sheets, and safety data sheets for the particular product you are using. Different sealants have unique requirements regarding surface preparation, priming, curing conditions, and tooling.

1. Site and Joint Preparation: The Foundation of Success

This is the most critical step. Poor preparation is the leading cause of sealant failure.

Cleanliness: The joint surfaces must be clean, dry, and free from any contaminants such as dust, dirt, grease, oil, loose material, old sealant residues, curing compounds, or efflorescence. Use appropriate cleaning methods (e.g., wire brushing, grinding, solvents – as recommended by the manufacturer).
Dryness: The substrate must be thoroughly dry. Moisture in the joint can lead to bubbling, poor adhesion, or prevent proper curing.
Priming (If required): Some substrates or sealants require joint sealant primers to improve adhesion. If a primer is specified by the manufacturer, apply it strictly according to their instructions.

2. Sealant application

Product preparation: Ensure the sealant cartridge is at the recommended application temperature. Cut the nozzle at an angle to achieve the desired bead size for the joint.
Filling the Joint: Apply the sealant with a steady, even pressure, pushing the sealant into the joint to ensure full contact and wet out with the joint faces. Work slowly and methodically to avoid trapping air bubbles.

Tooling: Tooling is essential, not just for aesthetics, but also for performance.

3. Curing and inspection

Curing: Allow the sealant to cure fully according to the manufacturer's specified curing times.

Inspection: Once cured, visually inspect the sealant for any defects, such as voids, pinholes, poor adhesion, or inadequate tooling.

Joint sealant failures: Why do they happen?

Even with careful planning, sealant failures can occur. They can result in costly repairs, damage to the surrounding areas, or even wider structural issues if they’re not addressed. By understanding how and why sealants may fail, you can take practical steps to avoid this from happening in any future projects and extend the life of joints and façades.

Adhesion loss (Delamination): This is the most common form of failure, where the sealant detaches from one or both joint surfaces. It can be caused by incorrect surface preparation, incompatible substrate, incorrect or no primer, excessive joint movement beyond the sealant's capability, or exposure to water during curing.
Cohesive failure: The sealant tears or splits within its own body, rather than detaching from the substrate. It can be caused by Joint movement exceeding the sealant's elastic limit, incorrect joint design, or three-sided adhesion (no backer rod or sealant bonded to the bottom of the joint).

Bubbling/blistering: Air or gas bubbles appear on or beneath the sealant surface. It can be caused by moisture in the joint during application or curing, rapid curing due to high temperatures, porous substrates releasing trapped air, or improper tooling.

Cracking/chalking: The sealant surface becomes brittle, develops cracks, or degrades into a powdery substance. The usual causes include UV degradation, ageing, exposure to harsh chemicals, or continuous overstretching.

Staining: Discolouration of the adjacent substrate caused by using an incompatible sealant with the substrate, using non-staining primers, or inadequate tooling, leaving excess sealant on porous surfaces.

Insufficient curing: The sealant remains soft, tacky, or does not fully harden, which is usually caused by the incorrect mixing of two-part systems, low temperature or humidity inhibiting moisture-cure sealants, or contamination.

Incorrect joint design: The joint width-to-depth ratio is incorrect, or a backer rod was not used. This can lead to excessive stress on the sealant and premature failure.

Improper tooling: Failure to tool the sealant adequately can lead to poor adhesion at the joint edges and trapped air, compromising the seal.

Find joint sealants for your project today!

Don't leave the integrity of your joints to chance. Browse our extensive range of high-quality joint sealants today, designed to meet the rigorous demands of various applications. If you have any questions about our products or want to learn more about how they can support your project, contact us today.

By Katie Taylor